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Title: Territory in Bird Life
Author: Howard, H. Eliot
Language: English
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*** Start of this LibraryBlog Digital Book "Territory in Bird Life" ***
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| TRANSCRIBER'S NOTE: |
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| There are a large number of compound words in |
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TERRITORY IN BIRD LIFE
[Illustration: A pair of Lesser Spotted Woodpeckers attacking a Great
Spotted Woodpecker
Emery Walker ph.sc.]
TERRITORY IN
BIRD LIFE
BY H. ELIOT HOWARD
WITH ILLUSTRATIONS BY
G. E. LODGE AND H. GRÖNVOLD
NEW YORK
E. P. DUTTON AND COMPANY
1920
PREFACE
When studying the Warblers some twenty years ago, I became aware of the
fact that each male isolates itself at the commencement of the breeding
season and exercises dominion over a restricted area of ground. Further
investigation, pursued with a view to ascertaining the relation of this
particular mode of behaviour to the system of reproduction, led to my
studying various species, not only those of close affinity, but those
widely remote in the tree of avian life. The present work is the outcome
of those investigations. In it I have endeavoured to interpret the
prospective value of the behaviour, and to trace out the relationships
in the organic and inorganic world which have determined its survival.
Much is mere speculation; much with fuller knowledge may be found to be
wrong. But I venture to hope that a nucleus will remain upon which a
more complete territorial system may one day be established.
I have to thank Mr. G. E. Lodge and Mr. H. Grönvold for the trouble they
have taken in executing my wishes; I also want to record my indebtedness
to the late E. W. Hopewell; and to Professor Lloyd Morgan, F.R.S., I am
beholden more than I can tell.
CONTENTS
PAGE
CHAPTER I
INTRODUCTION 1
CHAPTER II
THE DISPOSITION TO SECURE A TERRITORY 20
CHAPTER III
THE DISPOSITION TO DEFEND THE TERRITORY 73
CHAPTER IV
THE RELATION OF SONG TO THE TERRITORY 119
CHAPTER V
THE RELATION OF THE TERRITORY TO THE SYSTEM
OF REPRODUCTION 169
CHAPTER VI
THE WARFARE BETWEEN DIFFERENT SPECIES AND ITS
RELATION TO THE TERRITORY 216
CHAPTER VII
THE RELATION OF THE TERRITORY TO MIGRATION 259
INDEX 302
LIST OF ILLUSTRATIONS
_Face page_
A pair of Lesser Spotted Woodpeckers attacking a Great
Spotted Woodpecker _Frontispiece_
Territorial flight of the Black-tailed Godwit 54
Competition for territory is seldom more severe than
among cliff-breeding seabirds, and the efforts of
individual Razorbills to secure positions on the
crowded ledges lead to desperate struggles 64
Male Blackbirds fighting for the possession of territory.
The bare skin on the crown of the defeated bird shows
the nature of the injuries from which it succumbed 74
Male Cuckoos fighting before the arrival of a female 82
Two pairs of Pied Wagtails fighting in defence of their
territories 86
Long-tailed Tit: males fighting for the possession of
territory. The feathers have been torn from the crown
of the defeated and dying rival 96
A battle between two pairs of Jays 106
The Female Chaffinch shares in the defence of the territory
and attacks other females 110
Peregrine Falcon attacking a Raven 216
A battle between a pair of Green Woodpeckers and a
Great Spotted Woodpecker for the possession of a hole
in an oak-tree 238
Plans of the Water-meadow showing the Territories
occupied by Lapwings in 1915 and 1916 _Between_ 58 and 59
SCIENTIFIC NAMES OF BIRDS MENTIONED IN THE TEXT
Raven _Corvus corax._
Carrion-Crow _Corvus corone._
Hooded Crow _Corvus cornix._
Rook _Corvus frugilegus._
Magpie _Pica pica._
Jay _Garrulus glandarius rufitergum._
Chough _Pyrrhocorax pyrrhocorax._
Starling _Sturnus vulgaris._
Greenfinch _Chloris chloris._
Hawfinch _Coccothraustes coccothraustes._
House-Sparrow _Passer domesticus._
Chaffinch _Fringilla cœlebs._
Brambling _Fringilla montifringilla._
Linnet _Acanthis cannabina._
Corn-Bunting _Emberiza calandra._
Yellow Bunting _Emberiza citrinella._
Cirl Bunting _Emberiza cirlus._
Reed-Bunting _Emberiza schœniclus._
Sky-Lark _Alauda arvensis._
Pied Wagtail _Motacilla lugubris._
Tree-Pipit _Anthus trivialis._
Meadow-Pipit _Anthus pratensis._
Great Titmouse _Parus major newtoni._
Blue Titmouse _Parus cœruleus obscurus._
Long-tailed Titmouse _Ægithalus caudatus roseus._
Red-backed Shrike _Lanius collurio._
Whitethroat _Sylvia communis._
Lesser Whitethroat _Sylvia curruca._
Blackcap _Sylvia atricapilla._
Grasshopper-Warbler _Locustella nœvia._
Savi's Warbler _Locustella luscinioides._
Reed-Warbler _Acrocephalus scirpaceus._
Marsh-Warbler _Acrocephalus palustris._
Sedge-Warbler _Acrocephalus schœnobænus._
Willow-Warbler _Phylloscopus trochilus._
Wood-Warbler _Phylloscopus sibilatrix._
Chiffchaff _Phylloscopus collybita._
Song-Thrush _Turdus musicus clarkii._
Redwing _Turdus iliacus._
Blackbird _Turdus merula._
Redstart _Phœnicurus phœnicurus._
Redbreast _Erithacus rubecula melophilus._
Nightingale _Luscinia megarhyncha._
Stonechat _Saxicola rubicola._
Whinchat _Saxicola rubetra._
Wheatear _Œnanthe œnanthe._
Hedge-Sparrow _Accentor modularis._
Wren _Troglodytes troglodytes._
Spotted Flycatcher _Muscicapa striata._
Swallow _Hirundo rustica._
Martin _Delichon urbica._
Sand-Martin _Riparia riparia._
Great Spotted Woodpecker _Dryobates major anglicus._
Lesser Spotted Woodpecker _Dryobates minor._
Green Woodpecker _Picus viridis._
Cuckoo _Cuculus canorus._
Tawny Owl _Strix aluco._
Buzzard _Buteo buteo._
Sparrow-Hawk _Accipiter nisus._
Peregrine Falcon _Falco peregrinus._
Merlin _Falco æsalon._
Kestrel _Falco tinnunculus._
Shag _Phalacrocorax graculus._
Wild Duck _Anas boschas._
Snipe _Gallinago gallinago._
Dunlin _Tringa alpina._
Ruff _Machetes pugnax._
Redshank _Totanus totanus._
Black-tailed Godwit _Limosa limosa._
Curlew _Numenius arquata._
Whimbrel _Numenius phæopus._
American Golden Plover _Charadrius dominicus._
Lapwing _Vanellus vanellus._
Oyster-Catcher _Hæmatopus ostralegus._
Herring-Gull _Larus argentatus._
Kittiwake _Rissa tridactyla._
Razorbill _Alca torda._
Guillemot _Uria troille._
Puffin _Fratercula arctica._
Fulmar _Fulmarus glacialis._
Water-Rail _Rallus aquaticus._
Corn-Crake _Crex crex._
Moor-Hen _Gallinula chloropus._
Coot _Fulica atra._
Wood-Pigeon _Columba palumbus._
Turtle-Dove _Streptopelia turtur._
Partridge _Perdix perdix._
Black Grouse _Lyrurus tetrix britannicus._
Red Grouse _Lagopus scoticus._
TERRITORY IN BIRD LIFE
CHAPTER I
INTRODUCTION
In his _Manual of Psychology_ Dr Stout reminds us that "Human language
is especially constructed to describe the mental states of human beings,
and this means that it is especially constructed so as to mislead us
when we attempt to describe the working of minds that differ in a great
degree from the human."
The use of the word "territory" in connection with the sexual life of
birds is open to the danger which we are here asked to guard against,
and I propose, therefore, before attempting to establish the theory on
general grounds, to give some explanation of what the word is intended
to represent and some account of the exact position that representation
is supposed to occupy in the drama of bird life.
The word is capable of much expansion. There cannot be territories
without boundaries of some description; there cannot well be boundaries
without disputes arising as to those boundaries; nor, one would
imagine, can there be disputes without consciousness as a factor
entering into the situation; and so on, until by a simple mental
process we conceive of a state in bird life analogous to that which we
know to be customary amongst ourselves. Now, although the term "breeding
territory," when applied to the sexual life of birds, is not altogether
a happy one, it is difficult to know how otherwise to give expression to
the facts observed. Let it then be clearly understood that the
expression "securing a territory" is used to denote a process, or rather
part of a process, which, in order to insure success to the individual
in the attainment of reproduction, has been gradually evolved to meet
the exigencies of diverse circumstances. Regarded thus, we avoid the
risk of conceiving of the act of securing a territory as a detached
event in the life of a bird, and avoid, I hope, the risk of a conception
based upon the meaning of the word when used to describe human as
opposed to animal procedure.
Success in the attainment of reproduction is rightly considered to be
the goal towards which many processes in nature are tending. But what is
meant by success? Is it determined by the actual discharge of the sexual
function? So many and so wonderful are the contrivances which have
slowly been evolved to insure this discharge, that it is scarcely
surprising to find attention focused upon this one aspect of the
problem. Yet a moment's reflection will show that so limited a
definition of the term "success" can only be held to apply to certain
forms of life; for where the young have to be cared for, fostered, and
protected from molestation for periods of varying lengths, the actual
discharge of the sexual function marks but one stage in a process which
can only succeed if all the contributory factors adequately meet the
essential conditions of the continuance of the species.
Securing a territory is then part of a process which has for its goal
the successful rearing of offspring. In this process the functioning of
the primary impulse, the acquirement of a place suitable for breeding
purposes, the advent of a female, the discharge of the sexual function,
the construction of the nest, and the rearing of offspring follow one
another in orderly sequence. But since we know so little of the organic
changes which determine sexual behaviour, and have no means of
ascertaining the nature of the impulse which is first aroused, we can
only deal with the situation from the point at which the internal
organic changes reflect themselves in the behaviour to a degree which is
visible to an external observer. That point is reached when large
numbers of species, forsaking the normal routine of existence to which
they have been accustomed for some months, suddenly adopt a radical
change in their mode of behaviour. How is this change made known to us?
By vast numbers of individuals hurrying from one part of the globe to
another, from one country to another, and even from mid-ocean to the
coasts; by detachments travelling from one district to another; by
isolated individuals deserting this place for that; by all those
movements, in fact, which the term migration, widely applied, is held to
denote. Now the impulse which prompts these travelling hosts must be
similar in kind whether the journey be long or short; and it were
better, one would think, to regard such movements as a whole than to
fix the attention on some one particular journey which fills us with
amazement on account of the magnitude of the distance traversed or the
nature of the difficulties overcome. For, after all, what does each
individual seek? There may be some immature birds which, though they
have not reached the necessary stage of development, happen to fall in
with others in whom the impulse is strong and are led by them--they know
not where. But the majority seek neither continent nor country, neither
district nor locality is their aim, but a place wherein the rearing of
offspring can be safely accomplished; and the search for this place is
the earliest visible manifestation in many species of the reawakening of
the sexual instinct.
The movements of each individual are then directed towards a similar
goal, namely, the occupation of a definite station; and this involves
for many species a distinct change in the routine of behaviour to which
previously they had been accustomed. Observe, for example, one of the
numerous flocks of Finches that roam about the fields throughout the
winter. Though it may be composed of large numbers of individuals of
different kinds, yet the various units form an amicable society actuated
by one motive--the procuring of food. And since it is to the advantage
of all that the individual should be subordinated to the welfare of the
community as a whole there is no dissension, apart from an occasional
quarrel here and there.
In response, however, to some internal organic change, which occurs
early in the season, individuality emerges as a factor in the developing
situation, and one by one the males betake themselves to secluded
positions, where each one, occupying a limited area, isolates itself
from companions. Thereafter we no longer find that certain fields are
tenanted by flocks of greater or less dimensions, while acres of land
are uninhabited, but we observe that the hedgerows and thickets are
divided up into so many territories, each one of which contains its
owner. This procedure, with of course varying detail, is typical of that
of many species that breed in Western Europe. And since such a radical
departure from the normal routine of behaviour could scarcely appear
generation after generation in so many widely divergent forms, and still
be so uniform in occurrence each returning season, if it were not
founded upon some congenital basis, it is probable that the journey,
whether it be the extensive one of the Warbler or the short one of the
Reed-Bunting, is undertaken in response to some inherited disposition,
and probable also that the disposition bears some relation to the few
acres in which the bird ultimately finds a resting place. Whilst for the
purpose of the theory I shall give expression to this behaviour in terms
of that theory, and speak of it as a disposition to secure a territory,
using the word disposition, which has been rendered current in recent
discussion, for that part of the inherited nature which has been
organised to subserve a specific biological purpose--strict compliance
with the rules of psychological analysis requires a simpler definition;
let us therefore say "disposition to remain in a particular place in a
particular environment."
But even granting that this disposition forms part of the hereditary
equipment of the bird, how is the process of reproduction furthered? The
mere fact of remaining in or about a particular spot cannot render the
attainment of reproduction any less arduous, and may indeed add to the
difficulties, for any number of individuals might congregate together
and mutually affect one another's interests. A second disposition comes,
however, into functional activity at much the same stage of sexual
development, and manifests itself in the male's intolerance of other
individuals. And the two combined open up an avenue through which the
individual can approach the goal of reproduction. In terms of the theory
I shall refer to this second disposition as the one which is concerned
with the defence of the territory.
Broadly speaking, these two dispositions may be regarded as the basis
upon which the breeding territory is founded. Yet inasmuch as the
survival value of the dispositions themselves must have depended upon
the success of the process as a whole, it is manifest that peculiar
significance must not be attached to just the area occupied, which
happens to be so susceptible of observation; other contributory factors
must also receive attention, for the process is but an order of
relationships in which the various units have each had their share in
determining the nature and course of subsequent process, so that, as Dr
Stout says, when they were modified, it was modified.
Now the male inherits a disposition which leads it to remain in a
restricted area, but the disposition cannot determine the extent of that
area. How then are the boundaries fixed? That they are sometimes adhered
to with remarkable precision, that they can only be encroached upon at
the risk of a conflict--all of this can be observed with little
difficulty. But if we regard them as so many lines definitely delimiting
an area of which the bird is cognisant, we place the whole behaviour on
a different level of mental development, and incidentally alter the
complexion of the whole process. It would be a mistake, I think, to do
this. Though conscious intention as a factor may enter the situation,
there is no necessity for it to do so; there is no necessity, that is to
say, for the bird to form a mental image of the area to be occupied and
shape its course accordingly. The same result can be obtained without
our having recourse to so complex a principle of explanation, and that
by the law of habit formation. In common with other animals, birds are
subject to this law in a marked degree. An acquired mode of activity
becomes by repetition ingrained in the life of the individual, so that
an action performed to-day is liable to be repeated to-morrow so long as
it does not prejudice the existence or annul the fertility of the
individual.
Let us see how this may have operated in determining the limits of the
area acquired, and for this purpose let us suppose that we are observing
a male Reed-Bunting recently established in some secluded piece of marsh
land. Scattered about this particular marsh are a number of small
willows and young alder trees, each one of which is capable of providing
plenty of branches suitable for the bird to perch upon, and all are in a
like favourable position so far as the outlook therefrom is concerned.
Well, we should expect to find that each respective tree would be made
use of according to the position in which the bird happened to find
itself. But what actually do we find--one tree singled out and resorted
to with ever-increasing certainty until it becomes an important point in
relation to the occupied area, a headquarters from which the bird
advertises its presence by song, keeps watch upon the movements of its
neighbours, and sets out for the purpose of securing food. We then take
note of its wanderings in the immediate vicinity of the headquarters,
especially as regards the direction, frequency, and extent of the
journeys; and we discover not only that these journeys proceed from and
terminate in the special tree, but that there is a sameness about the
actual path that is followed. The bird takes a short flight, searches a
bush here and some rushes there, returns, and after a while repeats the
performance; we on our part mark the extreme limits reached in each
direction, and by continued observation discover that these limits are
seldom exceeded, that definition grows more and more pronounced, and
that by degrees the movements of the bird are confined within a
restricted area. In outline, this is what happens in a host of cases. By
repetition certain performances become stereotyped, certain paths fixed,
and a routine is thus established which becomes increasingly definite as
the season advances.
But while it would be quite untrue to say that this routine is never
departed from, and equally profitless to attempt to find a point beyond
which the bird will under no circumstances wander, yet there is enough
definition and more than enough to answer the purpose for which the
territory has, I believe, been evolved, that is to say the biological
end of reproduction. Again, however, the process of adjustment is a
complex one. Habit plays its part in determining the boundaries in a
rough and ready manner, but the congenital basis, which is to be found
in the behaviour adapted to a particular environment, is an important
factor in the situation. For example, if instead of resting content with
just a bare position sufficient for the purpose of reproduction, the
Guillemot were to hustle its neighbours from adjoining ledges, the
Guillemot as a species would probably disappear; or if instead of
securing an area capable of supplying sufficient food both for itself
and its young, the Chiffchaff were to confine itself to a single tree,
and, after the manner of the Guillemot, trust to spasmodic excursions
into neutral ground for the purpose of obtaining food, the Chiffchaff
as a species would probably not endure. All such adjustments have,
however, been brought about by relationships which have gradually become
interwoven in the tissue of the race.
The intolerance that the male displays towards other individuals,
usually of the same sex, leads to a vast amount of strife. Nowhere in
the animal world are conflicts more frequent, more prolonged, and more
determined than in the sexual life of birds; and though they are
acknowledged to be an important factor in the life of the individual,
yet there is much difference of opinion as to the exact position they
occupy in the drama of bird life. Partly because they frequently happen
to be in evidence, partly because they are numerically inferior, and
partly, I suppose, because the competition thus created would be a means
of maintaining efficiency, the females, by common consent, are supposed
to supply the condition under which the pugnacious nature of the male is
rendered susceptible to appropriate stimulation. And so long as the
evidence seemed to show that battles were confined to the male sex, so
long were there grounds for hoping that their origin might be traced to
such competition. But female fights with female, pair with pair, and,
which is still more remarkable, a pair will attack a single male or a
single female; moreover, males that reach their destination in advance
of their prospective mates engage in serious warfare. How then is it
possible to look upon the individuals of one sex as directly
responsible for the strife amongst those of the other, or how can the
female supply the necessary condition? As long as an attempt is made to
explain it in terms of the female, the fighting will appear to be of a
confused order; regard it, however, as part of a larger process which
demands, amongst other essential conditions of the breeding situation,
the occupation of a definite territory, and order will reign in place of
confusion.
But even supposing that the male inherits a disposition to acquire a
suitable area, even supposing that it inherits a disposition which
results indirectly in the defence of that area, how does it obtain a
mate? If the female behaved in a like manner, if she, too, were to
isolate herself and remain in one place definitely, that would only add
to the difficulties of mutual discovery. We find, however, in the
migrants, that the males are earlier than the females in reaching the
breeding grounds, and, in resident species, that they desert the females
and retire alone to their prospective territories, so that there is a
difference in the behaviour of the sexes at the very commencement of the
sexual process. What is the immediate consequence? Since the male
isolates itself, it follows, if the union of the sexes is to be
effected, that the discovery of a mate must rest largely with the
female. This of course reverses the accepted course of procedure. But
after all, what reason is there to suppose that, the male seeks the
female, or that a mutual search takes place; what reason to think that
this part of the process is subject to no control except such as may be
supplied by the laws of chance?
Now, clearly, much will depend upon the rapidity with which the female
can discover a male fit to breed; for if the course of reproduction is
to flow smoothly, there must be neither undue delay nor waste of energy
incurred in the search--some guidance is therefore necessary, some
control in her external environment. Here the song, or the mechanically
produced sound, comes into play, and assists in the attainment of this
end. Nevertheless if every male were to make use of its powers whether
it were in occupation of a territory or not, if the wandering individual
had an equal chance of attracting a mate, then it would be idle to
attempt to establish any relation between "song" on the one hand, and
"territory" on the other, and impossible to regard the voice as the
medium through which an effectual union of the sexes is procured. But
there is reason to believe that the males utilise their powers of
producing sound only under certain well-defined conditions. For
instance, when they are on their way to the breeding grounds, or moving
from locality to locality in search of isolation, or when they desert
their territories temporarily, as certain of the residents often do,
they are generally silent; but when they are in occupation of their
territories they become vociferous--and this is notoriously the case
during the early hours of the day, which is the period of maximum
activity so far as sexual behaviour is concerned. So that just at the
moment when the sexual impulse of the female is most susceptible to
stimulation, the males are betraying their positions and are thus a
guide to her movements. Nevertheless, even though she may have
discovered a male ready to breed, success is not necessarily assured to
her; for with multitudes of individuals striving to procreate their
kind, it would be surprising if there were no clashing of interests, if
no two females were ever to meet in the same occupied territory.
Competition of this kind is not uncommon, and the final appeal is to the
law of battle, just as an appeal to physical strength sometimes decides
the question of the initial ownership of a territory.
I shall try to make clear the relations of the various parts to the
whole with the assistance of whatever facts I can command. I shall do so
not only for the purposes of the theory, but because one so often finds
the more important features of sexual behaviour regarded as so many
distinct phenomena requiring separate treatment, whereas they are
mutually dependent, and follow one another in ordered sequence. I spoke
of the process as a series of relationships. Some of these relationships
have already been touched upon; others will become apparent if we
consider for a moment the purposes for which the territory has been
evolved. Indirectly its purpose is that of the whole process, the
rearing of offspring. But inasmuch as a certain measure of success could
be attained, and that perhaps often, without all the complications
introduced by the territory, there are manifestly advantages to be
gained by its inclusion in the scheme. The difficulties which beset the
path of reproduction are by no means always the same--all manner of
adjustments have to be made to suit the needs of different species.
There are direct relationships, such as we have been speaking of, which
are essential to the every-day working of the process, and others which
are indirect, though none the less important for they must have
exercised an influence throughout the ages. These latter are furnished
by the physical--the inorganic world, by climate, by the supply of the
particular kind of breeding stations, by the scarcity or abundance of
the necessary food and by the relative position of the food supply to
the places suitable for breeding. Why does the Reed-Bunting cling so
tenaciously to an acre or more of marshy ground, while the Guillemot
rests content with a few square feet on a particular ledge of rock? The
answer is the same in both cases--to facilitate reproduction. But why
should a small bird require so many square yards, whilst a very much
larger one is satisfied with so small an area? The explanation must be
sought in the conditions of existence. The Reed-Bunting has no
difficulty in finding a position suitable for the construction of its
nest; there are acres of waste land and reedy swamps capable of
supplying food for large numbers of individuals, and the necessary
situations for countless nests. But its young, like those of many
another species, are born in a very helpless state. For all practical
purposes they are without covering of any description and consequently
require protection from the elements, warmth from the body of the
brooding bird, and repeated supplies of nourishment. A threefold burden
is thus imposed upon the parents: they must find food for themselves,
they must afford protection to the young by brooding, and they must
supply them with the necessary food at regular intervals. And their
ability to do all this that is demanded of them will be severely taxed
by the brooding which must perforce curtail the time available for the
collection of food.
Let us then suppose that the Reed-Buntings inhabiting a certain piece of
marsh are divided into two classes, those which are pugnacious and
intolerant of the approach of strangers, and those which welcome their
presence. The nests of the former will be built in isolation, those of
the latter in close proximity. In due course eggs will be laid and
incubation performed, and thus far all alike will probably be
successful. Here, however, a critical point is reached. If the young are
to be freed from the risk of exposure, the parents must find the
necessary supply of food rapidly. But manifestly all will not be in a
like satisfactory position to accomplish this, for whereas the isolated
pairs will have free access to all the food in the immediate vicinity of
the nest, those which have built in proximity to one another, meeting
competition in every direction, will be compelled to roam farther
afield and waste much valuable time by doing so; and under conditions
which can well be imagined, even this slight loss of time will be
sufficient to impede the growth of the delicate offspring, or to lead
perhaps to still greater disaster. If any one doubts this, let him first
examine one of the fragile offspring; let him then study the conditions
under which it is reared, observing the proportion of time it passes in
sleep and the anxiety of the parent bird to brood; and finally let him
picture to himself its plight in a wet season if, in order to collect
the necessary food, the parents were obliged to absent themselves for
periods of long duration.
Now take the case of the Guillemot. Its young at birth are by no means
helpless in the sense that the young Reed-Bunting is, and food is
readily procured. But breeding stations are scarce, for although there
are many miles of cliff-bound coast, yet not every type of rock
formation produces the fissures and ledges upon which the bird rests.
Hence vast stretches of coast-line remain uninhabited, and the birds are
forced to concentrate at certain points, where year after year they
assemble in countless numbers from distant parts of the ocean. If, then,
different individuals were to jostle one another from adjoining
positions, and each one were to attempt to occupy a ledge in solitary
State, not only would the successful ones gain no advantage from the
additional space over which they exercised dominion, but inasmuch as
many members that were fitted to breed would be precluded from doing
so, the status of the species as a whole would be seriously affected.
The amount of space occupied by each individual is therefore a matter of
urgent importance. A few square feet of rock sufficient for the
immediate purpose of incubation is all that can be allowed if the
species is to maintain its position in the struggle for existence.
Our difficulty in estimating the importance of the various factors that
make for success or failure arises from our inability to see more than a
small part of the scene as it slowly unfolds itself. The peculiar
circumstances under which these cliff-breeding forms dwell does,
however, enable us to picture, on the one hand, the precarious situation
of an individual that was incapable of winning or holding a position at
the accustomed breeding station, and, on the other, the plight of the
species as a whole if each one exercised authority over too large an
area. With the majority of species it is difficult to do this. So many
square miles of suitable breeding ground are inhabited by so few
Reed-Buntings that, even supposing certain members were to establish an
ascendency over too wide an area, it would be impossible to discover by
actual observation whether the race as a whole were being adversely
affected. Competition doubtless varies at different periods and in
different districts according to the numerical standing of the species
in a given locality and according to the numerical standing of others
that require similar conditions of existence; at times it may even be
absent, just as at any moment it may become acute. These examples show
how profoundly the evolution of the breeding territory may have been
influenced by relationships in the inorganic world, and they give some
idea of the intricate nature of the problem with which we have to deal.
I mentioned that the first visible manifestation of the revival of the
sexual instinct was to be found in the movements undertaken by the males
at the commencement of the breeding season. Such movements are
characterised by a definiteness of purpose, whether they involve a
protracted journey of some hundreds of miles or merely embrace a parish
or so in extent, and that purpose is the acquirement of a territory
suitable for rearing offspring. They are thus directly related to the
territory, and the question arises as to whether their origin may not be
traced to such relatedness. So long as we fix our attention solely upon
the magnitude of the distance traversed the suggestion may seem a
fanciful one. Nevertheless, if the battles between males of the same
species _are_ directly related to the occupation of a position suitable
for breeding purposes, if those which occur between males of closely
related forms _can_ be traced to a similar source, if the females take
their share in the defence of the ground that is occupied, if, in short,
the competition is as severe as I believe it to be, and is wholly
responsible for the strife which is prevalent at the commencement of the
breeding season--then such competition must have introduced profound
modifications in the distribution of species; it must have even
influenced the question of the survival of certain forms and the
elimination of others; and since the powers of locomotion of a bird are
so highly developed it must have led to an extension of breeding range,
limited only by unfavourable conditions of existence.
CHAPTER II
THE DISPOSITION TO SECURE A TERRITORY
Those who have studied bird life throughout the year are aware that the
distribution of individuals changes with the changing seasons. During
autumn and winter, food is not so plentiful and can only be found in
certain places, and so, partly by force of circumstances and partly on
account of the gregarious instinct which then comes into functional
activity, different individuals are drawn together and form flocks of
greater or less dimensions, which come and go according to the
prevailing climatic conditions. But with the advent of spring a change
comes over the scene: flocks disperse, family parties break up, summer
migrants begin to arrive, and the hedgerows and plantations are suddenly
quickened into life. The silence of the winter is broken by an outburst
of song from the throats of many different species, and individuals
appear in their old haunts and vie with one another in advertising their
presence by the aid of whatever vocal powers they happen to possess--the
Woodpecker utters its monotonous call from the accustomed oak; the
Missel-Thrush, perched upon the topmost branches of the elm,
persistently repeats its few wild notes; and the Swallow returns to the
barn.
All of this we observe each season, and our thoughts probably travel to
the delicate piece of architecture in the undergrowth, or to the hole
excavated with such skill in the tree trunk; to the beautifully shaped
eggs; to the parent birds carrying out their work with devoted zeal--in
fact, to the whole series of events which complete the sexual life of
the individual; and the attachment of a particular bird to a particular
spot is readily accounted for in terms of one or other of the emotions
which centre round the human home.
But if this behaviour is to be understood aright; if, that is to say,
the exact position it occupies in the drama of bird life is to be
properly determined, and its biological significance estimated at its
true value, it is above all things necessary to refrain from appealing
to any one of the emotions which we are accustomed to associate with
ourselves, unless our ground for doing so is more than ordinarily
secure. I shall try to show that, in the case of many species, the male
inherits a disposition to secure a territory; or, inasmuch as the word
"secure" carries with it too much prospective meaning, a disposition to
remain in a particular place when the appropriate time arrives.
If the part which the breeding territory plays in the sexual life of
birds is the important one I believe it to be, it follows that the
necessary physiological condition must arise at an early stage in the
cycle of events which follow one another in ordered sequence and make
towards the goal of reproduction, and that the behaviour to which it
leads must be one of the earliest visible manifestations of the seasonal
development of the sexual instinct. When does this seasonal development
occur? For how long does the instinct lie dormant? In some species there
is evidence of this first step in the process of reproduction early in
February; there is reason to believe that in others the latter part of
January is the period of revival; and the possibility must not be
overlooked of still earlier awakenings, marked with little definiteness,
though nevertheless of sufficient strength to call into functional
activity the primary impulse in the sexual cycle. Here, then, we meet
with a difficulty so far as direct observation is concerned, for the
duration of the period of dormancy and the precise date of revival vary
in different species; and, if accurate information is to be obtained,
the study of the series of events which culminate in the attainment of
reproduction ought certainly to begin the moment behaviour is
influenced by the internal changes, whatever they may be, which are
responsible for the awakening of the sexual instinct.
In considering how this difficulty might be met, the importance of
migratory species as a channel of information was gradually borne in
upon me; for it seemed that the definiteness with which the initial
stage in the sexual process was marked off, as a result of the incidence
of migration, would go far towards removing much of the obscurity which
appeared to surround the earlier stages of the breeding problem in the
case of resident species. Recent observation has shown that I
exaggerated this difficulty, and that it is generally possible to
determine with reasonable accuracy the approximate date at which the
internal changes begin to exert an influence on the behaviour of
resident species also. Nevertheless, the specialised behaviour of the
migrants furnished a clue, and pointed out the direction which further
inquiry ought to take.
Those who are accustomed to notice the arrival of the migrants are aware
that the woods, thickets, and marshes do not suddenly become occupied by
large numbers of individuals, but that the process of "filling up" is a
gradual one. An individual appears here, another there; then after a
pause there is a further addition, and so on with increasing volume
until the tide reaches its maximum, then activity wanes, and the slowly
decreasing number of fresh arrivals passes unnoticed in the wealth of
new life that everywhere forces itself upon our attention. If now,
instead of surveying the migrants as a whole, our attention be directed
to one species only, this gradual arrival of single individuals in their
accustomed haunts will become even more apparent; and if the
investigation be pursued still further and these single individuals
observed more closely, it will be found that in nearly every case they
belong to the male sex. Males therefore arrive before females. This does
not mean, however, that the respective times of arrival of the males and
females belonging to any one species are definitely divided, for males
continue to arrive even after some of the females have reached their
destination; and thus a certain amount of overlapping occurs. A truer
definition of the order of migration would be as follows:--Some males
arrive before others, and some females arrive before others, but on the
average males arrive before females. This fact has long been known.
Gätke refers to it in his _Birds of Heligoland_. "Here in Heligoland,"
he says, "the forerunners of the spring migration are invariably old
males; a week or two later, solitary old females make their appearance;
and after several weeks, both sexes occur mixed, _i.e._, females and
younger males; while finally only young birds of the previous year are
met with." Newton alludes to it as follows:--"It has been ascertained by
repeated observation that in the spring movement of most species of the
northern hemisphere, the cock birds are always in the van of the
advancing army, and that they appear some days, or perhaps weeks, before
the hens"; and Dr Eagle Clarke, in his _Studies in Bird Migration_,
makes the following statement:--"Another characteristic of the spring
is that the males, the more ardent suitors, of most species, travel in
advance of the females, and arrive at their meeting quarters some days,
it is said in some cases even weeks, before their consorts." Some
interesting details were given in _British Birds_[1] in regard to the
sex of the migrants that were killed by striking the lantern at the
Tuskar Rock, Co. Wexford, on the 30th April 1914. In all, there were
twenty-four Whitethroats, nine Willow-Warblers, eight Sedge-Warblers,
and six Wheatears; and on dissection it was found that twenty
Whitethroats, seven Willow-Warblers, eight Sedge-Warblers, and one
Wheatear were males.
What a curious departure this seems from the usual custom in the animal
world! Here we have the spectacle afforded us of the males, in whom
presumably the sexual instinct has awakened, deserting the females just
at the moment when we might reasonably expect their impulse to accompany
them would be strongest; and this because of their inherited disposition
to reach the breeding grounds. If, in order to attain to reproduction,
the male depended primarily upon securing a female--whether by winning
or fighting matters not at the moment--if her possession constituted the
sole difference in his external environment between success and failure,
then surely one would suppose that an advantage must rest with those
individuals which, instead of rushing forward and inflicting upon
themselves a life of temporary isolation, remained with the females and
increased their opportunities for developing that mutual appreciation
which, by some, is held to be a necessary prelude to the completion of
the sexual act, and to which close companionship would tend to impart a
stimulus.
In thus speaking, however, we assume that the revival of the sexual
instinct in the migratory male is coincident in time with its return to
the breeding quarters; and we do so because the act of migrating is
believed to be the first step in the breeding process. But it is well to
bear in mind just how much of this assumption is based upon fact, and
how much is due to questionable inference. All that can be definitely
asserted is this, that appropriate dissection reveals in most of the
migrants, upon arrival at their destination, unquestionable evidence of
seasonal increase in the size of the sexual organs. Beyond this there is
nothing to go upon. Yet if the term "sexual instinct" is held to
comprise the whole series of complex relationships which are manifest to
us in numerous and specialised modes of behaviour, which ultimately lead
to reproduction, and which have gradually become interwoven in the
tissue of the race, there can be little doubt that the assumption is a
reasonable one. To some, the term may recall the fierce conflicts which
are characteristic of the season; to others, emotional response; to not
a few, perhaps, the actual discharge of the sexual function--all of
these, it is true, are different aspects of the one instinct; but at the
same time each one marks a stage in the process, and the different
stages follow one another in ordered sequence. However, we are not
concerned at the moment with the term in its wider application; we wish
to know the precise stage at which the disposition to mate influences
the behaviour of the male. Is the female to him, from the moment the
seasonal change in his sexual organs takes place, a goal that at all
costs must be attained? Or is it only when the cycle of events which
leads up to reproduction is nearing completion that she looms upon his
horizon? One would like to be in a position to answer these questions,
but there is nothing in the way of experimental evidence to go upon; and
if I say that there is reason to believe that, in the earlier stages,
the female is but a shadow in the external environment of the male, it
must be taken merely as an expression of opinion, though based in some
measure upon a general observation of the behaviour of various species.
Before attempting to explain the difference in the times of arrival of
the male and female migrant, let us examine the behaviour of some
resident species at a corresponding period. My investigations have been
made principally amongst the smaller species--the Finches and the
Buntings--which often pass the winter in or near the localities wherein
they brought up offspring or were reared. It is true that they wander
from one field to another according to the abundance or scarcity of
food; it is also true that, if the weather is of a type which precludes
the possibility of finding the necessary food, these wanderings may
become extensive or even develop into partial migrations. But under the
normal climatic conditions which prevail in many parts of Britain, these
smaller resident species seem to find all that they require without
travelling any great distance from their breeding haunts. Flocks
composed of Yellow Buntings, Cirl Buntings, Corn-Buntings, Chaffinches,
Greenfinches, etc., can be observed round the farmsteads or upon arable
land; small flocks of Reed-Buntings take up their abode on pieces of
waste land and remain there until the supply of food is exhausted,
deserting their feeding ground only towards evening when they retire to
the nearest reed-bed to pass the night; flocks of Hawfinches visit the
same holly-trees day after day so long as there is an abundance of
berries on the ground beneath; and so on.
I have mentioned the Reed-Bunting; let us take it as our first example
and try to follow its movements when the influence exerted by the
internal secretions begins to be reflected on the course of its
behaviour. First, it will be necessary to discover the exact localities
in any given district to which the species habitually returns for the
purpose of procreation; otherwise the earlier symptoms of any
disposition to secure a territory may quite possibly be overlooked in
the search for its breeding haunts.
In open weather Reed-Buntings pass the winter either singly, in twos or
threes, or in small flocks, on bare arable ground, upon seed fields, or
in the vicinity of water-courses; but in the breeding season they
resort to marshy ground where the _Juncus communis_ grows in abundance,
to the dense masses of the common reed (_Arundo phragmites_), and such
like places. During the winter, the male's routine of existence is of a
somewhat monotonous order, limited to the necessary search for food
during the few short hours of daylight and enforced inactivity during
the longer hours of darkness. But towards the middle of February a
distinct change manifests itself in the bird's behaviour. Observe what
then happens. When they leave the reed-bed in the morning, instead of
flying with their companions to the accustomed feeding grounds, the
males isolate themselves and scatter in different directions. The
purpose of their behaviour is not, however, to find fresh feeding
grounds, nor even to search for food as they have been wont to do, but
rather to discover stations suitable for the purpose of breeding; and,
having done so, each male behaves in a like manner--it selects some
willow, alder, or prominent reed, and, perching thereon, leads a quiet
life, singing or preening its feathers. Now if the movements of one
particular male are kept in view, it will be noticed that only part of
its time is spent in its territory. At intervals it disappears. I do not
mean that one merely loses sight of it, but that it actually deserts its
territory. As if seized with a sudden impulse it rises into the air and
flies away, often for a considerable distance and often in the same
direction, and is absent for a period which may vary in length from a
few minutes to an hour or even more. But these periodical desertions
become progressively less and less frequent in occurrence until the
whole of its life is spent in the few acres in which it has established
itself.
The behaviour of the Yellow Bunting is similar. In any roadside hedge
two or more males can generally be found within a short distance of one
another, and in such a place their movements can be closely and
conveniently followed. Under normal conditions the ordinary winter
routine continues until early in February; but the male then deserts the
flock, seeks a position of its own, and becomes isolated from its
companions. Now the position which it selects does not, as a rule,
embrace a very large area--a few acres perhaps at the most. But there is
always some one point which is singled out and resorted to with marked
frequency--a tree, a bush, a gate-post, a railing, anything in fact
which can form a convenient perch, and eventually it becomes a central
part of the bird's environment. Here it spends the greater part of its
time, here it utters its song persistently, and here it keeps watch upon
intruders. The process of establishment is nevertheless a gradual one.
The male does not appear in its few acres suddenly and remain there
permanently as does the migrant; at first it may not even roost in the
prospective territory. The course of procedure is somewhat as
follows:--At dawn it arrives and for a while utters its song, preens its
feathers, or searches for food; then it vanishes, rising into the air
and flying in one fixed direction as far as the eye can follow, until it
becomes a speck upon the horizon and is ultimately lost to view. During
these excursions it rejoins the small composite flocks which still
frequent the fields and farm buildings. For a time the hedgerow is
deserted and the bird remains with its companions. But one does not have
to wait long for the return; it reappears as suddenly as it vanished,
flying straight back to the few acres which constitute its territory,
back even to the same gate-post or railing, where it again sings. This
simple routine may be repeated quite a number of times during the first
two hours or so of daylight, with, of course, a certain amount of
variation; on one occasion the bird may be away for a few minutes only,
on another for perhaps half an hour, whilst sometimes it will fly for a
few hundred yards, hesitate, and then return--all of which shows clearly
enough that these few acres possess some peculiar significance and are
capable of exercising a powerful influence upon the course of its
behaviour. And so the disposition in relation to the territory becomes
dominant in the life of the bird.
Or take the case of the Chaffinch. In winter large or small flocks can
be found in many varied situations. But in the latter part of February,
or the early days of March, these flocks begin to disperse. At daylight
males can then be observed in all kinds of situations, either calling
loudly, uttering their spring note, or exercising their vocal powers to
the full; and it will be found that, in the majority of instances, these
males are solitary individuals, that they pass the early hours of the
morning alone, and that their normal routine of calling, singing, or
searching for food, is only interrupted by quarrels with their
neighbours. The same locality is visited regularly--not only the same
acre or so of ground, but even the same elm or oak, has, as its daily
occupant, the same cock Chaffinch. And temporary desertions from the
territory occur also, much like those referred to in the life of the
Bunting, but perhaps not so frequently. One has grown so accustomed
during the dark days of winter to the sociable side of Chaffinch
behaviour--to the large flocks searching for food, to the endless stream
of individuals returning in the evening to roost in the holly-trees, to
the absence of song--that this radical departure from the normal routine
comes as something of a surprise; for the days are still short, the
temperature is still low, the nesting season is still many weeks ahead,
and yet for part of the day, and for just that part when the promptings
of hunger must be strongest, the male, instead of joining the flock,
isolates itself and expends a good deal of energy in insuring that its
isolation shall be complete. And in place of the silence we hear from
all directions the cheerful song uttered with such marked persistency
that it almost seems as if the bird itself must be aware that by doing
so it was advertising the fact of its occupation of a territory. This is
surely a remarkable change, and the females in the meantime continue
their winter routine.
One other example. The monotonous call of the Greenfinch is probably
familiar to all. In winter these birds accompany other Finches and form
with them flocks of varying sizes, but in the spring the flocks
disperse, and the Greenfinch, in common with other units of the flock,
alters its mode of life. But whereas the Chaffinch or the Bunting begins
to acquire its territory in February, the Greenfinch only does so in
April. When the organic changes do at length begin to make themselves
felt, the male seeks a position of its own, and having found one remains
there, uttering its characteristic call. But owing probably to the fact
that it is much later than the aforementioned species in acquiring a
territory, temporary desertions are not so much in evidence. The species
is so very plentiful, and the bird is so prone to nest in gardens and
shrubberies surrounding human habitations, that this seasonal change in
its routine of existence cannot fail to be noticed. One can hear its
call in every direction, one can watch the same individual in the same
tree; and it is the male that is thus seen and heard, the female appears
later. Thus the behaviour falls into line with that of the Bunting or
the Chaffinch.
The behaviour of these resident species throws some light upon the early
arrival of the males which we are endeavouring to explain in the case of
the migrants. Let us see how their actions compare. The male resident
deserts the female early in the year and establishes itself in a
definite position, where it advertises its presence by song; the male
migrant travels from a great distance, arrives later, and also
establishes itself in a definite position, where it, too, advertises its
presence by song. The male resident passes only the earlier part of the
day in its territory at the commencement of the period of occupation;
the male migrant remains there continuously from the moment it arrives.
The male resident deserts its territory at intervals, even in the
morning; the male migrant betrays no inclination to do so. Thus there is
a very close correspondence between the behaviour of the two, and what
difference there is--slight after all--cannot be said to affect the main
biological end of securing territory. One is apt to think of the problem
of migration in terms of the species instead of in terms of the
individual. One pictures a vast army of birds travelling each spring
over many miles of sea and land, and finally establishing themselves in
different quarters of the globe; and so it comes about, I suppose, that
a country or some well-defined but extensive area is regarded as the
destination, the ultimate goal, of the wanderers. But the resident male
has a journey to perform, short though it may be; it, too, has a
destination to reach, neither a country nor a locality, but a place
wherein the rearing of offspring can be safely accomplished, and it,
too, arrives in that place in advance of the female.
With these facts at our disposal, we will endeavour to find an
explanation. It is unlikely that specialised behaviour would occur in
generation after generation under such widely divergent conditions,
and, moreover, expose the birds to risk of special dangers, if it were
but an hereditary peculiarity to which no meaning could be attached.
Hence the appearance of the males in their breeding haunts ahead of the
females becomes a fact of some importance, and suggests that the
extensive journey in the one case, and the short journey in the other,
may both have a similar biological end to serve.
Darwin evidently attached importance to this difference between the
males and the females in their times of arrival. In the _Descent of Man_
he referred to it as follows: "Those males which annually first migrated
in any country, or which in spring were first ready to breed, or were
the most eager, would leave the largest number of offspring; and these
would tend to inherit similar instincts and constitutions. It must be
borne in mind that it would have been impossible to change very
materially the time of sexual maturity in the females without at the
same time interfering with the period of the production of the young--a
period which must be determined by the season of the year." Newton
suggested the following explanation[2]: "It is not difficult to
imagine that, in the course of a journey prolonged through some 50° or
60° of latitude, the stronger individuals should outstrip the weaker by
a very perceptible distance, and it can hardly be doubted that in most
species the males are stouter, as they are bigger than the females."
Granting that the males are the stronger, how can this account for
their outstripping the females by a week, ten days, or even a fortnight,
in a journey of perhaps 1500 miles? To expect the birds to accomplish
such a distance in seven days is surely not estimating their
capabilities too highly, and any slight inequality in the power of
flight or endurance could give the males an advantage of a few hours
only. But this explanation, based upon inequalities in the power of
flight and endurance on the one hand, and the magnitude of the distance
traversed on the other, cannot afford a solution of the behaviour of the
resident males, and is less likely, therefore, to be a true solution of
that of the migrants.
There is another theory, simple enough in its way, which will probably
occur to many. It is based on the assumption that the males reach sexual
maturity before the females; and it is contended that the functioning of
the instincts which contribute towards the biological end of
reproduction depend upon the organic changes which the term "sexual
maturity" is held to embrace, and that, inasmuch as the migratory
instinct belongs to the group of such instincts, the males must be the
first to leave their winter quarters.
What is meant by the "migratory instinct"? To speak of it as one of the
instincts concerned in reproduction is not enough. Reproduction involves
the actual discharge of the sexual function, which involves the
females; but the first visible manifestation of organic change in the
male is its desertion of the females. Yet this is the behaviour which is
referred to as the "migratory instinct," and which comes into play,
according to this theory, because the bird has reached sexual maturity.
Manifestly we must have some clear understanding as to what these terms
represent. That organic changes determine the functioning of certain
definite instincts at certain specified times there can be no doubt;
that these changes may occur at a somewhat earlier date in the male than
in the female is more than probable, but that this explains the
behaviour in question I do not believe. One wants to know why the
changes should occur earlier in the male, what disposition it is which
first comes into functional activity, and to what such disposition is
related.
It may, however, be urged that, after all, this apparent eagerness to
reach the breeding grounds is but a modification of hereditary procedure
under the guiding hand of experience. What more likely result would
follow from the enjoyment associated with previous success in the
attainment of reproduction than a craving to repeat the experience? What
stronger incentive to a hurried return could be imagined? It must be
admitted that there are certain facts which might be used in support of
an appeal to experience as a reasonable explanation. For example, the
first males to arrive often display that richness of colouring which is
generally supposed to indicate a fuller maturity. Gätke even speaks of
the "most handsome old birds being invariably the first to hasten back
to their old homes." But if experience is a factor, if some dim
recollection of the past is held to explain the hurried departure of the
male migrant, one wants to know with what such recollection is
associated. Is it associated with the former female, or with the former
breeding place, or with both? I take it that any recollection, no matter
how vague, must be primarily associated with the particular place
wherein reproduction had previously been accomplished; and I grant that
if the first individuals to appear were invariably the older and
experienced birds, their early return might be explained on the basis of
such an association. But if there is reason to believe that a proportion
are young birds on the verge of carrying out their instinctive routine
for the first time, then we cannot appeal to past experience in
explanation of their behaviour.
The age of a bird is difficult to determine. Experience leads me to
believe that some of the males that arrive before the females are birds
born the previous season; one finds, for instance, individuals with
plumage of a duller hue, which denotes immaturity, amongst the first
batch of arrivals. But though plumage may sometimes be a satisfactory
guide, yet to rely upon it alone, or upon a more perfect development of
feather, is to exceed the limits of safety. How, then, can we ascertain
whether all the males that arrive before the females have had some
previous experience of reproduction? Well, we take a particular locality
and note the migrants that visit it year after year, and we find that
the respective numbers of the different species are subject to wide
annual fluctuations. Not every species lends itself to an inquiry of
this kind: some are always plentiful and fluctuation is consequently
difficult to discern; others are scarce and variation is easily
determined. Those which are of local distribution but conspicuous by
their plumage, or easily traced by the beauty or the peculiarity of
their song, afford the more suitable subjects for investigation. For
example, the Grasshopper-Warbler, Marsh-Warbler, Nightingale, Corncrake,
Red-backed Shrike, or Whinchat have each some distinctive peculiarity
which makes them conspicuous, and each one is subject to marked
fluctuation in numbers. The small plantation or wooded bank may hold a
Nightingale one year, but we miss its song there the next; the osier bed
or gorse-covered common which vibrates with the trill of the
Grasshopper-Warbler one April is deserted the following season; the
plantation which is occupied by a host of common migrants this summer
may be enlivened next year by the song of the rarer Marsh-Warbler also;
and so on. The fluctuation is considerable: we observe desertion on the
one hand, appropriation on the other, and yet males appear before
females whether the particular plantation, osier bed, or swamp had been
inhabited or not the previous season. This fact is not without
significance. It shows that similar conditions prevail both amongst the
males that appropriate breeding grounds new to them, and amongst those
that return to some well-established haunt; and on the assumption that
the earlier arrivals are experienced males, the same birds evidently do
not return to the same place year after year. Granting, then, that the
males which appropriate new breeding-grounds are young birds, how can
their earlier arrival be explained in terms of past experience; and
granting that they are old, and therefore experienced, how can it be
explained in terms of association?
Again, it may be urged that if there is some biological end to be
furthered by this hurried return, and if recollection of past experience
is a means towards that end, such recollection need not necessarily be
associated with a definite place, but only in a vague way with the whole
series of events leading up to reproduction--in which series the
migratory journey may even have acquired meaning. Whether there be any
recollection of a previous journey or of a nest with young, I do not
know. But the young bird is capable of performing its journey, of
building its nest, and of rearing its young antecedent to
experience--racial preparation has fitted it thus far; why then exclude
the other event in the series, the earlier departure of the male, from
hereditary equipment? If the journey were a casual affair without any
goal attaching to it, if the males upon arrival wandered about in search
of a mate, there would be some ground for thinking that a vague
recollection of the whole former experience was sufficient to explain
the hurried return; but since the pleasurable effect of association,
founded upon previous experience of a definite place, cannot well be
established, and since it is so difficult to study the objective aspect
of the behaviour in question without coming to the conclusion that the
journey is related to the appropriation of a place suitable for the
rearing of offspring, one is tempted to ask whether the hurried return
may not also be so related.
Now the males of some of the migratory species, especially of those
which are accustomed to return to their breeding haunts early in the
season, are called upon to face greater dangers and have a greater
strain imposed upon their strength by starting forth upon their journey
ten days or a fortnight before their prospective mates. The blizzards
which so often sweep across the northern parts of Europe in the latter
half of March, destroying in their course the all too scanty supply of
insect life, may take toll of their numbers; or the westerly gales,
which are not infrequent at that period, may meet them in mid-ocean and
add to the perils of their journey; or the temperature of the previous
weeks may have been sufficiently low to arrest the development of insect
life--and yet males are annually exposed to these risks in hurrying to
their breeding grounds. For what purpose? The answer will largely depend
upon the way in which we regard those few acres wherein a resting place
is ultimately found. For myself, I believe that they are of importance,
inasmuch as the securing of a place suitable for the rearing of
offspring is a primary condition of success in the attainment of
reproduction; and if this be so, it is evident that the interests of
the race will be better served by the males making good this first step
before the females are ready to pair, otherwise they might oscillate
between two modes of behaviour, created by the premature functioning of
conflicting impulses.
The different steps in the process seem to follow one another in ordered
sequence. The male inherits a disposition--which for us, of course, has
prospective meaning--to seek the appropriate breeding ground and there
to establish itself; and as early a functioning of this disposition as
possible, consonant with the conditions of existence in the external
environment, may have been evolved for the following reasons--firstly,
the earlier individuals will meet with less interference wherever they
may settle, every locality will be open to them, every acre free, their
only need being that particular environment for which racial preparation
has fitted them. In the second place, being already established when
other males appear upon the scene, and advertising their presence by
song, they will be less liable to molestation; thirdly, in those cases
in which a long journey is undertaken, they will have ample time to
recover from the fatigue, and, if attacked by later arrivals, will thus
be in a better position to defend their territories; and lastly, a
greater uniformity in their distribution will be insured before the
females begin their search.
There is, besides, another good reason for thinking that the earlier
males will have an advantage. We will assume--and from the abundant
evidence supplied by the marking of birds, it is quite a reasonable
assumption--that there is a tendency, generally speaking, for
individuals to return to the neighbourhood of their birthplace, or to
the place in which they had previously reared their offspring. Now the
earlier arrivals will have no difficulty in securing territories; those
that come later may have to search more diligently, still they will gain
all that they require so long as any available space remains. Then comes
the point when all suitable ground is occupied, and yet there are males
to be provided for. What will be the position of these males? Urged by
their inherited nature, they will leave the district and possibly
continue their search into those adjoining, only, however, to add to the
difficulties of the males there similarly situated; and even allowing
that they are at length successful in establishing themselves, what are
their prospects of securing mates? Since the earlier females will not
extend their wanderings farther than is absolutely necessary, but will
pair whenever the opportunity for doing so arises, it is to the later
females, forced onwards by competition, that the late males must look
for mates; so that when at length pairing does take place, much valuable
time will have been lost.
The disadvantages which the late arrivals have to face are therefore
great, and it is probable that the percentage which attain to
reproduction will on the average be somewhat lower than the percentage
in the case of the earlier arrivals. The district in which my
observations have been made lies well within the limits of the breeding
range of most of our common species, and it is not surprising that I
should have met with little evidence of failure to breed as a result of
failure to secure territory. Some interesting information was supplied
to me, however, by the late Robert Service. He found, in certain seasons
in Dumfriesshire, flocks of from ten to fifty unmated Sedge-Warblers,
which, from the time of their arrival in May until the middle of July,
haunted reed-filled spaces along stagnant streams. These flocks appeared
to him to be composed of loosely-attached individuals of a migrant flock
that had failed to find things congenial enough to entice them to
disperse. But may they not have been composed of males that had failed
to secure territories, or of females that had failed to discover males
in possession of territories, or of both?
We have seen that, in the case of many species, each male establishes
itself in a particular place at the commencement of the breeding season,
even though this may mean a partial or perhaps a complete severance from
former companions. We must now discuss this fact in greater detail
because it is opposed to the views often held regarding the sexual
behaviour of birds, and is manifestly of importance when considering
the theory of breeding territory.
First, however, there is a point which requires some explanation. I
speak of the _same_ male being in the _same_ place. How can I prove its
identity? In the first place it is highly improbable that a bird which
roams about within the same small area of ground, makes regular use of a
certain tree and a certain branch of that tree, and observes a similar
routine day after day, can be other than the same individual. But, apart
from this general consideration, are there any means by which
individuals of the same species can be identified? Well, there is
variation in the plumage. Supposing we take a dozen cock Chaffinches and
examine them carefully, we shall find slight differences in pattern and
in colour--more grey here or a duller red there, as the case may be--and
though these differences may not be sufficient to enable us to pick out
a bird at a distance, they are nevertheless conspicuous when it is close
at hand. Then again there is variation in the song; and the more highly
developed the vocal powers the greater scope there is for variation. But
even the phrases of a simple song can be split up and recombined in
different ways. If one were asked casually whether the different phrases
of the Reed-Bunting's song always followed one another in the same
sequence, the answer would probably be that they certainly did so,
whereas the bird is capable of combining the few notes it possesses in a
surprising number of different ways. And lastly, there are differences
in just the particular way in which specific behaviour, founded upon a
congenital basis, is adapted by each individual to its own special
environment. Racial preparation determines behaviour as a whole, but the
individual is allowed some latitude in the execution of details which
are in themselves of small moment--the selection of a particular tree as
a headquarters and a particular branch upon that tree, the direction of
the distant excursion, and the direction of the limited wanderings
within the small area surrounding the headquarters which in the course
of time determine the extent of the territory, are matters for each
individual to decide when the occasion for doing so arises. Moreover
instances of abnormal coloration or abnormal song are not rare, and they
are valuable since they place the identity of the individual beyond
dispute. I can recall the case of a Willow-Warbler whose song was unlike
that of its own or any other species, and of a Redbreast whose voice
puzzled me not a little. I can recollect also a male Yellow Bunting
whose foot was injured or deformed. Of this bird's behaviour I kept a
record for two months or so; and inasmuch as it inhabited a roadside
hedge, and was of fearless disposition, the deformed foot could plainly
be seen whenever it settled upon the road to search for food.
Identification is not, therefore, a difficulty. There is always some
small difference in colour or in song, or some well-defined routine
which makes recognition possible.
Owing to their great powers of locomotion, birds have generally been
regarded as wanderers more or less; anything in the nature of a fixed
abode, apart from the actual nest, having been accounted foreign to
their mode of life; and even the locality immediately surrounding the
nest has not been apprehended as possessing any meaning for the owner of
that nest. No doubt the supply of food determines their movements for a
considerable part of the year; they seek it where they can find it, here
to-day, there to-morrow--in fact few species fail to move their quarters
at one season or another, so that there is much truth in the notion that
birds are wanderers. Yet to suppose that every individual one sees or
hears--every Lapwing on the meadow, or Nightingale in the withy bed--is
in that particular spot just because it happens to alight there as it
roams from place to place, is to take a view which the observed facts do
not support. For as soon as the question of reproduction dominates the
situation, a new condition arises, and the habits formed during the
previous months are reversed, and the males, avoiding one another, or
even becoming actively hostile, prefer a life of seclusion to their
former gregariousness--all of which occurs just at the moment when we
might reasonably expect them to exhibit an increased liveliness and
restlessness as a result of their endeavour to secure mates; and so
universal is the change that it might almost be described as an
accompaniment of the sexual life of birds generally.
That the Raven and certain birds of prey exert an influence over the
particular area which they inhabit has long been known, and it has been
recognised more especially in the case of the Peregrine Falcon,
possibly because the bird lives in a wild and attractive country, and,
forcing itself under the notice of naturalists, has thus had a larger
share of attention devoted to its habits. Moreover, when a species is
represented by comparatively few individuals, and each pair occupies a
comparatively large tract of country, it is a simple matter to trace the
movements and analyse the behaviour of the birds. There is a rocky
headland in the north-west of Co. Donegal comprising some seven miles or
so of cliffs, where three pairs of Falcons and two pairs of Ravens have
nested for many years. Each year the different pairs have been more or
less successful in rearing their young; each year the young can be seen
accompanying their parents up to the time when the sexual instinct
arises; and yet the actual number of pairs is on the whole remarkably
constant, and there is no perceptible increase. It seems as if the
numbers of three and two respectively were the maximum the headland
could maintain. But this is no exceptional case; it represents fairly
the conditions which obtain as a rule amongst those species, granting,
of course, a certain amount of variation in the size of each territory
determined by the exigencies of diverse circumstances.
If we take a given district, and devote our attention to the smaller
migrants that visit Western Europe each returning spring for the purpose
of procreation, we shall find that the movements of the males are
subject to a very definite routine. This, however, is not true of every
male; some may be wending their way to breeding grounds at a distance;
others may be seeking the particular environment to which they may be
adapted; others again, having found their old haunts destroyed, may
consequently be seeking new.
Of all this there is evidence. Small parties of Chiffchaffs pass through
a district on their way to other breeding grounds, flitting from hedge
to hedge as they move in a definite direction with apparently a definite
purpose; Reed-Warblers settle in a garden or plantation, eminently
unsuited to their requirements, and disappear; Wood-Warblers arrive in
some old haunt, and finding it no longer suitable for their purpose,
seek new ground. So that plenty of individuals are always to be found,
which, for the time being at least, are wanderers.
In the district which I have in mind, the wandering males form only a
small part of the incoming bird population. The majority of individuals
that fall under observation are those that have made this particular
district their destination; and in doing so, they may possibly have been
guided by their experience as owners or inmates of former nests, for it
cannot be doubted that a return to the neighbourhood of the birthplace
would lead to a more uniform distribution and therefore be advantageous,
and the tendency to do so might consequently have become interwoven in
the tissue of the race. How, then, do they behave? A certain amount of
movement, an interchanging of positions, even though restricted to an
area defined, let us say, by experience, might be expected under the
circumstances--that, however, is not what we find; we observe the
available situations plotted out into so many territories, each one of
which is occupied by a male who passes the whole of his time therein.
Take whatever species we will--Whitethroat, Whinchat, Willow-Warbler,
Red-backed Shrike, it matters not which, for there is no essential
difference in the general course of procedure--this condition will be
found to prevail. Generally speaking, the behaviour in relation to the
territory can be studied more conveniently where a number of individuals
of the same species have established themselves in proximity to one
another. Such species as the Chiffchaff, Willow-Warbler, or Wood-Warbler
are often sufficiently common to allow of three or more of their
respective males being kept in view at the same time; and the
disposition to occupy a definite position can be readily observed. The
Reed-Warbler is a suitable subject for an investigation of this kind;
for since it is restricted by its habits to localities wherein the
common reed (_Arundo phragmites_) grows in abundance, and since such
localities are none too plentiful and often limited in extent, the area
occupied by each individual is necessarily small--if it were not so the
species would become extinct. Hence it is a simple matter to study the
routine of the different individuals and to mark the extent of their
wanderings.
In this way the males of all the Warblers that breed commonly in Great
Britain establish themselves, each one in its respective station at the
respective breeding ground; so, too, do those of many other
migrants--for example, the Whinchat, Wheatear, Tree-Pipit, and Red-backed
Shrike. All of these, it is true, are common species--numbers of
individuals can often be found in close proximity--and therefore it may
be argued that they keep to one position more from pressure of
population than from any inherited disposition working towards that end.
But the rarer species behave similarly. Districts frequented by the
Marsh-Warbler and offering plenty of situations of the type required by
the bird are often inhabited by a few members only, and yet the
disposition to remain in a definite position is just as marked.
You will say, however, that these smaller migrants have no exceptional
powers of flight; that they have besides just completed a long and
arduous journey; and you will ask why they should be expected to wander,
whether it is not more reasonable to expect that, in order to overcome
their fatigue, they should remain where they settle. The Cuckoo is a
wanderer in the wider sense of the term, and is gifted with considerable
powers of flight. Upon arrival the male flies briskly from field to
field, showing but little signs of weariness; yet we have only to follow
its movements for a few days in succession to assure ourselves that the
bird is no longer a wanderer; for just as the Warbler or the Chat moves
only within a definitely delimited area, so the male Cuckoo, strange as
it may seem, restricts itself to a particular tract of land. The area
over which it wanders is often considerable and consequently it is not
possible to keep the bird always in view, but inasmuch as the variation
in the voices of different individuals is quite appreciable,
identification is really a simple matter. If we cannot keep the bird in
sight, we can trace its movements by sound and mark the extent of its
wanderings, which by repetition become more and more defined, until a
belt of trees here, or an orchard there, mark a rough and rarely passed
boundary line.
Let us take another example from the larger migrants--the Black-tailed
Godwit, a bird common enough in the Dutch marshes but no longer breeding
in this country. On suitable stretches of marsh land, numbers will be
found in proximity one to another after the manner of the Lapwing, each
male occupying a definite space of ground wherein it passes the time
preening, searching for food, or in sleep--though at the same time
keeping a strict watch over its territory. Now the preference shown for
a particular piece of ground, and the determination with which it is
resorted to, is the more remarkable when we take into consideration the
specific emotional behaviour arising from the seasonal sexual condition.
This behaviour is expressed in a peculiar flight. The bird rises high in
the air, circles round with slowly beating wings above the marsh, and
utters a call which, as far as my experience goes, is characteristic of
the performance. The air is often full of individuals circling thus
even beyond the confines of the marsh, for a male does not limit its
flight to a space immediately above its territory; but nevertheless
careful observation will show how unerringly each one returns to its own
position on the breeding ground, no matter how extensive the aerial
excursion may have been. And so, when the males of the smaller migrants
confine their movements to an acre of ground at the completion of their
long journey, they are acting no more under the influence of fatigue
than the Cuckoo, which keeps within certain bounds yet flies about
briskly, or the Godwit which, though holding to its few square yards on
the ground, executes most tiring and extensive flights above the marsh.
Of all the migrants, however, the behaviour of the Ruff is perhaps the
most strange, and though it has long been known that these birds have
their special meeting places where they perform antics and engage in
serious strife, yet it is only within recent years that the primary
purpose of these gatherings has been ascertained--that purpose being the
actual discharge of the sexual function. Mr. Edmund Selous has carried
out some exhaustive investigations into their activities at the meeting
places, and he makes it clear that each bird has its allotted position.
He says, for example, that "It begins to look as though different birds
had little seraglios of their own in different parts of the ground,"
that "each Ruff has certainly a place of its own," or again that "this
Ruff indeed, which I think must be a tender-foot, does not seem to have
a place of its own like the others." Nevertheless it is only at the
meeting places that they have their special positions; there is no
evidence to show that each one has a special territory, wherein it seeks
its food, as the Warbler has, and therefore some may think that we are
here confronted with behaviour of a different order. But we must bear in
mind that the process has been adjusted to meet the requirements of
different species: the size of the territory, the period of its daily
occupation, the purpose which it serves--these all depend upon manifold
relationships and do not affect the principle. Why it has been
differentiated in different circumstances we shall have occasion to
discuss later; for the moment it is enough that at the end of its
migratory journey each Ruff occupies one position on the meeting ground.
[Illustration: Territorial flight of the Black-tailed Godwit
Emery Walker ph.sc.]
Now birds that are paired for life, whose food-supply is not affected by
alternations of climate, have no occasion to desert the locality wherein
they have reared their offspring, and so their movements, being subject
to a routine which would tend to become increasingly definite, must in
the course of time and according to the law of habit formation become
organised into the behaviour we observe. Is it necessary, therefore, to
seek an explanation of their tendency to remain in one place in anything
so complex as an inherited disposition? Again, since we have to confess
to so very much ignorance on so many points connected with the whole
phenomenon of migration, may there not be some condition, hitherto
shrouded in mystery, which might place so different a complexion on the
corresponding aspect of migrant behaviour as to rid us, in their case
also, of the necessity of appealing to an inherited disposition? Such
questions are justifiable. And if the life-histories of other species
gave no further support to our interpretation, if, in short, the
evidence were to break down at this point, then we should be forced to
seek some other explanation more in keeping with the general body of
facts.
But far from placing any obstacle in the way of an interpretation in
terms of inherited disposition, the behaviour of many of those residents
which are not paired for life gives us even surer ground for that
belief. Moreover in their case the initial stages in the process are
more accessible to observation. I will endeavour to explain why. In the
process of reproduction the environment has its part to play--whether in
the manner here suggested, or indirectly through the question of
food-supply, matters not at the moment. Now, migratory species are more
highly specialised than resident species as regards food, and are
affected more by variations of temperature, so that they can live for
only a part of the year in the countries which they visit for the
purpose of procreation. Hence the organic changes, which set the whole
process in motion, must be coincident in time with the growth of
appropriate conditions in the environment; for if it were not so, if the
internal organic changes were to develop prematurely, the bird would
undertake its journey only to find an insufficiency of food upon its
arrival, and this would scarcely contribute towards survival. Definite
limitations have therefore been imposed upon the period of organic
change. But in the case of many resident species the conditions are
somewhat different, for they remain in the same locality throughout the
year, and a gradual unfolding of the reproductive process cannot
therefore have a similarly harmful effect. Thus it comes about that the
behaviour of the migrant, when it arrives at the breeding ground and
first falls under observation, represents a stage in the process which,
in the case of the resident, is only reached by slow degrees; and by
closely observing the behaviour as it is presented to us in the life of
the resident male, we not only gain a better insight into the changes in
operation, but can actually witness the breaking down of the winter
routine, stereotyped through repetition, by the new disposition as it
arises.
The first visible manifestations, even though they may be characterised
by a certain amount of vagueness, are therefore of great importance if
the behaviour is to be interpreted aright; and in order to insure that
none of these earlier symptoms shall be missed, it is necessary to begin
the daily record of the bird's movements at an early date in the season.
As a rule the second week in February is sufficiently early for the
purpose, but the date varies according to the prevailing climatic
conditions. Even in species widely remote there is great similarity of
procedure, and the behaviour of the Buntings is typical of that of many.
With the rise of the appropriate organic state the male resorts at
daybreak to a suitable environment, occupies a definite position, and
singling out some tree or prominent bush, which will serve as a
headquarters, advertises its presence there by song. At first the bird
restricts its visits, which though frequent in occurrence are of short
duration, for the most part to the early hours of the morning; it
disappears as suddenly as it appeared, and one can trace its flight to
the feeding grounds--a homestead or perhaps some newly sown field. But
by degrees the impulse to seek the society of the flock grows less and
less pronounced, the visits to the territory are more and more
prolonged, and the occupation of it then becomes the outstanding feature
of the bird's existence. This in outline is the course of procedure as
it appears to an external observer.
But although much can be learnt from the lives of these smaller species,
there is no gain-saying the fact that a great deal of patient
observation is required, and the process is apt to become tedious. There
are others, however, which are more readily observed, whilst their
life-histories afford just as clear an insight into the effect produced
by the new disposition upon the developing situation; and among these
the Lapwing takes a prominent position, because it is plentiful and
inhabits open ground where it is easily kept in view.
There is a water meadow with which I am familiar, where large numbers
resort annually for the purpose of procreation. Here they begin to
arrive towards the end of February, and at first collect in a small
flock at one end of the meadow. A male, here and there, can then be seen
to break away from the flock, and to establish itself in a definite
position upon the unoccupied portion of the ground, where it remains
isolated from its companions. Others do likewise until the greater part
of the meadow is divided into territories. Six of these territories I
kept under observation for approximately two months in the year 1915.
The occupant of the one marked No. 6 upon the 1915 plan was a lame bird,
a fortunate occurrence as it enabled me to follow its movements with
some accuracy; and though it maintained its position for some weeks, it
ultimately disappeared, as a result, I believe, of the persistent
attacks of neighbouring males. The behaviour of the males during the
first fortnight or so after they broke away from the flock was
interesting. Though they retired to their territories and remained in
them for the greater part of their time, yet it was only by degrees that
they finally severed their connection with the flock, for so long as a
nucleus of a flock remained, so long were they liable to desert their
territories temporarily and to rejoin their companions.
[Illustration: Emery Walker Ltd. sc.
Plan of the water meadow showing the territories occupied by Lapwings in
the year 1915.]
[Illustration: Emery Walker Ltd. sc.
Plan of the water meadow showing the territories occupied by Lapwings in
the year 1916.
_Between pages 58 and 59._]
Lapwings, as is well known, collect in flocks during the winter months,
and these flocks, which sometimes reach vast proportions, are to be
found on tidal estuaries, water meadows, arable land, and such like
places, according to the prevailing climatic conditions. This flocking
may contribute towards survival, and may therefore be the result of
congenital dispositions which have been determined on biological
grounds. On the other hand, since food at that season is only to be
obtained in a limited number of situations, the birds may be simply
drawn together by accident. In the former case the behaviour would be
instinctive, in the latter, though accidental at first, recurrent
repetition would tend to make it habitual; but in either case the
impulse to accompany the flock must be a powerful one, for on the one
hand it would depend upon inherited, and on the other hand upon
acquired, connections in the nervous system. Now observe that soon
after the flock arrived in the meadow, single males detached themselves;
there was no hesitation, they just retired from their companions and
settled in their respective territories. They were not expelled, for if
their leaving had been compulsory much commotion would have preceded
their departure, and their return would certainly not have been
welcomed. A reference to the plan will make the position clearer; the
neutral zone inhabited by the flock is there shown as situated in one
corner of the meadow, the territories that fell under observation are
plotted out as far as possible to scale, and the more important zones of
conflict are also marked.
The males spent part of their time in their respective territories and
part with the flock, so long as it remained in existence. When a male
was in its territory it avoided companions and was openly hostile to
intruders; when it was with the flock it wandered about with companions
in search of food. The contrast between the two modes of behaviour was
very marked, and it was evident that the gregarious instinct was
gradually yielding its position of importance to the new factor--the
territory. If there had been no flock, if a few solitary individuals had
appeared here and there and had established themselves in different
parts of the meadow, one would have had no definite evidence of the
strength of the impulse in the male to seek a position of its own, one
could only have argued from the general fact of males flocking in the
winter and isolating themselves in spring that something more than
accident was required to explain so radical a change. But since the
birds returned in a flock to the ground upon which they intended to
breed, and since the flock occupied temporarily part of the ground
whilst the partitioning of the remainder was still proceeding, it was
possible to gauge the strength of the impulse, which was forcing the
males to isolate themselves in particular areas of ground, by comparing
it with the impulse to accompany the flock--and the measure of its
intensity was the rapidity with which the latter impulse yielded its
position of importance.
Like the Lapwing, the Coot and Moor-Hen are easily kept under
observation, and since many individuals often breed in proximity, more
than one can be watched at the same moment; moreover the area occupied
by each male generally embraces an open piece of water as well as part
of the fringe of reeds, so that the movements of the bird can be
followed without much difficulty. Under favourable conditions
manifestations of the developing situation become visible at a
comparatively early date in the season--the middle or the latter part of
February--and these manifestations resemble those of other species. But
the Moor-Hen passes summer and winter alike in the same situation, and
being therefore in a position to respond at once to internal
stimulation, however vague, the change from the one state to the other
is gradual. This, however, is a matter of detail; the main consideration
lies in the fact that the impulse to retire to a definite position, to
avoid companions, and to live in seclusion, is strongly marked, and
produces a type of behaviour similar on the whole to that of the
Lapwing. First of all there is the appropriation of a certain position,
the limits of which are fixed according to the law of habit formation,
and according to the pressure exerted by neighbouring individuals; then
there is the neutral ground over which the birds wander amicably in
search of food; and finally there is the contrast between the pugnacity
of the male whilst in its territory, and its comparative friendliness
when upon neutral ground.
Evidence of similar behaviour is to be found in the life of the Black
Grouse, a bird which has always excited the curiosity of naturalists on
account of the special meeting places to which both sexes resort in the
spring. Mr. Edmund Selous watched these birds in Scandinavia, where he
kept a daily record at one of the meeting places. In various passages he
refers to the appropriation of particular positions by particular males,
and concludes thus: "It would seem from this that, like the Ruffs, each
male Blackcock has its particular domain on the assembly ground, though
the size of this is in proportion to the much greater space of the
whole. On the other mornings, too, the same birds, as I now make no
doubt they are, have flown down into approximately the same areas."
The cliff-breeding species--Guillemots, Razorbills, and Puffins--are
difficult to investigate because individuals vary so little, and the
sexes resemble one another so closely; yet, despite these difficulties,
we can gain some idea of the general purport of their activities. But
when the ledges are crowded and the air is filled with countless
multitudes, how is it possible to keep a single bird in view for a
sufficient length of time to understand its routine? The difficulty is
not an insuperable one. The flights, undertaken seemingly for no
particular purpose, are often of short duration and are completed before
the strain of observation becomes too great; moreover an individual
sometimes possesses a special mark or characteristic which serves to
make it conspicuous. For example, there is a well-marked variety of the
Common Guillemot, the Ringed or Bridled Guillemot of science,
distinguished by an unusual development of white round the eye and along
the furrow behind it. One such individual I was fortunate in discovering
upon a crowded cliff, and, as in the case of the Lapwing with the broken
leg or the Yellow Bunting with the injured foot, the identity of the
bird was beyond dispute, and one could observe that it appropriated to
itself a particular position upon a particular ledge.
Guillemots and Razorbills return at intervals to the breeding stations
early in the season, and these visits are repeated with growing
frequency until the birds are finally established. I have witnessed
these periodic returns during March in the south of England, and during
April in the north-west of Ireland, and I am informed that in the latter
district such visits may occur as early as February. Gätke, who had
ample opportunity of observing the birds in Heligoland, puts their
return at an even earlier date. "They visit their breeding places," he
says, "in flocks of thousands at the New Year, often even as early as
December, as though they wanted to make sure of their former haunts
being well preserved and ready for their reception." Such visits,
however, are irregular in occurrence; the birds arrive, and, after
spending a short time upon the ledges, disappear. And since there is not
the same evidence in their coming and going of that method which we
observe in the periodical returns of the Bunting or the Finch, it may be
thought that needless importance is being attached to an episode in
their lives which is quite intelligible in terms of a feeble response
determined by a dawning organic change. While it may be quite
intelligible in such terms it is not thereby explained; for every
response must have as its antecedent an inherited connection in the
nervous system determined on biological grounds. Besides, these early
periodic returns conform in general to the type of behaviour displayed
by other species, the males of which return to their breeding grounds
many weeks before the real business of reproduction begins. Are we then
justified in regarding them as accidents of the developing situation?
Are we not rather bound to admit that they have some definite biological
end to serve?
[Illustration: Competition for territory is seldom more severe than
amongst cliff-breeding sea birds, and the efforts of individual
Razorbills to secure positions on the crowded ledges lead to desperate
struggles.
Emery Walker ph.sc.]
These examples show that the males of many species reverse their mode of
life at the commencement of the breeding season and proceed to isolate
themselves, each one in a definitely delimited area.
There are three ways in which we may attempt to interpret this
particular mode of male behaviour. We may regard it as an accidental
circumstance, nowise influencing the course of subsequent procedure; or,
appealing to the law of habit formation, we may regard it as an
individual acquirement; or again, we may invest it with a deeper
significance and seek its origin in some specific congenital disposition
determined on purely biological grounds.
Which of these three shall we choose? The first by itself requires but
little consideration; for though it might explain the initial visit, it
cannot account for the persistency with which the plot of ground is
afterwards resorted to. Supposing, however, that we combine the first
and the second; supposing, that is to say, we assume, for the purpose of
argument, that the initial visit is fortuitous, and that constancy is
supplied by habit formation--would that be a satisfactory
interpretation? It is a simple one, inasmuch as it only requires that a
male shall alight by chance in a particular place for a few mornings in
succession in order that the process may be set in motion. Now an
essential condition of habit formation is recurrent repetition; given
this repetition and, it is true, any mode of activity is liable to
become firmly established. But how can we explain the repetition? Even
if we are justified in assuming that the initial visit is purely an
accidental occurrence, we cannot presume too far upon the laws of chance
and assume that the repetition, at first, is also fortuitous.
So that we come back to the congenital basis, the last of our three
propositions. And it will, I think, be admitted that the facts give us
some grounds for believing that the securing of the territory has its
root in the inherited constitution of the bird. In comparing the
behaviour of the migratory male with that of the resident, attention was
drawn to the manner in which the occupation of a territory was effected:
the former bird, it may be remembered, established itself without delay,
whereas the latter did so only by degrees, and the difference was
attributed to the incidence of migration which required a closer
correspondence between organic process and external environment. But the
significance for us just now lies in the fact that the definiteness,
which accompanies the initial behaviour of the migratory male in
relation to the territory, cannot have been acquired by repetition; for
this reason, that when the male occupies its space of ground at the end
of its long and arduous journey, it does so without preparation or
experiment, even without hesitation, as if aware that it was making good
the first step in the process of reproduction. No doubt, if it happened
to be an individual that had already experienced the enjoyment of
reproduction, it might be aware of the immediate results to be achieved
and act accordingly. But among the hosts of migrants that one observes,
there must be many males which have not previously mated; and yet, upon
arrival, they all behave in a similarly definite manner--so that
experience cannot well be the primary factor in the situation. If, then,
the essential condition of habit formation is absent and experience is
eliminated, there is nothing left but racial preparation to fall back
upon.
Nevertheless, it is true that many resident males seem to pass through
a period of indecision before they establish themselves permanently in
their respective territories; they come and go, their visits grow more
and more prolonged, and only after the lapse of some considerable time
does the process of establishment attain that degree of completeness
which is represented in the initial behaviour of the migratory male.
Their whole procedure seems therefore to bear the stamp of individual
acquirement; and, if it stood alone, we might be content to construe it
thus, but the example of the migratory male necessitates our looking
elsewhere for the real meaning of the indecision.
Let me first of all give some instances of the persistence with which a
male remains in one spot, and this despite the fact that it has no mate.
A Reed-Bunting occupied a central territory in a strip of marshy ground
inhabited annually by four or five males of this species. Throughout
April, May, and until the 19th June, it clung to its small plot of
ground, tolerated no intrusion, and sang incessantly.
Two Whitethroats arrived at much the same time--the 30th April
approximately--and occupied the corner of a small plantation; the one
obtained a mate the day following its arrival, the other remained
unpaired for a fortnight.
A Reed-Warbler established itself amongst some willows and alders
adjoining a reed-bed and made its headquarters in a small willow bush.
Not more than fifteen yards away, on the edge of the main portion of the
reeds, another male was established and was paired on the 22nd May. Each
morning the single male behaved in much the same way, singing
continuously whilst perched upon the bush. And so the days passed by
until it seemed improbable that it would ever secure a mate, but one
appeared on the 20th June, and a nest was built forthwith.
Now it is difficult to believe that a chance visit, even though repeated
for a few mornings in succession, could have accounted for the
Reed-Bunting remaining so persistently in the marsh, or the Whitethroat
in one corner of the osier bed, or the Reed-Warbler in that one
particular willow. Not only so, but if a habit of such evident strength
can be acquired so readily, we have a right to ask why it should only be
acquired in the spring--why not at every season? Considerations such as
these lead to the belief that there must be some congenital basis to
account for such persistent endeavour; the more so since it is difficult
not to be impressed with the conative aspect of the male's behaviour. To
a stranger, unacquainted with its previous history, the bird might
appear to be leading a life of hesitation, whereas, if carefully
watched, its whole attitude will be found to betray symptoms of a
striving towards some end; and the frequent departure and return, which
might be pointed to as the material from which a definite mode of
procedure would be likely to emerge, is in reality behaviour of a
determinate sort.
My interpretation, then, of the apparent indecision in the behaviour of
the resident male is this. During the winter most species live in
societies, together they seek their food and together they retire in the
evening to the accustomed roosting places; and the association of
different individuals confers mutual benefits upon the associates. The
movements of these societies are dominated by the question of food; all
else is subservient, and the supply of the necessary sustenance may,
under certain conditions, become a difficulty which can only be met by
energy and resource. After the long night the sensation of hunger is
strong, and the birds, on awakening, fly to the accustomed feeding
grounds, returning again in the evening to the selected spot, and by
frequent repetition a routine becomes established. Thus the behaviour of
each individual is determined not only by the powerful gregarious
impulse but also by the habits formed in connection therewith during
many weeks in succession. Now with the rise of the appropriate organic
state, the disposition to seek the breeding ground and there to
establish itself becomes dominant in the male. But the process is a
gradual one. There is no need, as happens amongst the migrants, for the
period of organic change to conform rigidly to the growth of any
particular condition in the environment, and hence for a time the bird
oscillates between two modes of behaviour--between that one organised by
frequent repetition and that one determined by the functioning of this
new disposition.
To look at the matter broadly, it is scarcely likely that so definite a
mode of behaviour would recur with such regularity, generation after
generation, in the individuals belonging to so many widely divergent
forms, if it had no root in the inborn constitution of the bird. But the
law of habit formation has its part to play also. By itself it is
inadequate; yet it probably does assist very materially in adding still
greater definition, and it probably is responsible in a large measure
for determining the limits of the territory according to the conditions
of existence of the species--thus the Falcon seeks its prey over wide
tracts of land, and, by hunting over certain ground repeatedly,
establishes a routine, which broadly fixes the area occupied; the
Woodpecker cannot find food upon every tree, and every forest does not
contain the necessary trees, and therefore the bird regulates its flight
according to the position of the trees; and the Warbler, finding food
close at hand, does not need to travel far, and the area it occupies is
consequently small.
So that the most likely solution of the problem will be found in a
combination of our second and third propositions; that is to say, in an
initial responsive behaviour provided for in the inherited constitution
of the nervous system, and in a definiteness acquired by repetition and
determined by relationships in the external environment.
CHAPTER III
THE DISPOSITION TO DEFEND THE TERRITORY
In the previous chapter I endeavoured to show that each male establishes
a territory at the commencement of the breeding season, and there
isolates itself from members of its own sex. And further I gave my
reasons for believing that this particular mode of behaviour is
determined by the inherited nature of the bird, and that we are
justified in speaking of it as "a disposition to secure a territory"
because we can perceive its prospective value. But the act of
establishment is only one step towards "securing." By itself it can
achieve nothing; for any number of different individuals might fix upon
the same situation, and if there were nothing in the inherited
constitution of the bird to prevent this happening, where would be the
security, or how could any benefit accrue to the species?
In withdrawing from its companions in the spring, the male is breaking
with the past, and this action marks a definite change in its routine of
existence. But the change does not end in attempted isolation; it is
carried farther and extends to the innermost life and affects what,
humanly speaking, we should term its emotional nature, so that the bird
becomes openly hostile towards other males with whom previously it had
lived on amicable terms.
The seasonal organic condition is responsible for the functioning of the
disposition which results in this intolerance, just as it is for the
functioning of the disposition which leads to the establishment of the
territory; and the effect of these two dispositions is that a space of
ground is not only occupied but made secure from intrusion. The process
is a simple one. There is no reason to believe, there is no necessity to
believe, that any part of the procedure is conditioned by anticipatory
meaning; the behaviour is "instinctive" in Professor Lloyd Morgan's
definition of the word, since it is of a "specific congenital type,
dependent upon purely biological conditions, nowise guided by conscious
experience though affording data for the life of consciousness."
That the males of many animals are apt to become quarrelsome during the
mating period is notorious. Darwin collected a number of facts, many of
which related to birds, showing the nature and extent of the strife when
the sexual instinct dominated the situation. And pondering over these
facts, he deduced therefrom a "law of battle," which, he believed, bore
a direct relation to the possession of a female. And it must be admitted
that he had excellent ground for his conclusion in the fact not only
that the conflicts occur mainly during the pairing season, but that
the female is often a spectator and seems even to pair with the victor.
I accepted it, therefore, as the most reasonable interpretation of the
facts. But, as time passed by, incidents of a conflicting character led
me to think that after all there might be another solution of the
problem. And when it was no longer possible to doubt that there was a
widespread tendency to establish territories, it at once became manifest
that the battles might have an important part to play in the whole
scheme. But how was this to be proved? What sort of evidence could show
whether the proximate end for which the males were fighting had
reference to the female or to the territory? Clearly nothing but a
complete record of the whole series of events leading up to reproduction
could supply the necessary data upon which a decision might rest. In the
present chapter I shall give, in the first place, the reasons which lead
me to think that the origin of the fighting cannot be traced to the
female; afterwards, the evidence which seems to show that it must be
sought in the territory; and finally, I shall make a suggestion as to
the part the female may play in the whole scheme.
[Illustration: Male Blackbirds fighting for the possession of territory.
The bare skin on the crown of the defeated bird shows the nature of the
injuries from which it succumbed.
Emery Walker ph.sc.]
The facts upon which the "law of battle" was founded were ample to
establish the truth of its main doctrine. But the evidence upon which
the interpretation of the battles was based was somewhat superficial. It
was based mainly upon the general observation that one or more females
could frequently be observed to accompany the combatants; and if this
were the sole condition under which the fighting occurred, one must
admit that this view would have much to recommend it. But it is not
merely a question of males disputing in the presence of a female; for
males fight when no female is present, pair attacks pair, or a male may
even attack a female--in fact there is a complexity of strife which is
bewildering.
In attributing the rivalry to the presence of the female, it is assumed
that males are in a preponderance, and that consequently two or more are
always ready to compete for a mate. Her presence is presumably the
condition under which his pugnacious nature is rendered susceptible to
its appropriate stimulus, the stimulus being, of course, supplied by the
opponent. There would be nothing against this interpretation if it were
in accord with the facts; but it can, I think, be shown that the males
are just as pugnacious and the conflicts just as severe even when the
question of securing a mate is definitely excluded; and I shall now give
the evidence which has led me to this conclusion.
In the previous chapter we had occasion to refer to the difference in
the times of arrival of the male and female migrants, and we came to the
conclusion, it may be remembered, that this was a fact of some
importance, because it gave us a clue to the meaning of much that was
otherwise obscure in their behaviour. But it is also of importance in
connection with the particular aspect of the problem which we now have
in view, for if it can be shown that males, when they first reach their
breeding grounds, are even then intolerant of one another's presence, if
their actions and attitudes betray similar symptoms of quasi-conation,
if disputes are rife and the struggles of a kind to preclude all doubt
as to their reality, then it is manifest that in such cases their
intolerance cannot be due to the presence of the female.
Here, however, I must refer to a view which is held by some
psychologists, namely, that amongst the higher animals, even on the
occasion of the first performance of an instinctive act, there is some
vague awareness of the proximate end to be attained. Discussing the
nature of instincts, Dr M'Dougall[3] says, "Nor does our definition
insist, as some do, that the instinctive action is performed without
awareness of the end towards which it tends, for this, too, is not
essential; it may be, and in the case of the lower animals no doubt
often is, so performed, as also by the very young child, but in the case
of the higher animals some prevision of the immediate end, however
vague, probably accompanies an instinctive action that has often been
repeated." A similar view seems to be held by Dr Stout.[4] "As I have
already shown," he says, "animals in their instinctive actions do
actually behave from the outset as if they were continuously interested
in the development of what is for them one and the same situation or
course of events; they actually behave as if they were continuously
attentive, looking forward beyond the immediately present experience in
preparation for what is to come. They apparently watch, wait, search,
are on the alert. They also behave exactly as if they appreciated a
difference between relative success and failure, trying again when a
certain perceptible result is not attained and varying their procedure
in so far as it has been unsuccessful. All these characters are found in
the first nest-building of birds as well as in the second; they are
found also in courses of conduct which occur only once in the lifetime
of the animal." Both these writers would, I imagine, contend that, even
when a female is absent, the idea of the female, as the end in view
throughout, is present; and they would argue that the fact of her
absence during the fighting in no way disposes of the belief that she is
the condition under which the pugnacious instinct of the male is
rendered susceptible to stimulation. What reason is there to think that
this interpretation is applicable to the case under consideration? When
a female is present, we observe that the males are pugnacious, and, when
she is absent, that they still continue to be hostile--that is to say,
they behave _as if_ she were present. Now, as far as I can ascertain,
the "_as if_" is the only ground there is for supposing that the female
is represented in imaginal form--there is no evidence of the fact, if
fact it be. On the contrary, the behaviour of the male affords some
fairly conclusive evidence that no such image is the primary factor in
exciting the instinctive reaction. For if it be the actual presence of
the female, or, in the absence of such, a mental image, that renders the
pugnacious nature of the male responsive; provided the usual stimulus
were present, the instinct ought surely to respond, not only under one
particular circumstance, but under all circumstances. Yet, as we shall
presently see, a male is by no means consistently intolerant of other
males. It may be sociable at one moment or pugnacious at another, but
the pugnacity is always peculiar to a certain occasion--the occupation
of a territory. What shall we say then--that a mental image is a
situational item only when the territory is occupied? It may be so; it
may be that the fact of occupation gives rise to the mental image which,
in its turn, renders the fighting instinct explosive, which again
renders the possession of the territory secure. That such an
interpretation is possible we must all admit. But if it were true,
though it would not affect the main consideration, namely, whether the
fighting has reference to the possession of a particular female, or to
the protection of the territory, it would make further discussion as to
which of these is the condition of the fighting unprofitable, for each
would have its part to play in the process, the territory remaining,
however, the principal factor in the situation.
Now the difference in the times of arrival of the male and female
migrant varies in different species from a few days to a fortnight or
even more. It is most marked in those that return to their breeding
grounds early in the season, and the greater the margin of difference
the greater scope is there for observation. In my records for the past
twelve years, there are frequent references to these initial male
contests in the life of the Willow-Warbler and of the Chiffchaff; and in
the district which I have in mind, these two species arrive early in the
season, the males preceding the females by a week or even as much as a
fortnight. Suppose, then, that two Chiffchaffs establish themselves in
adjoining territories; or suppose that a male settles in a territory
already occupied; what is the result? Well, scenes of hostility soon
become apparent; as the birds approach one another they become more and
more restive, their song ceases, they no longer search for food in the
usual methodical manner, but instead their movements are hurried and
their call-notes are uttered rapidly--all of which betrays a heightened
emotional tone. Then the climax is reached, there is a momentary
fluttering of tiny wings, a clicking of bills, and for the time being
that may be all. But unless one or other of the combatants retires, this
scene may be repeated many times in the course of a few hours, and
repeated with varying degrees of severity. Yet the fighting, even in the
most extreme form, when the birds locked together fall slowly to the
ground, is seldom of an impressive kind, and one has to bear in mind the
capabilities of the actors, remembering that the most severe struggle
might readily be interpreted as a game if it were not for certain
symptoms which reveal its inner nature.
The males of many other migrants can frequently be observed to fight
when there was every reason to believe that females had still to arrive.
The Blackcap is notoriously pugnacious, but not more so than the
Marsh-Warbler or the Whinchat. Here in Worcestershire, the _Arundo
phragmites_ grows mainly on certain sheets of water which are
comparatively few and far between, and the Reed-Warbler is consequently
restricted to isolated and more or less confined areas. The males arrive
early in May before the new growth of reeds has attained any
considerable height, and each one has its own position in the reed-bed,
sings there, and throughout the whole period of reproduction actively
resists intrusion on the part of other males. I have kept watch upon a
small area of reeds daily from the date of the first arrival; each
individual was known to me, and as the growing reeds were only a few
inches in height, a female could scarcely have escaped detection. Yet
time and again disputes arose, and males pursued and pecked one
another, striving to attain that isolation for which racial preparation
had fitted them.
But on account of their violence, or their novelty, or because the
absence of a female was beyond question, some battles stand out in one's
memory more prominently than others. An instance of this was a struggle
between two Whitethroats which happened in the latter part of April and
lasted for three successive days. The scene of its occurrence was more
or less the same on each occasion, and the area over which the birds
wandered was comparatively small. The fighting was characterised by
persistent effort and was of a most determined kind, and so engrossed
did the assailants become that they even fluttered to the ground at my
feet. No trace of a female was to be seen at any time during these three
days, nor, during the pauses in the conflict, was the emotional
behaviour of a kind which led me to suppose that a female was anywhere
in the vicinity. And, if she had been near, she must have made her
presence known, for the belief that she is a timid creature, skulking on
such occasions in the undergrowth, is by no means borne out by
experience.
Even more impressive was a battle between two male Cuckoos. It occurred
high up in the air above the tops of some tall elm-trees which roughly
marked the boundary line between their respective areas, and the actions
of the birds were plainly visible. At the moment of actual collision
the opponents were generally in a vertical position, and wings, feet,
and beaks were made use of in turn; one could plainly see them strike at
one another with their feet, and one could observe the open bill which
generally denotes exhaustion, but may of course have been due to anger,
or used as a means of producing terror. Yet no female appeared in the
locality until six days after the occurrence of this struggle--and
she certainly is not easily overlooked, for her note is unmistakable
even when the behaviour of the male does not betray her arrival.
[Illustration: Male Cuckoos fighting before the arrival of a female
Emery Walker ph.sc.]
That the actual presence of the respective females exercised any
influence on the course of these struggles is more than doubtful. Not
only did one fail to detect them, but one's failure to do so was
confirmed by the knowledge that they had not yet arrived in those
particular localities. Hence the fact of the male preceding the female
is a valuable aid to the interpretation of subsequent behaviour; and one
appreciates it the more after having experienced the difficulty of
deciding whether she is present during the conflicts between resident
males, for no matter how carefully we may observe the conditions which
lead up to, and which accompany, such conflicts, or how closely we may
scrutinise the surrounding trees, undergrowth, or ground, there always
remains the possibility that she may, after all, have been overlooked.
But this must not be taken to imply that in such cases direct
observation alone can lead to no serviceable result, or that the
evidence gained therefrom is worthless. Far from it. Failure to detect a
female is so very common an occurrence that, even if we lacked the
corroborative evidence supplied in the life of the migratory male, it
would still be unreasonable to suppose that it were solely due to
mistaken observation. We mark her absence during the conflicts between
the respective males of many common species--the Finches, Buntings, and
Thrushes that occupy their territories early in the season when the
hedgerows and trees are still bare; but more frequently amongst those
that inhabit open ground, because the movements of the birds are there
more accessible to observation. For instance, half a dozen or more
Lapwings can be kept in view at the same time, and as they stand at dawn
in solitary state, keeping watch upon their respective territories, they
are conspicuous objects on the short, frosted grass; no stranger can
enter the arena without the observer being aware of it, no commotion can
occur but one detects it, no movement however small need be missed. And
so they fight, in a manner which leaves no doubt as to the reality of
the struggle, when their prospective mates are absent not only from the
particular territories in which the conflicts take place, but absent too
from those adjoining.
If the fact that males fight before they are paired and in the absence
of a female could be placed beyond all question, it would no longer be
possible to regard her possession as the end for which they are
contending, and consequently there would be no need to produce further
evidence. But the examples which I have given refer, of course, to only
a few migrants and a few residents--and moreover it must be admitted
that a female _is_ often conspicuous during the battles--so that by
themselves they must be regarded, and rightly so, as inconclusive. We
must therefore pass on to consider evidence of a somewhat different
character.
I spoke of the complexity of the strife. By this I mean that it is not
merely a matter of disputes between adjoining males, but that it is a
far more comprehensive business involving both sexes. Thus female fights
with female and pair with pair, or a male will attack a female, or,
again, a pair will combine against a single male or a single female. And
from all this complexity of strife we gain much valuable evidence in
regard to the question immediately before us. For when one pair attacks
another, or males that are definitely paired fight with one another, or
an unpaired male attacks either sex of a neighbouring pair
indiscriminately, there is surely little ground for supposing that the
possession of a mate is the reason of it all.
The battles between pairs of the same species are by no means uncommon.
Observe, for example, the central pair of three pairs of Reed-Buntings
occupying adjoining territories, and keep a daily record of the routine
of activity practised by both sexes during the early hours of the
morning; then, at the close of the season, summarise all the fighting
under different headings, and it will be found that the number of
occasions upon which the central pair attacked, or was attacked by,
neighbouring pairs will form a considerable portion of the whole.
Or watch the Moor-Hen, and for the purpose choose some sheet of water
large enough to accommodate three or more pairs, and so situated that
the birds can always be kept in view. Early in February the pool will be
haunted by numbers of individuals of both sexes, all swimming about
together, and, if the pool is surrounded by arable land, wandering over
that land subject to no territorial restrictions, apparently free to
seek food where they will. But as time goes by, their number gradually
decreases until a few pairs only remain, and these will occupy definite
areas. If careful watch is then kept and the relations of the pairs
closely studied, there will be no difficulty in observing the particular
kind of warfare to which I am alluding, and it will be noticed that the
encounters are of a particularly violent description. Thus two pairs
approach one another, and, when they meet, throw themselves upon their
backs, each bird striking at its adversary with its feet or seizing hold
of it with its beak; and though, in the commotion that ensues, it is
almost impossible to determine what exactly is happening, there is
reason to believe that the sexes attack one another indiscriminately.
A struggle between two pairs of Pied Wagtails is worth mentioning. It
impressed itself upon my memory because of the unusual vigour with which
it was conducted. The battle lasted for fifteen minutes or more, and
the four birds, collecting together, pursued and attacked one
another--at one moment in the air, at another upon the roof of a house
where they would alight and flutter about on the slates, uttering their
call-note without ceasing--until finally they disappeared from view,
still, however, continuing the struggle.
[Illustration: Two pairs of Pied Wagtails fighting in defence of their
territories.
Emery Walker ph.sc.]
Such is the nature of the warfare which prevails between neighbouring
pairs, and which can be observed in the life of many other species--the
Chaffinch, Stonechat, Blackbird, Partridge, Jay, to mention but a few.
The conflicts between males that are definitely paired are of such
common occurrence that it is scarcely necessary to mention specific
instances. But the occasions on which a male attacks either sex of a
neighbouring pair indiscriminately, or on which a pair combine to attack
a female, are less frequent.
Now if it be true that males fight for no other purpose than to gain
possession of a mate, what meaning are we to attach to the battles
between the pairs, or what explanation are we to give of the fact that
paired males are so frequently hostile? Those who hold this view will
probably argue thus: "The presence of the female is the condition under
which the pugnacious instinct of the male is rendered susceptible to
appropriate stimulation, and the stimulus is supplied by a rival male;
we admit that all the fighting which occurs after pairing has taken
place has nothing to do strictly speaking with gaining a mate, but,
inasmuch as the fact of possession is always liable to be
challenged--and no male can differentiate between a paired and an
unpaired intruder--we contend that it would add to the security of
possession if the pugnacious instinct remained susceptible to
stimulation so long as there were any possibility of challenge from an
unpaired male; and we think that the waste of energy involved in the
struggles between paired birds, and which we grant is purposeless, would
be more than balanced by the added security." This is a possible
explanation and requires consideration. It cannot account for all the
diverse ways in which the sexes are mixed up in the fighting--it cannot,
for instance, explain the fact that an unpaired male will attack either
sex of an adjoining pair indiscriminately--but nevertheless it appears
at first sight to be a reasonable explanation of some of them. We must
remember, however, that fighting continues throughout the whole period
of reproduction. Even after the discharge of the sexual function has
ceased, and the female is engaged in incubation or in tending her young,
the male is still intolerant of intruders; and it is difficult to
believe that, at so late a stage in the process, a female could be any
attraction sexually to an unpaired male. But apart from any theoretical
objection, there remains the fact--namely that there is no evidence
that a male, after having once paired, is liable to be robbed of its
mate. And in support of this fact I have only to state that I have met
with no single instance of failure to obtain and hold a mate when once a
territory had been secured. Bearing in mind then that both sexes
participate in the fighting, and that individuals of the opposite sex
frequently attack one another; that all such conflicts are characterised
by persistent effort, and that they are not limited to just the
particular period when the sexual instinct is dominant but continue
throughout the breeding season; bearing in mind that in at least one
form of this promiscuous warfare the influence of the female can be
definitely excluded, and that, in the remaining forms, the evidence
which is required to link them up with the biological end of securing
mates is lacking--can it be denied that the complexity of the strife
makes against the view that the possession of a female is the proximate
end for which the males are fighting?
We started with the most simple aspect of the whole problem, the
fighting of two males in the presence of one female--the aspect upon
which attention has usually been fixed. And if it remained at that, if
observation failed to disclose any further development in the situation,
then there would be no need to probe the matter deeper, there would be
no reason to doubt the assertion that the quarrel had direct reference
to the female. But assuredly no one can ponder over the diversity of
battle and still believe that the possession of a mate furnishes an
adequate solution of the mystery. Clearly such an hypothesis cannot
cover all the known facts; there are conflicts between separate pairs,
and there are conflicts between males when females are known to be
absent and when their mates are even engaged in the work of
incubation--these cannot be due to an impulse in a member of one sex to
gain or keep possession of one of the other sex. So that taking all
these facts into consideration, we are justified, I think, in hesitating
to accept this view, and must look elsewhere for the real condition
under which the pugnacious nature of the male is rendered susceptible to
appropriate stimulation.
What then is the meaning of all this warfare? The process of
reproduction is a complex one, built up of a number of different parts
forming one inter-related whole; it is not merely a question of
"battle," or of "territory," or of "song," or of "emotional
manifestation," but of all these together. The fighting is thus one link
in a chain of events whose end is the attainment of reproduction; it is
a relationship in an inter-related process, and to speak of it as being
even directly related to the territory is scarcely sufficient, for it is
intimately associated with the disposition which is manifested in the
isolation of the male from its companions, and forms therewith an
_imperium in imperio_ from which our concept of breeding territory is
taken. But let me say at once that it is no easy matter to prove this,
for since so many modes of behaviour, which can be interpreted as
lending support to this view, are likewise interpretable on the view
that the presence of a female is a necessary condition of the fighting,
it is difficult to find just the sort of evidence that is required.
Nevertheless, after hearing the whole of the evidence and at the same
time keeping in mind the conclusion which we have already reached, I
venture to think that the close relationship between the warfare on the
one hand and the territory on the other will be fully admitted.
Formerly I deemed the spring rivalry to be the result of accidental
encounters, and I believed that an issue to a struggle was only reached
when one of the combatants succumbed or disappeared from the locality, a
view which neither recognised method nor admitted control. Recent
experience has shown, however, that I was wrong, and that there is a
very definite control over and above that which is supplied by the
physical capabilities of the birds.
Let us take some common species, the Willow-Warbler being our first
example; and, having found three adjoining territories occupied by
unpaired males, let us study the conflicts at each stage in the sexual
life of the three individuals, observing them before females have
arrived upon the scene, again when one or two of the three males have
secured mates, and yet again when all three have paired. Now we shall
find that the conditions which lead up to and which terminate the
conflicts are remarkably alike at each of these periods. A male
intrudes, and the intrusion evokes an immediate display of irritation on
the part of the owner of the territory, who, rapidly uttering its song
and jerking its wings, begins hostilities. Flying towards the intruder,
it attacks viciously, and there follows much fluttering of wings and
snapping or clicking of bills. At one moment the birds are in the
tree-tops, at another in the air, and sometimes even on the ground, and
fighting thus they gradually approach and pass beyond the limits of the
territory. Whereupon a change comes over the scene; the male whose
territory was intruded upon and who all along had displayed such
animosity, betrays no further interest in the conflict--it ceases to
attack, searches around for food, or sings, and slowly makes its way
back towards the centre of the territory.
Scenes of this kind are of almost daily occurrence wherever a species is
so common, or the environment to which it is adapted so limited in
extent, that males are obliged to occupy adjacent ground. The Moor-Hen
abounds on all suitable sheets of water, and it is a bird that can be
conveniently studied because, as a rule, there is nothing, except the
rushes that fringe the pool, to hinder us from obtaining a panoramic
view of the whole proceedings, and moreover the area occupied by each
individual is comparatively small. Towards the middle of February,
symptoms of sexual organic change make themselves apparent, and the pool
is then no longer the resort of a peaceable community; quarrels become
frequent, and as different portions of the surface of the water are
gradually appropriated, so the fighting becomes more incessant and more
severe. Each individual has its own particular territory, embracing a
piece of open water as well as a part of the rush-covered fringe, within
which it moves and lives. But in the early part of the season, when the
territories are still in process of being established, and definiteness
has still to be acquired, trespassing is of frequent occurrence, and the
conflicts are often conspicuous for their severity.
Now these conflicts are not confined to unpaired individuals, nor to one
sex, nor to one member of a pair--every individual that has settled upon
the pool for the purpose of breeding will at one time or another be
involved in a struggle with its neighbour. If then we single out certain
pairs and day by day observe their actions and their attitude towards
intruders, we shall notice that, instead of their routine of existence
consisting, as a casual acquaintance with the pool and its inmates might
lead us to believe, of an endless series of meaningless disputes, the
behaviour of each individual is directed towards a similar goal--the
increasing of the security of its possession; and further, if we pay
particular attention to the circumstances which lead up to the quarrels
and the circumstances under which such quarrels come to an end, we shall
find, when we have accumulated a sufficient body of observations, that
the disputes always originate in trespass, and that hostilities always
cease when the trespasser returns again to its own territory. By careful
observation it is possible to make oneself acquainted with the
boundaries--I know not what other term to use--which separate this
territory from that; and it is the conduct of the birds on or near these
boundaries to which attention must be drawn. A bird may be feeding
quietly in one corner of its territory when an intruder enters. Becoming
aware of what is happening it ceases to search for food, and approaching
the intruder, at first swimming slowly but gradually increasing its
pace, it finally rises and attacks with wings and beak, and drives its
rival back again beyond the boundary. Thereupon its attitude undergoes a
remarkable change; ceasing to attack, but remaining standing for a few
moments as if still keeping guard, it betrays no further interest in the
bird with which a few seconds previously it was fighting furiously. On
one occasion I watched a trespasser settle upon a conspicuous clump of
rushes situated near the boundary. The owner, who was at the moment some
distance away, approached in the usual manner, and, having driven off
the trespasser, returned immediately to the clump, where it remained
erect and motionless.
A feature which marks all the fighting, and which we cannot afford to
disregard, is the conative aspect of the behaviour of the owner of the
territory. The bird attacks with apparent deliberation _as if_ it were
striving to attain some definite end. I recollect an incident which was
interesting from this point of view. A pair of Reed-Buntings were
disturbed by a Weasel which had approached their nest containing young.
Both birds betrayed symptoms of excitement; as the Weasel threaded its
way amongst the rushes, so they fluttered from clump to clump or clung
to the stems, uttering a note which is peculiar to times of distress,
and followed it thus until finally it disappeared in a hedge. The
rapidly uttered note and the excitement of the birds caused some
commotion, and the male from an adjoining territory approached the
scene. Now one would have expected that the presence of this bird, and
possibly its aid in driving away a common enemy, would have been
welcomed; one would have thought that all else would have been
subservient to the common danger, and that so real a menace to the
offspring would have evoked an impulse in the parent powerful enough to
dominate the situation and subordinate all the activities of the bird to
the attainment of its end. But what happened? Three times during this
incident, the male, whose young were in danger, abandoned the pursuit of
the Weasel and pursued the intruder. It was not merely that he objected
to the presence of this neighbouring male in a passive way, nor even
that he had a momentary skirmish with it, but that he determinedly drove
the intruder beyond the boundary and only then returned to harass the
Weasel.
Thus it seems clear that the proximate end to which the fighting is
directed is not necessarily the defeat of the intruder, but its removal
from a certain position. And inasmuch as this result will be obtained
whether the retreat is brought about by fear of an opponent or by
physical exhaustion, it is manifest that too much significance need not
be attached to the amount of injury inflicted. It is necessary to bear
this in mind, because it is held by some, who have carefully observed
the actions of various species, that overmuch importance is attached to
the conflicts, that in a large number of instances they are mere
"bickerings" and lead to nothing, and that they are now only "formal,"
which means, I suppose, that they are vestigial--fragments of warfare
that determined the survival of the species in bygone ages. But if the
conclusion at which we have just arrived be correct, if we can recognise
a single aim passing through the whole of the warfare--and that one the
removal of an intruder from a certain position, then we need no longer
concern ourselves as to the degree of severity of the battles--we see it
all in true perspective. Neither exhaustion nor physical inability are
the sole factors which determine the nature and extent of the fighting;
there is a more important factor still--position. According, that is to
say, to the position which a bird occupies whilst fighting is in
progress, so its pugnacious nature gains or loses susceptibility, and it
is this gain or loss of susceptibility which I refer to when I speak of
the fighting as being controlled.
What we have then to consider is the relation of "susceptibility" to
"position." We can explain the relationship in two ways. We can say that
the part of the nature of the male which leads to the occupation of a
territory, and is partly hereditary and partly acquired, is stronger
than the part which leads the bird to fight, and which is conditioned by
the presence of a female, and that consequently when the male passes the
boundary, the impulse to return asserts itself and the conflict ceases;
or we can say that the occupation of a territory is the condition under
which the pugnacious instinct is rendered susceptible to stimulation,
that the stimulus is supplied by the intruder, and that when the male
passes outside the accustomed area its instinct is no longer so
susceptible and it therefore retires from the conflict.
[Illustration: Long-tailed Tit Males fighting for possession of
territory. The feathers have been torn from the crown of the defeated
and dying rival.
Emery Walker ph.sc.]
Of these explanations, the first is not altogether satisfactory. It
requires the presence of a female and, as we have seen, a female is by
no means always present. Then it attributes to the one side of the
inherited nature an influence which is not borne out by the facts, for
in the ordinary routine of existence, without the incentive of battle,
every individual is liable to wander occasionally beyond its boundary
and to intrude temporarily upon its neighbours; and this it could
scarcely do, providing its nature to remain within the territory were
powerful enough to dominate its movements and curtail its activities
even during the excitement of an encounter. But there is nothing
inherently improbable in the alternative hypothesis, nor anything that
is at all inconsistent with the behaviour as observed; on the contrary,
if it is admitted, the facts become connected together and exhibit a
meaning which they otherwise would not have possessed.
So much for the controlling influence of "position," which alone seems
to me sufficient ground for believing that the fighting has reference to
the territory. But it is not the whole of the evidence.
Now if it were possible to demonstrate by actual observation that those
males which had not established territories were not pugnacious, we
should have something in the nature of proof of the correctness of this
view. Demonstrative evidence of this kind is, however, unattainable. Yet
we can come very near to obtaining it by reason of a peculiar feature
which marks the process of acquiring territory--the neutral ground. The
Lapwing will serve as an illustration. In the previous chapter I
referred to the small flocks that appeared in the accustomed water
meadow early in February, and I described how they settled day after day
in that meadow, but only in a limited part of it, where they passed
their time in rest, in preening their feathers, or in running this way
and that lazily searching for food; and how, at length, the flock
dwindled by reason of individuals breaking away in order to secure
positions on the remaining part of the meadow. Here the neutral ground
is adjacent to the territories, and, while still occupied by the flock,
is resorted to by the males that had deserted that flock in order to
establish those territories.
Suppose now that we have the whole meadow in view from some point of
vantage. In front of us are the territories, in the distance the neutral
ground; and in each territory there is a solitary male, while on the
neutral ground a number of individuals of both sexes are assembled, and
move about freely one amongst another. So that the scene presented to
view is somewhat as follows: a flat meadow, at one end of which, and at
fairly regular intervals, a few solitary individuals are dotted about,
each one keeping at a distance from its neighbours; while at the other
end a number of individuals are collected together in a comparatively
small space, apparently deriving some satisfaction from their close
association. That surely is a very remarkable contrast. But let us
continue our investigation, first fixing our attention upon the solitary
individuals; one is standing preening its feathers, another is squatting
upon the ground, a third runs a few yards in this direction then a few
yards in that, stimulated apparently by the sight of food, and so on.
Moreover, each one keeps strictly to a well-defined area and makes no
attempt to associate with its fellows. One of the males, however, whilst
roaming backwards and forwards approaches the limit of its territory,
and this brings the neighbouring bird, whose boundary is threatened,
rapidly to the spot. In an upright position both stand face to face, and
the battle then begins; with their wings they attempt to beat one
another about the body, with their beaks they aim blows at the head, and
in the mêlée wings and legs seem to be inextricably mixed; whilst at
intervals, driven backwards by the force of the collision, they are
compelled to separate, only, however, to return to the charge--and the
sound of beating wings and the feathers that float in the air are tokens
of earnestness. Such scenes are of frequent occurrence; but the
conflicts vary in intensity, and the circumstances under which they
occur vary too, and females come and go without leaving any clue as to
their ultimate intentions.
Turning now to the flock one is impressed with the friendship that seems
to exist between the various members. There are, it is true, occasional
displays of pugnacity which never seem to develop into anything very
serious; for instance, one bird will fly at another, and a momentary
scuffle is followed by a short pursuit but nothing more--nothing, that
is to say, in the least comparable with the battle previously described.
Of what is the flock composed? Of members of both sexes. There is no
difficulty in assuring oneself that this is so. But is it entirely
composed of individuals in whom development has not reached a stage
adequate for the functioning of the primary dispositions? No, not
entirely; for it will be observed that its number is a fluctuating one,
that birds come and go, and, if a close watch is kept upon the different
individuals as they leave, it will be noticed that some at least are
inmates of the territories at the opposite end of the meadow--the
solitary members whose behaviour we were recently watching. This fact is
an important one. We were impressed, it may be remembered, with the
contrast between the general behaviour of the birds at the opposite
ends of the meadow. But now it appears as if the contrast were not
between this individual and that, but between the behaviour of the same
one under different circumstances. The male, that is to say, which,
while in its territory, tolerates the approach of no other male, flies
to the flock and is there welcomed by the very individuals with whom a
short time previously it had been engaged in serious conflict.
But if the conditions are reversed and the flock happens to settle in an
occupied territory, the attitude of the owner towards the flock is very
different. In the year 1916 an incident of this kind occurred in the
meadow to which reference has already been made. The weather had been
exceptionally severe--very cold easterly and north-easterly winds,
frost, and frequent falls of snow had affected the behaviour of the
Lapwings, and seemed to have checked the normal development of their
sexual routine. The males would attempt to establish themselves, and
then, when the temperature fell and the ground was covered with snow,
would collect again in flocks and follow their winter routine. It was on
the 9th March, during one of the spells of milder weather, that the
flock on the neutral ground was disturbed and settled mainly in the
territory marked No. 3 on the 1916 plan, but partly on that marked No.
2. The owners thereupon began to attack the different members of the
invading flock. Fixing attention upon a particular bird whilst ignoring
the remainder, the No. 3 male drove it away, and then after a pause
drove another away, and so on until by degrees all the invaders were
banished, and the No. 2 male did likewise. The interest of this incident
lies, however, in the behaviour of the different individuals of which
the flock was composed; when attacked they made no real show of
resistance, but accepted the situation and left. The will to fight was
clearly lacking, yet their presence was a source of annoyance to the
owners of the territories. A short time previously a female had
accompanied one of the males and was at that time somewhere in the
vicinity, but beyond this there was no evidence to show that either of
them were paired, and even if the presence of the female were the reason
of the pugnacity of the one, it could not well account for that of the
other.
The neutral ground does not always happen to be so close at hand as in
the case of the meadow referred to. Sometimes the birds will resort to a
particular field, attracted probably by a plentiful supply of food, and
here they collect and behave as they do during the winter, running this
way and that as the fancy takes them, meeting together by accident at
one moment, parting at another, according to the direction in which they
happen to wander. Of animosity there is little sign; the season might be
the middle of winter instead of the middle of March for all the
indication there is of sexual development, and yet one knows that they
will behave differently when they leave this ground, as presently they
will, and return to their territories in the surrounding neighbourhood,
and that there each one will fight if necessary to preserve its acre
from intrusion.
It would seem, then, from this that the fighting must bear some relation
to the particular area of ground in which it occurs; and unless it can
be shown that there is some other factor in the external environment of
the male, that is the direction in which we must look for the condition
under which the instinct is rendered susceptible. One's thoughts turn,
of course, to the female, but she too passes backwards and forwards
between the territories and the neutral ground, and if her presence were
really a _conditio sine qua non_ of the strife, one would like to know
why, when she leaves those territories and joins the flock and the males
do likewise, similar conflicts should not prevail there also.
Other species have their neutral ground, but the environment seldom
affords such facilities for observation as does that of the Lapwing.
Even though the Moor-Hens, who are so conspicuously intolerant upon the
pool, _do_ feed together amicably upon the meadows adjoining; and the
Chaffinch that is so pugnacious in the morning, _does_ seek out the
flock later in the day; yet their conditions of existence prevent our
obtaining a panoramic view of the whole proceeding, and we have to study
each scene separately before discovering that the relationship between
intolerance and the territory on the one hand, and friendship and the
neutral ground on the other, is just as strong a feature as it is in the
behaviour of the Lapwing.
I shall now give a brief account of the conduct of a male Reed-Bunting
which by persistent effort established itself late in the season, and I
shall do so because its behaviour tends to confirm much that has been
said in the preceding pages.
Early in March three male Reed-Buntings occupied a small water meadow
overgrown with the common rush, and by the third week all of them were
paired. On the 30th March two of the males were unusually pugnacious,
and on the following day fighting continued and at times was very
severe. Now I knew that the occupants of the ground in which the
fighting was taking place were paired, and not doubting that the
combatants were the owners of two territories marked for convenience
sake Nos. 1 and 2, I was at a loss to understand the meaning of so
determined and persistent a struggle. My attention, however, was
presently drawn to a third bird, which also joined in the conflict and
made the whole situation still more perplexing. This bird, as it soon
became clear, was none other than the owner of No. 2 territory, and the
one that I had previously regarded as such was a new arrival. On the
following day, the 1st April, fighting continued, and in my record for
that day there is a note to the effect that "No. 2 female seems to be of
no interest to No. 5 male (the new arrival); its purpose seems to be to
drive away intruders." On the 2nd April and subsequent days, this bird
attacked every other male that approached, and not only maintained its
position but ultimately succeeded in securing a mate. Here then we have
two territories occupied by two males, both of which had obtained a
mate. The relation of these two birds was normal, a month's routine had
defined their boundaries, and conflicts were less frequent than
formerly. But upon this comparatively peaceful scene a strange male
intrudes. Observe the manner of the intrusion. The stranger does not
wander about first in this direction and then in that, but acts _as if_
it had some definite end in view, and establishing itself in a small
alder bush which it uses as a base or headquarters, it gradually extends
its dominion, gains the mastery over the surrounding ground, part of
which belonged to No. 1 male and part to No. 2, and finally drives a
wedge, so to speak, between the two territories.
How is its behaviour to be explained, and why did its presence cause
such commotion? No one could have watched the gradual unfolding of this
incident day by day and not have been impressed by the persistent
endeavour with which this male maintained its position in one small part
of the meadow. This is the first and most important consideration. Then
there is the attitude, also significant, which it adopted towards the
females; for I take it that, apart from the question of territory, the
explanation of its intrusion must be sought in the necessity for
securing a mate--that it was attracted by the presence of the females,
and that the proximate end of its behaviour was the possession of one of
them. But if there is one thing that emerges from the facts more clearly
than another it is that the course of its behaviour was in no way
influenced by the presence or absence of either of the females. My
reasons for saying so are the following: in the first place, it made no
attempt to pursue or to thrust its attention upon either one or the
other of them; secondly, it even went so far as to attack and drive them
away when they approached too closely; and in the third place, when an
unpaired female did at length appear, it adopted a different attitude
and forthwith paired. And bearing in mind that these two females had
already been with their respective mates for some considerable time, and
that there was reason to believe that coition had actually taken place,
is it likely that any counter-attraction would have proved successful in
tempting either of them away from its mate, or probable, if they were
the sole attraction, that the intruding male would have been so
persistent in remaining? How very much simpler it is to fit the pieces
together, if for the time being we ignore the female and fix our
attention upon the territory. Each item of behaviour then falls into its
proper place, and the fighting which seemed so perplexing and
meaningless becomes a factor of prime importance. First of all the male
arrives; then it establishes itself in a small alder bush and advertises
its presence by song; next, by persistent effort in attacking the
neighbouring males, it frees a piece of ground from their dominion; and
finally, in proper sequence, a female arrives, pairing takes place, and
reproduction is secured.
[Illustration: A battle between two pairs of Jays
Emery Walker ph.sc.]
How then does the whole matter stand? If it were males only that engaged
in serious conflict, and if they fought only in the presence of a
female, the problem would resolve itself into one simply of obtaining
mates. But the warfare extends in a variety of directions, it is not
confined to one sex, nor to unpaired individuals, nor need the opponents
necessarily be of the same sex; it involves both sexes alike singly or
combined. Now the view that the biological end of battle is, in its
primary aspect, related to the female, cannot, as we have seen, apply to
the conflicts between different pairs, and only by much stretching of
the imagination can it be held responsible for the hostility that males
frequently display towards females or _vice versa_. It is valid only for
a certain form of warfare. But that form represents, you will say, a
large proportion of the whole, which is true; and so long as we ignore
the remainder, we might rest content in the belief that we had solved
the major part of the problem. But can we ignore the remainder? Can we
say that the conflicts between paired males, for example, are simply
offshoots of the pugnacious disposition, and have no part to play in the
process of reproduction? They recur with marked persistency season after
season and generation after generation; they are to be found in species
widely remote; they are frequent in occurrence; and no one who had
observed them and noted the vigour with which they are conducted, could,
I think, conclude that they were meaningless--and be satisfied. They
must somehow be explained. So that if anyone thinks fit to maintain that
possession of a mate is an adequate explanation of part of the
hostilities, it is clearly impossible to regard all the fighting as a
manifestation of one principle directed towards a common biological end.
But wherever we extend our researches, we find that the facts give
precision to the view that the occupation of a territory is the
condition under which the pugnacious instinct is rendered susceptible to
stimulation. The Lapwing, when in its territory, displays hostility
towards other males of its own species, but when upon neutral ground,
treats them with indifference; the Chiffchaff pursues its rival up to
the boundary and is then apparently satisfied that its object has been
achieved; the cock Chaffinch in March permits no other male to intrude
upon its acre or so of ground during the early hours of the morning, but
for the rest of the day it joins the flock and is sociable; the
Herring-Gull resents the approach of strangers so long as it occupies
its few square feet of cliff, but welcomes companions whilst it is
following the plough--all of which points to a relation between the
territory and the fighting. And this view has at least one merit--it
accounts for all the fighting no matter what degree of severity may be
reached or in what way the sexes may be involved. The complexity of the
strife presents no obstacle; for if the biological end of the fighting
is to render the territory, which has already been established, secure
from intrusion, each sex will have its allotted part to play at the
allotted time: thus the battles between the males before females appear
on the scene will decide the initial question of ownership; those
between the females will give an advantage to the more virile members
and insure an even distribution of mates for the successful males; the
constant struggles between paired males will roughly maintain the
boundaries and prevent such encroachment as might hamper the supply of
food for the young; and the co-operation of male and female in defence
of the territory will be an additional safeguard. Each form of battle
will contribute some share towards the main biological function of
reproduction.
Hitherto we have dealt principally with the male. We have referred, it
is true, to the fact that the female co-operates with her mate in order
to drive away intruders, but beyond this, we have made no attempt to
trace what part, if any, she plays in the whole scheme. We must do so
now.
The various steps by which the territory is not only established but
made secure from invasion, imply an inherited nature nicely balanced in
many directions--first of all the male must be so attuned as to be ready
to search for a territory at the right moment; then it must be capable
of selecting a suitable environment; and, having established itself, it
must be prepared to defend its area from a rival, and to resist
encroachment by its neighbours--and if it failed in any one of these
respects, it would run the risk of failure in the attainment of
reproduction. Each individual has therefore to pass, so to speak,
through a number of sieves--the meshes of which are none too
wide--before it can have a reasonable prospect of success. This being
so, we ask, in the first place, whether the female, too, may not have an
eliminating test to pass; and in the second place, whether she may not
also assist in furthering the biological end of securing the territory.
Now the answer to the first of these questions will be found to be in
the affirmative. Just as, in the securing of a territory, the ultimate
appeal is to the physical strength of the male, so, in the course of her
search for a mate, the female may be called upon to challenge, or may be
challenged by a rival, and the issue is decided by force. My attention
was first drawn to this fact by a struggle between two female
Whitethroats, which I have described elsewhere. The scene of its
occurrence was the corner of a small osier bed occupied by one male, and
the females that took part in it had only recently arrived, but the
male, an unpaired bird, had been in possession of its territory for some
days. The sequel to this struggle, which was protracted and severe, was
the disappearance of both females, the male being left without a mate
for a further ten days.
[Illustration: Emery Walker ph.sc.
The female Chaffinch shares in the defence of the territory and attacks
other females.]
Numerous instances have since come under my notice. Hen Chaffinches
become so absorbed that they fall to the ground and there continue the
struggle. Seizing hold of one another by the feathers of the head, they
roll from side to side, and then, without relaxing their grip, lie
exhausted--the quickened heart-beat, altered respiration, tightly
compressed feathers and partially expanded wings betraying the intensity
of the conflict.
As the breeding season approaches, hen Blackbirds grow more pugnacious.
Individuals that early in the year have frequented the same spot daily
and have even shown every sign of friendship, become openly hostile. For
two years in succession I had an opportunity of observing females under
such conditions, and of studying the gradual change in their
relationship. Each morning at break of day and for some hours afterwards
they could be seen in the same place, one following the other as they
searched for food first in this direction and then in that, as if they
derived some special pleasure from the fact of their companionship. Then
a change began to manifest itself. Indications of animosity became
apparent; one would run towards the other in a threatening attitude and,
in a half-hearted manner, peck at it; and gradually the hostility grew,
until the tentative pecking developed into a scuffle and the scuffle
into a conflict.
Much fighting also occurs between the females of the Reed-Bunting, and
likewise between those of the Moor-Hen, and because these two species
are not only common but inhabit respectively open stretches of marshy
ground or large sheets of water, the fighting can be readily observed.
Why do the females fight before they are definitely paired? To obtain
mates? This certainly seems to be the obvious explanation because any
question of securing territory can be excluded; yet if it be true that
their sex is numerically inferior, it is difficult to understand the
necessity for such strenuous competition. But what is the condition
under which the pugnacious instinct of the female is rendered
susceptible to stimulation? It cannot be merely the presence of a male
ready to breed, for then there would be endless commotion amongst the
flocks of Chaffinches or of Lapwings which in March are composed of both
sexes, including even males that have secured territories. There must be
some other circumstance; and, judging by experience, it is to be found
in the territory--a male, that is to say, in occupation of one, is the
condition under which the inherited nature of the female is allowed free
play. We must bear in mind, however, that the competition between the
males is very severe, that large numbers probably fail to pass even this
preliminary test, and that only a proportion are in a position to offer
to the female the condition under which her process can successfully run
its course; so that the presumption is--though it is incapable of
demonstration--that there is a competition for such males each recurring
season, and that, on the average, the weaker females fail to procreate
their kind.
But apart from any direct assistance she may give in driving away
intruders, does she in any way help to further the biological end of
reproduction? This is a difficult question to answer, and the suggestion
I have to make can only apply in those cases in which the territory is
occupied throughout the breeding season. Much of the fighting between
the males occurs in her presence, and it must be admitted--though it is
difficult to speak with any degree of certainty--that such fighting,
taken as a whole, bears the stamp of exceptional determination. Let us
then grant that the excitement of a male does, under these
circumstances, reach a higher level of intensity, and let us see how
this will add to the security of the territory. The fact that the male
has established itself and obtained a mate is not alone sufficient to
accomplish the end for which the territory has been evolved. During the
period between the initial discharge of the sexual function and the time
when incubation draws to a close, much may happen to prejudice the
future of the offspring; there is always the possibility of invasion by
an individual whose development is backward or which has been
unsuccessful in making good the first step, and, as we saw in the case
of the Reed-Bunting, a portion of the ground won may be lost; there is
always the danger of gradual encroachment by neighbouring owners; and
there is even a possibility that a pair may be so persistently harassed
by more virile neighbours as to forsake the locality permanently. If
then a male is to attain a full measure of success it must be capable
of keeping its boundaries intact up to the time when the young are able
to fend for themselves, and consequently it is important that its
intolerant nature should remain susceptible to stimulation throughout
the greater part of the season.
Does the presence of a female serve to promote this end? Now we know
very little of the influence exerted by one sex upon the other.
Professor Lloyd Morgan has suggested that the male raises the emotional
tone of the female, a suggestion which seems to me in accordance with
the facts. There is reason to believe, however, that the converse is
also true--namely that the excitement of the male reaches a higher level
of intensity when a female is present. Granting then that his emotional
tone is raised, how will this affect the question? So great is the
difference of opinion as to the part that the emotions play in
furthering the life of the individual that one hesitates to accept any
particular one. But it seems to be generally admitted that emotion adds
to the efficacy of behaviour, and this is the view of Professor Lloyd
Morgan. "Whatever may be the exact psychological nature of the emotions,
it may be regarded," he says, "as certain that they introduce into the
conscious situation elements which contribute not a little to the energy
of behaviour. They are important conditions to vigorous and sustained
conation." Therefore, if it be true that the female raises the emotional
tone of the male, the result will be an increased flow of energy into
all the specific modes of behaviour connected with reproduction, amongst
which those directly concerned in the securing and defence of the
territory will receive their share; so that instead of a progressive
weakening of just those elements in the situation which make for
success, the level of their efficiency will be maintained as a result of
such reinforcement. But the female becomes intolerant of her own sex
when she has discovered a male ready to breed, and, later, assists her
mate in resisting intrusion; and by raising her emotional tone, he may
be the means of furthering more strenuous behaviour on her part. Each
member of the pair would in this way contribute towards the energy of
behaviour of its mate, and hence add indirectly to the security of the
territory.
It may be well to illustrate the foregoing remarks. Suppose that there
is a small piece of woodland barely sufficient to hold three pairs of
Willow-Warblers, and suppose that the male and female in the middle
territory did not respond to one another's influence quite as readily as
the adjoining males and females, what would be the result? The emotional
tone of the central pair would stand at a lower level of intensity; and,
since their congenital dispositions would lack the necessary
reinforcement, the birds would tend to become less and less punctilious
in keeping their boundaries intact, whereas the adjoining pairs, always
on the alert and meeting with little opposition, would encroach more and
more and gradually extend their dominion. And so, by the time the young
were hatched, the parents would be in occupation of an area too limited
in extent to insure the necessarily rapid supply of food, and would be
compelled to intrude upon the adjoining ground. But knowing how routine
becomes ingrained in the life of the individual, knowing that for weeks
this pair had submitted to their neighbours, can we believe that they
would be capable of asserting their authority and that the young would
be properly cared for? Or suppose that different pairs of Kittiwake
Gulls on the crowded ledges, or different pairs of Puffins in the
crowded burrows, varied in like manner, would they all have equal
chances of rearing their offspring? The struggle for reproduction is
nowhere more severe than amongst the cliff-breeding sea birds; it is not
for nothing that one sees Kittiwake Gulls, locked together, fall into
the water hundreds of feet below and struggle to the point of
exhaustion, or, as has been reported, to the point of death; it is not
for nothing that Puffins fight with such desperation. And surely success
will be attained by that pair whose emotional tone stands high and whose
impulse to fight is therefore strong, rather than to the ill-assorted
couple.
The argument, then, is briefly this. In the spring, a marked change
takes place in the character of the males of very many species; instead
of being gregarious they either avoid one another and become hostile,
or, if their conditions of existence require that they shall still live
together, they become irritable and pugnacious. This change is made
known to us by the battles of varying degrees of severity which are such
a feature of bird life in the spring; and since a female can commonly be
observed to accompany the combatants, the possession of a mate appears
at first sight to be the proximate end for which the males are
contending. But when the circumstances which lead up to the quarrels are
investigated closely, the problem becomes more difficult; for it is not
merely a question of males fighting in the presence of a female, as is
generally supposed to be the case, but on the contrary there is a
complexity of strife which is bewildering--males attack females or _vice
versa_; female fights with female; or a pair combine to drive away
another pair, or even a solitary individual no matter of which sex. This
complexity of strife makes against the view that the possession of a
mate is the reason of the fighting. But an even stronger objection is to
be found in the fact that males are hostile when no female is
present--and hence we must seek elsewhere for the true explanation.
Now if the behaviour of a male be closely observed, it will be found
that its pugnacious instinct gains or loses susceptibility according to
the position which it happens to occupy--when its ground is trespassed
upon, the impulse to fight is strong; but when it crosses the boundary
it seems to lose all interest in the intruder. Moreover, in some
species, the male rejoins the flock at intervals during the early part
of the season and for a time leads a double existence, passing backwards
and forwards between its territory and the neutral ground. Its behaviour
under these circumstances affords some valuable evidence, for the bird
displays little if any hostility when accompanying the flock, yet when
it returns to the ground over which it exercises dominion, no male can
approach without being attacked. The conclusion, therefore, seems to be
inevitable, namely that the actual occupation of a territory is the
condition under which the pugnacious nature of the male is rendered
susceptible to appropriate stimulation.
CHAPTER IV
THE RELATION OF SONG TO THE TERRITORY
If we listen to the voices of the Waders as, in search of food, they
follow the slowly ebbing tide, we shall notice that each species has a
number of different cries, some of which are uttered frequently and
others only occasionally. Not only so, but if we study the circumstances
under which they are uttered, we shall in time learn to associate
certain specific notes with certain definite situations.
The Curlew, when surprised, utters a cry with which most of us, I
suppose, are familiar; but when with lowered head it drives away another
individual from the feeding ground, it gives expression to its feelings
by a low, raucous sound, which again is different from its cry when a
Common Gull steals the _arenicola_ that has been drawn out of the mud
with such labour.
Thus we come to speak of "alarm notes," "notes of anger," "warning
notes"--naming each according to the situations which normally accompany
their utterance. And so, all species, or at least a large majority of
them, have, in greater or lesser variety, cries and calls which are
peculiar to certain seasons and certain situations; and since on many
occasions we have indisputable evidence of the utility of the sound
produced--as when, upon the alarm being given by one individual, the
flock of Lapwing rises, or when, in response to a particular note of the
parent, the nestling Blackcap ceases to call--so are we bound to infer
that all the cries are, in one way or another, serviceable in furthering
the life of the individual.
But besides these call-notes, birds produce special sounds during the
season of reproduction--some by instrument, others by voice, others
again by the aid of mechanical device. And not only is this the case,
but many accompany their songs with peculiar flights, such as soaring to
a great height, or circling, or floating in the air upon outstretched
wings. These special sounds and special flights are those with which I
now propose to deal, including under the heading "song" all sounds
whether harsh or monotonous or beautiful, and whether vocally or
otherwise produced; and I shall endeavour to show not only that they are
related to the "territory," but that they contribute not a little to the
successful attainment of reproduction.
The vocal productions are infinite in variety and combination. At the
one extreme we have songs composed of a single note repeated slowly or
rapidly as the case may be, whilst at the other we have the complex
productions of the Warblers; and between these two extremes, notes and
phrases are combined and recombined in ways innumerable. And just as
there is a rich variety of combination, so there is a very wide
variation in the purity and character of the notes--some are harsh,
others melodious, some flute-like, others more of a whistle, and others
again such as can only be likened to the notes of a stringed instrument.
Hence in variety of phrase combination added to variety in the character
of the note, there is a possibility of infinite modes of expression.
If, in the latter part of May, we take up a position at dawn in some
osier bed, we listen to songs which have reached a high degree of
specialisation, songs, moreover, which appeal to us on account of their
beauty; if, on the other hand, we climb down the face of the sea cliff,
we hear an entirely different class of songs--harsh, guttural, weird,
monotonous sounds, which, appeal to us though they may, lack the music
of the voices in the osier bed. And just as, in the osier bed, we can
recognise each species by its voice, so we can distinguish the "cackle"
of the Fulmar, the "croak" of the Guillemot, or the "grunt" of the Shag.
In the osier bed, however, there is considerable variation in the song
of different individuals of the same species, so much so that we can
recognise this one from that; whereas on the cliff we cannot distinguish
between the voices of different individuals. And the more highly
developed the song, the greater the range of variation appears to be;
but notwithstanding this--notwithstanding the fact that the pitch may
differ, the phrase combination may differ, and the timbre may
differ--the song remains nevertheless specific. So that the two
principal features of "song," broadly speaking, are "diversity" and
"specific character."
In contrast with the call-notes, the majority of which can be heard at
all times of the year, the song is restricted as a rule to one season,
and that one the season of reproduction. It is true, of course, that
some birds sing during the autumn, and, if the climatic conditions are
favourable, in the winter also, just as others betray, in the autumn,
symptoms of emotional manifestation peculiar to the spring; but just as
the manifestation of the latter is feeble and vestigial, so, too, does
the song of the former lack the vigour and persistency which is
characteristic of the spring. Again, in contrast with the call-notes,
which are common alike to both sexes, song is confined to one sex--a
peculiar property of the males.
Now all, I think, will agree that it must serve some biological
purpose--this at least seems to be the conclusion to be drawn from the
two outstanding features of "diversity" and "specific character"; and
since the voices of different individuals of the same species vary, it
has been suggested that, by creating a more effective pairing situation,
it is serviceable in furthering the life of the individual. I do not
propose at the moment to enquire whether this doctrine be true, but
rather to direct attention to other ways in which the song may be
useful.
Is the instinct susceptible to stimulation under all conditions during
the season of reproduction, or only under some well-defined condition?
This is the question to which we will first direct inquiry.
Song in its full development belongs, as we have seen, to the season of
reproduction; it is heard at the dawn of the seasonal sexual process,
and is the most conspicuous outward manifestation of the internal
organic changes which ultimately lead to reproduction. These changes
would appear, at first sight, to be the primary condition which renders
the instinct susceptible to appropriate stimulation. But while this is
true up to a point, in so far, that is to say, as organic changes are a
necessary antecedent of all behaviour connected with the attainment of
reproduction, closer acquaintance with the circumstances under which the
instinct is allowed full play leads to the belief that they are not
alone sufficient to account for the facts as observed. In order to
arrive at a decision we must seek out the specific factors in the
external environment with which "song" is definitely related.
Some birds cross whole continents on their way to the breeding grounds,
others travel many miles, others again find suitable accommodation in a
neighbouring parish--nearly all have a journey to perform, it may be
short or it may be long. The flocks of Finches gradually decrease and we
observe the males scattering in different directions in search of
territories; we watch the summer migrants on their way--small parties
halting for a few hours in the hedgerows and then continuing their
journey, single individuals alighting on trees and bushes and resting
there for a few minutes, and the constant passage of flocks of various
dimensions at various altitudes; and we see Fieldfares, Redwings, and
Bramblings slowly making their way from the south and the west to their
homes in the far north. Occasionally we hear their song, not the
emotional outburst customary at this season, but, except in isolated
cases, a weak and tentative performance. Gätke speaks of the absence of
song on the Island of Heligoland, and refers to the Whitethroat as one
of the few migrants that enliven that desolate rock with their melody.
On the other hand, many migrants that rest temporarily on the Isle of
May sing vigorously.[5] But on the whole there is, I think, no question
that the male whilst travelling to its breeding grounds, and, even after
its arrival, whilst in search of a territory, sings but little--and that
little lacks the persistency characteristic of the period of sexual
activity. Yet, when a suitable territory is eventually secured, the
nature of the bird seems to change; for, instead of being silent and
retiring, as if aware of some end not fully attained, it not only makes
itself conspicuous but advertises its presence by a song uttered with
such perseverance as to suggest that that end is at length attained.
Hence, in a general way, the instinct of song seems to be related to the
establishment of a territory.
Now the subsequent course of behaviour tends to confirm this view. We
have already had occasion to refer to the fact that the males of some
species desert their territories temporarily and join together on ground
which is regarded by the birds that associate there as neutral, and that
they do so not merely for the purpose of securing food but because they
derive some special pleasure from the act of association, and we shall
find that the altered behaviour of the male when it leaves its territory
to seek food or to join the flock is an important point for us just now.
Buntings desert their territories temporarily and collect in flocks on
the newly sown fields of grain. Some of the males are single, others are
paired, and accompanied, it may be, by their mates; they wander over the
ground in search of food, uttering their call-notes from time to time,
or, settling upon the hedges and trees surrounding the field, rest there
and preen their feathers. But even though a male may be surrounded by
other males, even though it may occupy a position where it is
conspicuous to all around, even though, that is to say, it is
apparently in contact with just those stimulating circumstances which
will evoke a response when it returns to its territory, yet it makes no
attempt to sing.
Lapwings, when they resort to the neutral ground, run this way and that
in full enjoyment of one another's companionship, behaving as they do
when they flock in autumn and winter. Specific emotional manifestation
is, however, absent, and their actions seem to be in nowise affected by
the powerful impulse which only a few minutes previously determined
their conduct, for of the characteristic flight with its accompanying
cry there is no sign.
Early in the season Turtle Doves often collect from the surrounding
country at certain spots where their favourite food is abundant. The
croak of this Dove--its true song--is a familiar sound during the
summer, but in addition the bird has a sexual note characteristic of the
race. I watched a flock of upwards of one hundred on some derelict
ground approximately eight acres in extent. Here, in May, the birds were
attracted by the seeds of _Stellaria media_ which was growing in
profusion. After 5 A.M. there was continuous traffic between this piece
of ground and the surrounding neighbourhood, a constant arrival and
departure of single individuals or pairs; and, as they fed, the sexual
note could be heard in all directions. Now some of the males occupied
territories close at hand, and one could watch their passage to and
fro; yet in no single instance did I hear the true song uttered on the
feeding ground, although the moment a male returned to its territory its
monotonous croak could be heard, uttered moreover with that persistence
which is so marked a feature of all song or of the sounds that
correspond to it.
Thus it will be seen that, even after the internal organic changes have
taken place, the instinct of song is not susceptible to stimulation at
all times and under all circumstances, but only at certain specified
times and under special circumstances which can be observed to
correspond with the occupation of the territory.
In many species each male singles out within its territory some
prominent position to which it resorts with growing frequency. This
position is an important feature of the territory, and exercises a
dominating influence on the life of the bird. I have referred to it as
the "headquarters," and it may be a solitary tree or bush, an
outstanding mound or mole hillock, a gatepost or a railing--anything in
fact that supplies a convenient resting place so long as it fulfils one
condition, namely that the bird when it is there is conspicuous. It need
not, however, be a tree or a mound or indeed anything upon which the
bird can perch, for there is reason to think that the soaring flight
undertaken at this season by so many males, since it is generally
accompanied by the specific sexual sound, answers the same purpose as
the topmost branch of a tree.
Now there is nothing in the external environment to which the song is
more definitely related than to the "headquarters"--this at least is the
conclusion to be drawn from the behaviour, and I will indicate the sort
of evidence upon which such conclusion is based. There is, first of all,
the persistency with which the male resorts to the same tree, even to
the same branch, and, as it seems, solely for the purpose of
advertisement. We know by experience the approximate routine of the
male's behaviour; we know where to seek it, where to hear it, and when
once we have discovered its headquarters, we know that there it will
sing day after day for weeks or it may be for months together--perhaps
the most striking feature of its behaviour at this season. Next, we find
that other trees, though made use of, are not made use of to a similar
extent for the purpose of song. The area occupied varies much according
to the nature of the environment; it is sometimes extensive, and seldom
less than half an acre or so in extent; but in most instances it
contains plenty of trees and bushes which could, one would imagine,
serve the purpose of a "headquarters" just as well as the particular one
selected, and yet the bird, when there, betrays no inclination to sing
at all comparable with that which can be observed when it occupies its
accustomed perch. Further evidence is afforded in the behaviour of those
species that make temporary excursions from their territories. The male,
on its return, flies as a rule direct to its special tree and sings.
Sometimes, however, it settles upon the ground, not unfrequently
accompanied by the female, and while there remains silent; but presently
rising from the ground and deserting its mate, it flies to the
headquarters and sings. Again, nearly every male at one time or another
in the course of the season is aroused to action by the intrusion of a
rival. The emotional tone of the owner of the territory is then raised,
and the intruder is pursued and attacked; but this alone is not
sufficient, it seems as if the chain of instinctive activities, when
once aroused by appropriate stimulation, must pursue its course to the
end--and the end in such a case is only reached and complete
satisfaction only gained when the bird has not merely returned to his
"headquarters" but has given vocal expression to his emotion. Finally,
we must bear in mind these two facts, that the "headquarters" is
occupied solely by the male--it forms no part of the life of the
female--and that it is the male only that sings.
Many such subtle incidents of behaviour as the foregoing can be
perceived but not readily described, and trifling though they may seem
to be in themselves, yet in the aggregate they yield full assurance of a
close relationship.
The distant song of a male, or the presence of an intruding male, have
also stimulating effects, though in somewhat different ways. The former
evokes the normal reply, that is to say the bird, if silent, is liable
to utter a corresponding reply; the latter arouses hostility into which
is infused much feeling tone, the bird sings hurriedly while in pursuit
of its rival, and, which is more remarkable still, even in the midst of
an encounter. Both the normal reply and the emotional song must be
similar in origin--different aspects of the same situation--and both are
clearly related to the other male.
The arrival of a female may also be followed by an emotional outburst
which can be heard at intervals for some days; on the other hand, the
song may continue as before or, for a time, entirely cease.
To take the emotional outburst first. This would appear to be
susceptible of explanation on the hypothesis that the voice contributes
to a more effective pairing situation; an hypothesis which admittedly,
at first sight, gains some support from the fact that a second or a
third male is frequently present. But, in truth, the presence of a
second male makes the situation, so far as the relationship between the
song and the female is concerned, all the more perplexing; for, as we
have already seen, the instinct of pugnacity, when aroused by the
appearance of an intruder, is also liable to be accompanied by a
similarly extravagant song. On each occasion the vocal effort is infused
with much feeling tone, and it would be impossible to point to any one
feature which is peculiar to only one occasion. The question therefore
arises as to whether the emotional outburst which we are attributing to
the arrival of a female may not after all be due to the presence of an
intruding male. It may be so. But although I can recall no single
instance in which the presence of an intruder could be definitely
excluded, yet I should hesitate to base upon this any broad
generalisation.
When the normal course of the song is not interrupted by the arrival of
a female, when, that is to say, the male still pursues the routine to
which he has all along been accustomed, and still sings at stated
intervals in stated places with a voice that betrays no heightened
emotional tone, even though the song may convey some meaning to the
delicate perceptual powers of the female, we have nothing to lay hold
upon which can be construed as an indication of direct relationship
between the song and the presence of the female.
The partial or complete suspension of the song after pairing has taken
place is the most interesting, as it is the most noticeable, feature.
Not that it is by any means universal--if it were so, some of the
difficulties that beset the path of interpretation would be removed, but
it is sufficiently widespread to demand explanation. In nearly every
case it is, however, only temporary, the period during which the male is
silent varying from a few days to a few weeks. The male
Grasshopper-Warbler, when it first reaches us, sings persistently, but
when it is joined by a female a change becomes apparent; instead of the
incessant trill, there are spasmodic outbursts of short duration, and in
the course of a few days the bird lapses into a silence which may be
broken for a short while at dawn, or late in the evening, but is often
complete. More striking still is the change in the case of the
Marsh-Warbler, and the sudden deterioration, or even suspension, of
strains so beautiful and so varied, at a moment, too, when it might
least be expected, at once arrests the attention. The Reed-Warbler that
had its headquarters in a willow sang vigorously from the middle of May
until a female arrived on the 20th June, when its voice was hushed,
except for occasional outbursts which lacked force and were of short
duration. When the Wood-Warbler secures a territory it repeats its
sibilant trill with unwearying zeal, yet no sooner does a mate appear
than its emotion is manifested in other directions. The Reed-Bunting is
vociferous during February and March; but when a female arrives, periods
of silence are frequent and the instinct of the bird becomes
progressively less susceptible to stimulation. After the manner of the
race the male makes temporary excursions from its territory accompanied
by his mate, and it is noteworthy that when he returns and she is absent
he sings, but that the moment she joins him, or even comes into sight,
he is silent. In fact, in greater or less degree, a change is noticeable
in the song of many resident and migratory species under similar
circumstances, a deterioration so marked that we learn by experience to
regard it as a certain indication of the arrival of a mate.
Thus it becomes clear that there are certain specific factors in the
external environment with which the instinct can be definitely related,
and in the order of their importance they are (1) the territory as a
whole; (2) the headquarters; (3) an intruding male; (4) the female.
To what extent are these relationships interrelated? Are they all
mutually dependent upon one another, or is there one which conditions
the remainder?
In the first place it is evident that if a male were not to establish a
territory, no opportunity would be afforded for making use of any
special post or for acquiring a habit in relation to it, and so without
further consideration we may say that the connection between the song
and the headquarters, whatever it may be, is primarily dependent upon
the establishment of a territory.
Next, we have the fact that the distant voice, or still more so the
presence, of another male has an exciting influence and evokes a
corresponding reply. Here we have a direct relationship, and one which
at first sight appears to be exclusive of cross-correlation. But is it
really so; does no circumstance arise under which even the proximity of
a rival fails to evoke response? The reply is not doubtful. Such a
circumstance _does_ arise--when a male for one reason or another passes
outside the limits of its accustomed area. This aspect of behaviour has
already been fully discussed in connection with the question of
hostility, and everyone, I imagine, must by now be pretty well familiar
with the facts. However, it does not often happen that we are given
such an aid to interpretation as is vouchsafed to us in the altered
behaviour of the male when it joins the flock, and if, as I believe,
song and hostility are intimately associated, forming part of an
inter-related whole which, for biological interpretation, has, as its
end, the attainment of reproduction, it is not surprising that
circumstances which lead to the modification of the one should likewise
affect the other; I offer no apology, therefore, for adverting to this
aspect of behaviour once again.
Now a male may leave its territory for three reasons--to pursue an
intruder, to join the flock on neutral ground, or to find the necessary
means of subsistence on other feeding grounds. On each of these
occasions it hears the song of, and is in close contact with, other
males; and if the relationship of which we are speaking be really
exclusive of cross-correlation, its instinct ought to respond with the
customary freedom. But what happens? A male pursues its rival, betraying
much emotion and singing extravagantly, until the boundary is passed,
when emotion subsides and it is silent; or, it flies to the flock on
neutral ground, and, although surrounded by the very males that a short
time previously evoked response, is there unresponsive; or again, it
goes in search of food and collects with other males bent on a similar
errand, and in presence of what we know would be an exciting influence
under other circumstances, it nevertheless remains silent. Hence the
relationship between the song and a male rival seems, as in the case of
the headquarters, to depend in the first instance upon the occupation
of a territory.
So that the relationship between the song and the territory as a whole
is clearly of a different order from that which obtains between the song
and the headquarters, or the song and a male rival; for the first, as
far as can be judged by observation, is exclusive of, whilst the second
and the third involve, cross-correlation. How are these facts to be
explained? We have already seen that it belongs to the nature of the
male during the season of reproduction to establish itself in a definite
place, and this action is just as much a part of its hereditary nature
as the building of the nest is of that of the female, and it is just as
necessary for the successful attainment of reproduction. What exactly
the stimulus is to this mode of behaviour we do not know; we can go no
further back than the internal organic changes which are known to occur
and which we assume, not without some reason, are responsible for its
initiation. Granting, then, that there is this congenital disposition,
what relation does it bear to the song? Without a doubt the song is
likewise founded upon a congenital basis; it is truly instinctive, and
as such requires appropriate stimulation; furthermore the male sings
only when in occupation of its territory. Having regard to these two
facts we might say that the territory is the stimulus to the song. But
this can scarcely be a true interpretation, for inasmuch as the stimulus
would be relatively constant, a relatively constant response ought to
follow, and even a slight acquaintance with the daily round of
behaviour will furnish plenty of evidence to the contrary, seeing that
the song, though persistent, is never continuous--in fact there are long
periods of silence during the daytime, and only in the morning and the
evening does the male become really vociferous. What then is the
stimulus? Through awareness of something in the environment the male
responds to stimulation, and the only reply we can give is that the
headquarters, or a distant song, or the proximity of another male--with
all of which, as we have seen, the instinct is definitely related--are
the specific factors which normally evoke response--and experience
teaches us that the periods of quiescence are just those when life is at
its lowest ebb and these stimulating factors less in evidence. Bearing
this in mind, bearing in mind the fact that when a male joins the flock
or crosses the boundary its instinct ceases to respond, bearing in mind,
that is to say, that there is evidence of relationship between these
specific factors and the song only when the territory is actually
occupied, the conclusion seems inevitable that we have here the
determining condition which renders the instinct susceptible to
appropriate stimulation.
There remains the female. I place her last in order of importance, not
because I regard her influence as of small consequence, but because the
evidence is of a varied and complex kind, so much so that it is
difficult to ascertain by observation just how far she is a situational
item. It will be remembered that the only direct evidence we had of such
influence was a deterioration or, in some instances, a complete
cessation of vocal manifestation. Clearly then we are confronted with a
relationship of a different kind from that which we have been
discussing; for not only is anything in the nature of stimulation
absent, but, and this is a remarkable fact, the other items in the
environment which formerly evoked response no longer do so in quite the
same way. Is there any awareness on the part of the male of the relation
between his voice and the mate that is to be, or is it merely that as
the sexual situation increases in complexity some inhibiting influence
comes into play? These are questions which lead up to difficult
problems. But it is no part of my task to discuss the psychological
aspect of the behaviour; my purpose is merely to show that the situation
on the arrival of a female undergoes marked modification, that the
instinct of the male is then less susceptible to stimulation, and that
the factors in the external environment which formerly elicited response
become relatively neutral.
Hence the appearance of the female on the scene marks the opening of a
new stage in the life-history of the male, and, to judge by the course
of events, it would seem as if the song with its network of
relationships had now served its main biological purpose.
And now, what is the purpose, and what the origin, of song? Is it, as
some naturalists have conceived, a means of raising the emotional tone
of the female, of creating a more effective pairing situation, and so
of removing a barrier to the successful discharge of the sexual
function; or, is the emphasis here too much upon the emotional, too
little upon the strictly utilitarian, aspect? All, I think, will agree
that it must serve some biological purpose, and the position we have so
far reached is that the determining condition of its manifestation is
not merely the establishment, but the actual occupation of a territory,
and that there are no factors in the external environment which can
evoke response in the absence of such condition. This being so, the
further questions arise as to whether it contributes towards the
attainment of the end for which the whole territorial system has been
built up, and what precisely is the way in which it does so.
Everyone knows that in the spring the shyest of birds no longer practise
the art of concealment. The Curlew soars to a great height, and upon
outstretched wings hovers in the air whilst uttering its plaintive wail;
the cock Grouse, as if dissatisfied with its "crowing," springs into the
air and becomes a conspicuous object of the moor; the wary Redshank,
poised on flickering wings, forgets its mournful alarm cry, and finds
again its melodious song; and even the secretive Grasshopper-Warbler
crawls out of the midst of the thicket in order to "reel," just as, for
a similar reason, Savi's Warbler climbs to the top of a tall reed. In
fact the males of most species, when they are finally established on the
breeding grounds, make themselves as conspicuous as possible by sight
and by sound. And since the sounds produced by no two species are
exactly alike, the females are able to recognise their prospective
mates, and the males that are still in search of ground have ample
warning if that upon which they are treading is already occupied. So
that you see, from the remarkable development of the vocal powers in the
male, there follow two important results--"recognition" and "warning."
We here turn from song as the expression of an instinctive disposition,
and the question of what calls forth this expression, to the impression
produced by the song on the hearer.
Most birds have a call-note or a number of call-notes, which, generally
speaking, are specifically distinct. But to the human ear they are not
always so, perhaps because our power of hearing is less sensitive than
that of a bird, and unable to appreciate delicate differences of tone.
Be this as it may, however, the fact remains that we often find it
difficult, and in not a few cases impossible, to recognise a bird merely
by its call. The plaintive notes of the Willow-Warbler and of the
Chiffchaff are to our ears very closely akin, so, too, are those of the
Marsh-Warbler and of the Reed-Warbler, and there is a great resemblance
between the hissing sound produced by the two Whitethroats. In Co.
Donegal I have been deceived by the spring-call of the Chaffinch which,
owing possibly to the humidity of the atmosphere, is, there, almost
indistinguishable from the corresponding note of the Greenfinch. The
Yellow Bunting and the Cirl Bunting frequently make use of a similar
note, so do the Curlew and the Whimbrel. In fact, numberless instances
could be quoted in which notes appear to us identical, and, as a rule,
the more closely related the species, the more difficult it becomes to
distinguish the sounds--alike in plumage, alike in behaviour, alike in
emotional manifestation, it would be surprising if they were not alike
in voice. But the moment we pass from the call-notes to a consideration
of the songs we are faced with a very remarkable fact, for not only are
these readily distinguished, but in many cases they bear no resemblance
in any single characteristic. What could be more unlike than the songs
of the Willow-Warbler and of the Chiffchaff, of the Marsh-Warbler and
the Reed-Warbler, or of the Yellow Bunting and the Cirl Bunting?
Now when different individuals collect in flocks at certain seasons,
they assist one another in finding food, and afford mutual protection by
giving timely warning of the approach of a common enemy, and the
gregarious instinct is thus of great advantage to the species; but no
matter how powerful the impulse to flock might be, if there were no
adequate means of communication, the different units would frequently
fail to discover their neighbours. Here the specific cries and calls
come into play, enabling them as they move about in search of food, or
change their feeding grounds, or whilst they are on migration, to keep
constantly in touch with one another; and hence one purpose that these
call-notes serve is that of recognition. Moreover, they convey their
meaning to individuals of other species and are acted upon, and are thus
in every sense socially serviceable; but on the other hand, whilst
there is much evidence to show that the song is of great individual
value, there is none to show that it is in any like manner of direct
advantage to the community.
If, then, there is in the call-notes an adequate means of communication
and of recognition, why do I suggest that the song has also been evolved
primarily for the purpose of recognition?
What, first of all, are the conditions in the life behaviour during the
season of reproduction that make the intervention of the voice a
consideration of such importance? The general result of our
investigation might be summed up thus: we found that the male inherits a
disposition to secure a territory, that at the proper season this
disposition comes into functional activity and leads to its
establishment in a definite place, and that it cannot search for a mate
because its freedom of action in this respect is forbidden by law; that
the female inherits no such disposition, that she is free to move from
place to place, free to satisfy her predominant inclination, and to seek
a mate where she wills; and, since the appropriate organic condition
which leads to pairing must coincide with appropriate conditions in the
environment, that the union of the sexes must be accomplished without
undue delay. Furthermore we found that a territory is essential if the
offspring are to be successfully reared; that, since the available
breeding ground is limited, competition for it is severe, and that the
male is precluded from leaving the ground which he has selected, and is
obliged, in order to secure a mate, to make himself conspicuous. That
was our general result. Now there are two ways by which the male can
make himself conspicuous--by occupying such a position that he can be
readily seen, or by producing some special sound which will be audible
to the female and direct her to the spot. The former, by itself, is
insufficient; in the dim light of the early dawn, when life is at its
highest, and mating proceeds apace, what aid would it be for a male to
perch on the topmost branch of a tree, how slender a guide in the depth
of the forest? But whether in the twilight or in the dark, in the
thicket or the jungle, on the mountain or on the moor, the voice can
always be heard--and the voice is the principal medium through which the
sexes are brought into contact.
Well now, we come back to the question, why, if all species have a
serviceable recognition call, that call should not be sufficient for the
purpose, just as, without a doubt, it is adequate for all purposes at
other seasons? The answer is, I think, clear. The recognition call is
not confined to one sex, nor only to breeding birds; it is the common
property of all the individuals of the species, and if the female were
to rely upon it as a guide she might at one moment pursue another
female, at another a non-breeding male; she might even be guided to a
paired female or to a paired male, and time would be wasted and much
confusion arise. So that no matter how much a male might advertise
himself by cries and calls which were common alike to all the
individuals of the species, it would not assist the biological end which
we have in view. Something else is therefore required to meet the
peculiar circumstances, some special sound bearing a definite meaning by
which the female can recognise, amongst the host of individuals of no
consequence to her, just those particular males in a position to breed
and ready to receive mates. Hence the vocal powers, the power of
producing sounds instrumentally, and the power of flight, have been
organised to subserve the biological end of "recognition."
And this view is strengthened, it seems to me, by the erratic behaviour
of certain species, more particularly by one remarkable case, the case
of the Cuckoo. The male, after having established himself, utters his
call persistently from the day of arrival until approximately the middle
of June; but, in contrast with the large majority of species, the female
has a characteristic call which she, too, utters at frequent intervals.
The female is polyandrous and has a sphere of influence embracing the
territories of a number of males; she wanders from place to place, is
often silent, and not unfrequently is engaged in dealing with her egg or
in searching for a nest in which to deposit it, and therefore she is not
always in touch with a male, still less with any particular one. Now
there is much evidence to show that the discharge of the sexual function
amongst birds is subject to control, and that this control operates
through the female--through her physiological state becoming
susceptible to stimulation only at certain periods. So that we have
these considerations, that the female is polyandrous, that she has a
territory distinct from that of the male, and that her sexual impulse is
periodical; and the further consideration that the impulse, since it is
periodical, is of limited duration and must receive immediate
satisfaction. Such being the circumstances of the case, would the voice
of the male serve to insure the union of the sexes at the appropriate
moment? Well, the fact that she is polyandrous implies that every male
in her sphere of influence is not always capable of satisfying her
sexual instinct. Is, then, the male's call an indication of his
readiness to yield to stimulation? Without a doubt it is an index of the
general physiological state which generates the sexual impulse, without
a doubt it denotes a general preparedness to breed, but there is no
evidence to show that it denotes the degree of ardour of the male at any
particular moment, and much that proves the contrary. So that only by
the female producing some special sound which will attract the males
that are eager and bring them rapidly to the spot where she happens to
be, only thus is it possible to insure the consummation of the sexual
act. This, it seems to me, is the purpose of the peculiar call of the
female--a call which, so far as biological interpretation is concerned,
is just as much a song as the melody of the Marsh-Warbler--and its
interest for us just now lies in this, that here we have a special case
in which the sexes have separate territories, the female is polyandrous,
and the voice of the male is not sufficient by itself to bring to pass
the union of the sexes; and in which, consequently, if the purpose of
song be that of recognition, we should expect to find, as we do find,
that the female had a distinct and penetrating call.
We now come to the question of "warning," by no means the least
important purpose of song. I pointed out that one of the chief
differences between the call-notes and the song was that the former were
socially serviceable, whereas the latter was only serviceable to certain
individuals; and in making this statement, I had in mind the direct
benefits to the community which proceeded from an appreciation of sounds
having a mutually beneficial meaning, not the indirect, though none the
less beneficial, consequences to the species as a whole. Biologically
considered, song, if it acts as a warning and thereby leads in one way
or another to more complete success in the rearing of offspring, may be
spoken of as socially serviceable; but it is legitimate to draw a
distinction between the prospective value of remote relationships which
we can foresee, and the mutual assistance which the individuals of a
community derive from their close association.
If there were always sufficient breeding ground to support the offspring
of all the individuals of each species, if the individuals were always
so distributed that there was no possibility of overcrowding in any
particular area, and if the conditions of existence of different
species were so widely divergent that the presence of this one in no way
affected the interests of that, no opportunity would be afforded for the
development of so complex a system as is involved in the "territory" and
all that appertains to it. But the available breeding ground is by no
means unlimited. The supply of food, which is a determining factor in
the environment, is always fluctuating according to the climate and
according to the changes in the earth's surface; and so the distribution
of the bird population in any given area, though it may be suitably
adjusted for one year or even for a period of years, is bound in the
course of time to require readjustment. Now there cannot be readjustment
without competition, nor competition without combat. But the appeal to
physical force is only a means to an end, and, since no male can endure
incessant warfare and the perpetual strain of always being on the alert,
without experiencing such physical exhaustion as might affect his power
of reproduction, its direct effect upon the combatants cannot be
otherwise than harmful--in fact it is a necessary evil which for the
good of the species must be kept strictly within bounds. Bearing in
mind, then, these two facts, namely that the distribution of the males
is never stable and that overmuch fighting may defeat the end in view,
we can appreciate the importance of any factor which will lead to a more
uniform distribution and at the same time insure security by peaceable
means.
The proximate end of the male's behaviour is isolation--how is it to be
obtained? If, after having occupied a territory, the bird were to remain
silent, it would run the risk of being approached by rivals; if, on the
other hand, it were merely to utter the recognition call of the species,
it would but attract them. In neither case would the end in view be
furthered, and isolation would solely depend upon alertness and the
capacity to eject intruders. Supposing, however, that the song, just as
it serves to attract the females, serves to repel other males, a new
element is introduced deserving of recognition; for those males that had
established themselves would not only be spared the necessity of many a
conflict, but they would be spared also the necessity of constant
watchfulness, and so, being free to pursue their normal routine--to seek
food, to rest, and, if migrants, to recover from the fatigue of the
journey, they would be better fitted to withstand the strain of
reproduction; and those that were still seeking isolation in an
appropriate environment, instead of settling first here and then there
only to find themselves forestalled, would avoid and pass by positions
that were occupied, establishing themselves without loss of time in
those that were vacant. Without the aid of something beyond mere
physical encounter to regulate dispersal, it is difficult to imagine how
in the short time at disposal anything approaching uniformity of
distribution could be obtained. Hence, both in the direction of limiting
combat, of insuring accommodation for the maximum number of pairs in the
minimum area, and of conserving energy, the song, by conveying a
warning, plays an important part in the whole scheme.
And if this be so, if the song repels instead of attracting, it follows
that the more distinct the sounds, the less likelihood will there be of
confusion; for supposing that different species were to develop similar
songs, whole areas might be left without their complement of pairs just
because this male mistook the voice of that, and avoided it when there
was no necessity for doing so. So that just as from the point of view of
"recognition" each female must be able to distinguish the voice of its
own kind, so likewise the warning can only be adequate providing that
the sounds are specifically distinct. A point, however, arises here in
regard to closely related forms. Some species require similar food and
live under similar conditions of existence; they meet in competition and
fight with one another; and, if they did not do so, the food-supply of a
given area would be inadequate to support the offspring of all the pairs
inhabiting that area. Generally speaking, the more closely related the
forms happen to be, the more severe the competition tends to become; and
it may be argued that in such cases a similar song would contribute to
more effective distribution and in some measure provide against the
necessity of physical encounter; that, in fact, it would stand in like
relation to the success of all the individuals concerned, as does the
song to the individuals of the same species. But we must bear in mind
that the primary purpose of song is to direct the females to those males
that are in a position to breed; and to risk the possibility of prompt
recognition in order that the males of closely related species should
fight the less, would be to sacrifice that which is indispensable for a
more remote and less important advantage.
What meaning does the song convey to a male that is unestablished? Does
the bird recognise that it is forestalled; does it foresee and fear the
possibility of a conflict, and conclude that the attempt to settle is
not worth while? I do not imagine that it thinks about it at all. How
then does the warning warn? We will endeavour to answer this question,
but, in order to do so, we must review the stages by which a territory
is secured.
We take as our starting point the internal organic changes which are
known to occur. These changes are correlated with other changes,
manifested by a conspicuous alteration in behaviour--to wit, the
disappearance of sociability and its replacement by isolation. Having
found a station which meets the requirements of its racial
characteristics, the male establishes itself for a season, becomes
vociferous, displays hostility towards others of its kind, and in due
course is discovered by a female. The whole is thus an inter-related
whole, a chain of activities which follow one another in ordered
sequence. Now we have seen that it is neither pugnacious nor vociferous
until the territory is actually occupied; we have seen that the fact of
occupation is the condition under which the instincts of pugnacity and
of song are rendered susceptible to appropriate stimulation; we have
discussed the nature of the stimulus in each case, and we wish to know
the sort of meaning that the song conveys to an individual which is
still in the preliminary stage of seeking a station. In sequential order
we have the following: (1) internal organic changes which lead to
isolation, (2) the appropriate environment which gives rise to an
impulse to remain in it, (3) the occupation of a territory which is the
condition under which the instincts are rendered susceptible to
stimulation, (4) the various stimuli. Each is dependent upon that which
precedes it, and no part can be subtracted without failure of the
biological end in view, neither can the different stages be combined in
different order. So that, in considering the significance of song to an
unestablished male, we are dealing with the situation at a point at
which all the latent activities have not been fully felt, for all that
so far has occurred is the change from sociability to isolation
determined by internal organic changes. The bird has not established a
territory because it has not come into contact with the appropriate
environment, and it is not pugnacious because the condition which
renders its instinct susceptible is absent; and so, as it wanders from
place to place and hears the voices of males here or males there, it
merely behaves in accordance with that part of its nature which
predominates just at that particular moment--the impulse to avoid them.
But given the appropriate environment, given, that is to say, just that
combination of circumstances which might bring into functional activity
all the latent instincts of the intruder, and no matter how vociferous
the occupant of a territory might be, it would not be preserved from
molestation. The advantage of the song, biologically considered, is then
this, that it will often prove just sufficient to preclude males in
search of isolation from coming into contact with the environmental
conditions adequate to supply the stimulus to their latent activities
and to convert them into rivals.
If this interpretation be correct, if we are right in attributing the
withdrawal solely to the fact that the first stage only in the
relational series has been reached, it follows that the effect of song
upon males that have reached subsequent stages in that series must be of
a very different kind. We have dealt with the male when in the
preliminary stage of seeking isolation, we must deal with it now when
eventually it occupies a territory. How does it behave when it hears, as
it is bound to do, the voices of rivals in its neighbourhood? You may
remember that some allusion was made to the fact that an outburst of
song from one individual was followed, not unfrequently, by a similar
outburst on the part of other individuals in the immediate locality. For
example, silence may reign in the reed-bed except for an occasional note
of the Reed-Warbler or Sedge-Warbler. Suddenly, however, a dispute
arises between two individuals, accompanied by a violent outburst of
song, and forthwith other males in the vicinity begin to sing excitedly
and continue doing so for some minutes in a strangely vigorous manner,
the tumult of voices affording a striking contrast to the previous
silence. Spasmodic outbursts of this kind, stimulated by an isolated
utterance, are by no means uncommon. But not only does song stimulate
song; under certain conditions it has the still more remarkable effect
of arousing hostility. The boundary that separates two adjoining
territories is by no means a definite line, but rather a fluid area
wandered over by this owner at one moment, by that at another. Now so
long as the bird is silent while in this area, the probability is that
it will escape detection and remain unmolested; let it however sing--it
often does so--and it will not merely be approached but attacked, and
consequently this area is the scene of much strife. The point to be
noticed here is that the song brings about no withdrawal; it elicits a
response, attracts instead of repelling, and, in short, arouses the
impulse that is always predominant in the nature of the male when
eventually it occupies a territory--the impulse of self-assertiveness.
Therefore it seems clear that the different stages in the process of
reproduction mark the appearance of different conditions, each of which
renders some new impulse susceptible to stimulation, and that the
significance of song depends upon the stage which happens to have been
reached. Hence when we speak of song acting as a "warning," we do not
mean that it arouses any sensation of fear; it is but a stimulus to
that part of the inherited nature of the hearer which predominates at
the moment.
Are we then justified in the use of such terms as "warning,"
"significance," or even "meaning," when it is but a matter of stimulus
and response? In what does the impulse to avoid other males consist?
There is no reason to suppose that there is any sensation of fear in the
first stage, and the course of behaviour demonstrates that there is none
in the later stages. But it is difficult to conceive of an impulse which
has, as its end, the isolation of the individual from members of its own
sex and kind, without some feeling-tone, the reverse of pleasurable,
entering into the situation; just as it is difficult to believe that the
female experiences no pleasurable sensation when she hears the voice of
the male that directs her search. So that the song may be actually
repellent in the one case and attractive in the other; and it is none
the less repellent when, as in the later stages, it attracts a
neighbouring male, for the attraction is then of a different order,
determined by the presence of the condition which renders the pugnacious
nature susceptible and leads to attack. In a sense, therefore, we can
speak of "meaning"--though not perhaps of "significance"--and of
"warning," when we refer to the prospective value of the behaviour.
So much for the purpose of "song"; there still remains the more
difficult question--the question of origin. Let me make clear what I
mean by origin. As we have already seen, there is infinite diversity in
the sexual voice of different species; some are harsh and others
monotonous, and some strike the imagination by their novelty whilst
others are melodious; and to the naturalist each, in its particular way
and in a particular degree, probably makes some appeal according to the
associations that it arouses. But just why a Marsh-Warbler is gifted
with a voice that is so beautiful and varied, whilst the
Grasshopper-Warbler must perforce remain content with a monotonous
trill; just why the tail feathers of the Snipe have developed into an
instrument, whilst the Pied Woodpecker has developed muscles which
enable it to make use of a decayed branch as an instrument--we know no
more than we do of the nature of the forces which lead the Reed-Warbler
to weave its nest to reeds, or the caterpillar of the Elephant Hawk Moth
to assume so peculiar an attitude when disturbed. When therefore I speak
of the origin, I do not refer to the mode of origin of variation; I take
for granted that variations somehow arise, and I seek to ascertain
whether there is anything in the phenomena which we have explored which
might reasonably be held to determine the survival of this one in
preference to that.
When we reflect upon the problem of song and consider the numerous and
diverse forms in which it is manifested, we are apt to draw a
comparison between the sounds we hear and those produced by musical
instruments, and hence to conclude that each bird is gifted with a
special instrument in virtue of which it produces its characteristic
melody. But there is a very remarkable phenomenon connected with the
singing of birds which shows that this is really not the case--I mean
the phenomenon of imitation. There are plenty of good imitators amongst
our native species, and the power of imitation is not the exclusive
property of those which have reached a high degree of vocal development,
nor, for the matter of that, of song-birds at all. Even the Jay, than
which few birds have a more raucous voice, that "hoots" like the
Wood-Owl, or copies the sounds produced by the tail feathers of the
Snipe, will occasionally imitate the most melodious strains of some
other species; and the Red-backed Shrike, whose sexual call is
principally a few harsh notes rapidly repeated, bursts at times into
perfect imitations of the song of the Swallow, Linnet, or Chaffinch.
Nevertheless it is amongst such typical songsters as the Warblers that
we find the greatest volume of imitation, and no limit seems to be
placed upon their capacity. The Marsh-Warbler can utter the call of the
Green Woodpecker, or sing as the Nightingale does, with as much facility
as it sings its own song; and the Blackcap is well-nigh as proficient in
copying the cries and melodies of surrounding species--and so, if it
were necessary, we might proceed to add to the list.
These examples demonstrate that different songs are not represented by
a corresponding number of different physiological contrivances; for if
the difference were really attributable to some structural peculiarity,
then the range of sounds embraced in the call-notes and the sexual call
of any given species, must be the measure of the capacity of its
instrument; and no matter how great its power of imitation may be, it
follows that it will only be capable of copying those sounds which fall
within that range. There is plenty of evidence to show that the power of
imitation is almost unlimited, at all events that it is not confined
within such narrow limits as are here demanded. Hence it seems clear
that the diversity of song is not to be sought in structure, but in some
innate capacity to play one tune in preference to another; and if this
be so, and if out of the same instrument, which has been primarily
evolved to further the biological end of intercommunication, all manner
of diverse sounds can be made to proceed, the problem of the origin of
song is to that extent simplified.
We must next inquire into the nature of song, and endeavour to ascertain
whether all the individuals of a species are alike proficient, or,
failing this, whether there is any quality which can be observed to be
constant under all conditions. I watch the Reed-Buntings in a marsh and
find that there are three males occupying adjoining territories. Two of
them are fully mature and their plumage is bright: that is to say the
crown is black, the collar and breast are white, the flanks are dull
white spotted with black, and the mantle is reddish-brown. The third is
immature: the crown, instead of being black, is suffused with brown; the
collar, instead of being white, is mottled with brown; and the flanks
are more heavily streaked with brown. These three birds take up their
positions in February, and, as is their wont, sing incessantly each day
at daybreak. The song of the first two is normal, including the usual
number of phrases which flow in no definite sequence, but are combined
and recombined in different order, and the tone is pure; that of the
third, the immature bird, is, however, very different; for just as in
comparison its plumage is dull, so the phrases of its song are limited
and reiterated with great monotony, the tone is impure, and the whole
performance is dull and to our ears unmusical. I watch them from
February to June, and observe the order in which they are mated--first a
mature male; next, after a short interval, the immature male; and
finally, after a still longer interval, the remaining bird gets a mate.
As the season advances, still keeping watch on the development of the
plumage and of the voice of the immature male, I observe that no very
definite change takes place--that the colours remain dull, that there is
a conspicuous absence in the song of certain phrases, and that the notes
lack purity of tone.
If now, instead of Reed-Buntings in a marsh, I watch Yellow Buntings on
a furze-covered common, I find that, establishing themselves early in
February, they sing persistently, and in a few weeks are paired. But
what arrests my attention more particularly is the quality of the song;
for although the voice is unmistakably the voice of the Yellow Bunting,
yet it is incomplete and lacks the variety of phrases and musical
notation which we customarily associate with the bird. Nevertheless, as
the season advances, there is a progressive development in both these
directions, and by the end of March or the beginning of April the song
possesses all those qualities which appeal to us so forcibly.
There is one other fact to which attention must be drawn--the variation
in the song of the same species in different districts. As an
illustration let us take the case of the Chaffinch. In Worcestershire
the bird sings what I imagine to be a normal song--the notes are clear
and the phrases are distinct and combined in numerous ways. With the
notes fresh in mind I leave them and go to the west of Donegal, where I
am at once conscious of a difference; not a subtle difference that
perplexes the mind and is difficult to trace, but a change so remarkable
that one is conscious of a passing doubt as to whether after all the
voice is the voice of the Chaffinch; the song is pitched in a lower key,
certain phrases are absent, the notes lack tone and are sometimes even
harsh, and the bird seems wholly incapable of reaching the higher notes
to which I am accustomed.
Now the immature Reed-Bunting, though to our ears its song is but a poor
representation of that of the adult, gains a mate; the Yellow Bunting
pairs, and the discharge of the sexual function may even have taken
place before its voice attains what we judge to be its full development;
and there are no grounds for supposing that the Donegal Chaffinch, with
its less musical notes, has on that account any the less chance of
procreating its kind--facts which demonstrate that the biological value
of song is neither to be sought in the purity of tone, nor in the
variety and combination of phrases, nor, indeed, in any of those
qualities by which the human voice gains or loses merit, and which leave
us with no alternative but to dismiss from our minds all æsthetic
considerations in the attempt to estimate its true significance.
What, then, determines its value? Are there any qualities which, whether
the bird is mature or immature, whether it is untrained or has acquired
fuller expression by practice, whether it inhabits this district or
that, are alike constant? Well, no matter how great the variation, no
matter how much this voice falls below or exceeds the standard, judged
from the human standpoint, attained by that, even we, with our duller
perception, have no difficulty in recognising the species to which the
owner of the voice belongs; in other words, the song is always specific,
and this is the most noticeable, as it is the most remarkable,
characteristic.
There is still, however, another quality to which I would draw
attention--that of loudness. The sounds produced are on the whole alike
penetrative, and the individuals of any given district, even though the
climate by affecting their vocal muscles may have modified the character
of the song, are at no disadvantage in this respect; neither are the
females on the same account the less likely to hear the undeveloped
voice of the immature male.
We have then the following considerations: firstly, there is the
widespread and remarkable phenomenon of imitation, from which we can
infer that the diversity of song is not due to structural differences
but must be sought in some innate capacity to play one tune in
preference to another; secondly, not all the individuals of the same
species play a similar tune--we find that there is in certain directions
a noticeable variation which nevertheless does not seem to affect the
question of success or failure in the attainment of reproduction; in the
third place, in contrast with this variation, we can observe a striking
uniformity in two important particulars, namely in the specific
character and penetrative power of the song--qualities which we know are
essential for the purposes of "recognition" and "warning"; and finally,
from the general course of our investigation, we can infer that if a
male had no certain means of advertising its position, the territory
would not be brought into useful relation in its life. Have we here
sufficient ground on which to construct a theory of origin; in other
words, has the evolution of song been incidental to, and contributory
to, the evolution of the territory?
We have all along spoken of the song and of the call-notes as if they
were manifestations of separate emotional states having their respective
and well-defined spheres of usefulness; and while, speaking generally,
this is a true statement of the case, there is much evidence to show
that the relationship between them is nevertheless very close. There
are, for example, quite a number of cases in which a particular
call-note is uttered with unusual energy during sexual emotion, and is
attached to the song, of which it may be said to form a part; but a
still closer connection can be traced in many simple melodies which are
merely compositions of social and family calls repeated many times in
succession, and even in some of the more complex productions there will
be found indications of a similar construction. And since this is so,
since moreover, in the seasonal vocal development of such a bird as the
Yellow Bunting, we can observe the gradual elaboration from simple to
complex--from the repetition of single notes to phrases and from phrases
to the complete melody--we have every reason to suppose that it is along
these lines that the evolution of the voice has proceeded.
In all probability there was a time when vocal expression was limited
to primitive social and family cries which would be called into play
with special force during times of excitement, more particularly during
the sexual season which is the period of maximum emotional excitement.
But the excitement would express itself in all the congenital modes of
behaviour peculiar to the season, and thus the repetition of these cries
would become associated with combat, with extravagant feats of flight,
and with other forms of motor response. Now the more emotional
individuals would be the more pugnacious, and all the more likely
therefore to secure territory and so to procreate their kind; and, being
of an excitable disposition, they would at the same time be the more
vociferous. Hence variations of the hereditary tendency to vocal
expression, even though in themselves they were not of survival value,
would be fostered and preserved, so long as they were not harmful, in
virtue of their association with pugnacity. But if, instead of being
neutral, they helped to further the biological end of combat, the
relationship between the voice and pugnacity would be of a mutually
beneficial kind; and those individuals in which variation in both
directions happened to coincide, would have a better chance of success
in the attainment of reproduction.
A territorial system, closely corresponding to that which we have
discussed, forms part of the life behaviour of certain mammals, and of
its existence much lower in the scale of life evidence is not wanting;
from which we can infer that it is not of recent origin, but that the
conditions in the external environment demanded such a system at a
remote period of avian development. Now even in its incipient stages the
system must have involved a separation of the sexes, and howsoever
slight the degree of separation may have been in comparison with that
which can be observed to-day, inasmuch as the power of locomotion was
then less highly developed, mating could only have proceeded
satisfactorily providing that males fit to breed had some adequate means
of disclosing their positions. Thus there is reason to think that from
the very commencement of the process variations of emotional disposition
expressed through the voice would have been of survival value.
But expressed in what direction, in loudness and persistency of
utterance, these are the qualities which, I imagine, would have been
more likely to have facilitated the search of the female? Yet if she
were uncertain as to the owner of the voice, neither loudness nor
persistent repetition would avail much; and as species multiplied and
the competition for the means of living became increasingly severe, so
the necessity of a territory would have become intensified, and so, too,
with the extension of range, would the separation of the sexes have been
an ever-widening one; and as with their multiplication, irregularities
and delays in mating, arising from the similarity of the calls, would
have increased in frequency, so a distinctive call, which would have
tended to minimise these risks, would have come to possess biological
value.
Here we have a theory of origin, but origin of what? Of certain
characteristics of song--nothing more; and therefore to suppose that it
furnishes a complete explanation, which satisfies all the requirements
of scientific logic, of so wonderful an intonation as that, for example,
of the Marsh-Warbler, or that no other relationships, except that of the
territory, enter into the total emotional complex, simplifying here or
elaborating there to meet the exigencies of diverse circumstances--to
suppose this would be foolish. That there are many relationships which
even to-day are leading to modifications in important particulars, but
which at the present time are beyond our cognisance, of this there can
be no doubt.
There is one process by which song may have attained a fuller
development, and which would account in some measure for the
elaboration, inexplicable merely in terms of "recognition." It is this:
the effect of the sexual call upon the female cannot well be neutral, it
must be either pleasurable or the reverse--it must, that is to say, be
accompanied by some suggestiveness, and by suggestion I mean the
arousing of some emotion akin to that of the male; and if there are
degrees of suggestiveness, which well there may be, some males will mate
sooner than others and some will remain mateless--this is the theory of
sexual selection. The question to be decided here is whether the
biological emphasis is on loudness, or specific distinctness, or pitch,
or modulation, or the manner in which the phrases are combined--that is,
on some qualities in preference to others--or whether the emphasis is on
the whole. We have already seen, and it is well known, that there is
much variation in the voices of different individuals of the same
species, and thus the first condition of the theory is fulfilled. Now
the conditions which lead to variation are threefold--immaturity,
seasonal sexual development, and isolation. Of the three, the variation
in the case of the immature bird is the most instructive; the tone is
not so pure, the combination of phrases is incomplete, and elaboration
is imperfect, and yet, notwithstanding all these imperfections, we can
observe that the bird pairs as readily as does the adult. But even if we
lacked this demonstrative evidence, we should still be justified in
assuming that such must be the case, for we know from experience in the
preservation of game, where there is no surer way of reducing the stock
than by leaving too high a percentage of old cocks, that for the young
bird to be at any disadvantage in competition with the adult is
detrimental, if not disastrous, to the species. So that while there is
plenty of evidence of variation in those particular qualities which
appeal to our æsthetic faculties, there is at the same time evidence
which demonstrates that such variations exercise no influence on the
course of mating; and inasmuch as it is difficult to conceive of any
voice departing more from the normal type in these particular qualities
than the immature does from the adult, if there be degrees of
suggestive influence, we must seek it in some other direction. There
remain the two other characteristics which we found to be constant under
all circumstances, namely, loudness and specific distinctness; and if,
in addition to serving the purpose of disclosing the positions of the
males, they serve to evoke some emotion in the female, which helps to
further the biological end of mating, so much the more reason is there
for their survival.
There can be no question that this ingenious and attractive theory, if
it were true in its special application to song, would immensely
simplify interpretation, and moreover that preferential mating would
contribute not a little to the success of the whole territorial system.
No one can deny the strength of the argument: that the sexual instinct,
like all other instincts, must require a stimulus of an appropriate
kind; that the effect of the sexual call upon the female cannot be
neutral; and hence the probability that stimulation varies too; no one,
I say, can question the strength of this evidence, and, one might add,
of the evidence derived from the analogy of the human voice. But when we
have said this, we have said all; and our acceptance of the hypothesis,
so far as song is concerned, must remain provisional so long as the
evidence remains but secondary evidence.
CHAPTER V
THE RELATION OF THE TERRITORY TO THE SYSTEM OF REPRODUCTION
In the first two chapters I tried to show that the inherited nature of
the male leads it to remain in a definite place at a definite season and
to become intolerant of the approach of members of its own sex, and that
a result is thus attained which the word "territory" in some measure
describes. But the use of this word is nevertheless open to criticism,
for it denotes a human end upon which the highest faculties have been
brought to bear, and consequently we have to be on our guard lest our
conception of the "territory" should tend to soar upwards into regions
which require a level of mental development not attained by the bird. It
is necessary to bear this in mind now we have come to consider the
meaning of the territory, or rather the position that it occupies in the
whole scheme of reproduction.
Relationship to a territory within the interrelated whole of a bird's
life serves more than one purpose, and not always the same purpose in
the case of every species. We have only to glance at the life-histories
of divergent forms to see that the territory has been gradually adjusted
to suit their respective needs--limited in size here, expanded there, to
meet new conditions as they arose. Now some may think that the theory
would be more likely to be true if the territory had but one purpose to
fulfil, and that one the same for every species; and they may see
nothing but weakness in the multiplication of ways in which I shall
suggest it may be serviceable. But such an objection, if it were raised,
would arise from a mistaken conception, a conception which, instead of
starting with a relationship and working up to the "territory," sees in
the "territory" something of the bird's own selection and thence works
back to its origin. Holding the view that it is nothing but a term in a
complex relationship which has gradually become interwoven in the
history of the individual, I see no reason why the fact of its serving a
double or a treble purpose should not be a stronger argument for its
survival. I now propose to examine the various ways in which the
territory may have been of use in furthering the life of the individual,
and the circumstances in the inorganic world which have helped to
determine its survival.
The purpose that it serves depends largely upon the conditions in the
external environment--the climate, the supply of food, the supply of
breeding-stations, and the presence of enemies. Hence its purpose varies
with varying conditions of existence. But before we proceed to examine
the particular ways in which it has been modified to suit the needs of
particular classes of species, and the reason for such modifications, we
must inquire whether there is not some way in which it has been
serviceable alike to every species, or at least to a large majority of
them.
Success in the attainment of reproduction depends upon the successful
discharge of the sexual function; and the discharge of the sexual
function depends primarily upon an individual of one sex coming into
contact with one of the opposite sex at the appropriate season and when
its appropriate organic condition arises. Now the power of locomotion is
so highly developed in birds that it may seem unreasonable to suppose
that males and females would have any difficulty in meeting when their
inherited nature required that they should do so, still less reasonable
to suggest that this power might even act as a hindrance to successful
mating. Nevertheless, if we try to picture to ourselves the conditions
which would obtain if the movements of both sexes were in no wise
controlled, and mating were solely dependent upon fortuitous gatherings,
we shall come, I fancy, to no other conclusion than that much loss of
valuable time and needless waste of energy would often be incurred in
the search, and that many an individual would fail to breed just because
its wanderings took it into districts in which, at the time, there
happened to be too many of this sex or too few of that. And as the power
of locomotion increased and the distribution of the sexes became more
and more irregular, so the opportunity would be afforded for the
development of any variation which would have tended to facilitate the
process of pairing, and by so doing have conferred upon the individuals
possessing it, some slight advantage over their fellows.
What would have been the most likely direction for variation to have
taken? Any restriction upon the freedom of movement of both sexes would
only have added to the difficulties of mating; but if restriction had
been imposed upon one sex, whilst the other had been left free to
wander, some order would have been introduced into the process. That the
territory serves to restrict the movements of the males and to
distribute them uniformly throughout all suitable localities, there can
be no question; and since the instinctive behaviour in relation to it is
timed to appear at a very early stage in the seasonal sexual process,
the males are in a position to receive mates before the impulse to mate
begins to assert itself in the female.
We will take the Ruff as an example. According to Mr. Edmund Selous,
pairing, in this species, is promiscuous--the Ruffs are polygamous, the
Reeves polyandrous. Suppose, then, that upon this island of some few
miles in circumference, whereon his investigations were made, the
movements of neither Ruff nor Reeve were subject to control, that the
birds wandered in all directions, and that the union of the sexes were
fortuitous, would the result have been satisfactory? We must remember
that the Reeve requires more than one Ruff to satisfy her sexual
instinct; we must also bear in mind the possibility that the functioning
of her instinct may be subject to some periodicity, and we ask whether,
under these circumstances, accidental gatherings would meet all the
requirements of the situation. Now, manifestly, she must be in a
position to find males when her appropriate organic condition arises.
But in the absence of any system in the distribution of the sexes, how
could delay be avoided, or how could a uniform discharge of the sexual
function be assured? There is, however, a system. In the first place,
there are the assembly grounds to which the birds repair season after
season; and then, on the assembly grounds, there are the territories,
represented, as Mr. Selous tells us, by depressions where the grass by
long use has been worn away, and each depression is owned by one
particular Ruff. The assembly grounds have the effect of splitting up
and scattering the birds, and the number of Ruffs at any one particular
meeting place is limited by the territories; with the result that Ruffs
fit to breed are evenly distributed and always to be found in certain
definite places, and the Reeves know by experience where to find them.
The advantage of this territorial system is therefore apparent. Instead
of this district being overcrowded and that one deserted; instead of
there being too many of one sex here and too few of the other sex there;
instead of a high percentage of individuals failing to procreate their
kind, just because circumstances over which they have no control prevent
their discovering one another at the appropriate time--each sex has its
allotted part to play, each district has its allotted number of
inhabitants, and the waste of energy and the loss of time incurred in
the process of mating is reduced to a minimum.
Let us return again to the question of fortuitous mating, and consider
the position of a male and female that have discovered one another by
accident and have paired; what will be the subsequent course of their
behaviour? We are assuming, of course, that a territory forms no part of
their life-history. If the discharge of the sexual function takes place
immediately and the ovaries of the female are in an advanced state of
seasonal development, the construction of the nest will proceed without
delay--and the nest will answer the same purpose as the territory in so
far as it serves to restrict the movements of the birds and tends to
make them remain in, or return to, its vicinity; but if not, there will
be an interval during which both sexes will continue to wander as
before, guided only by the scarcity or abundance of food. In the first
case, there will be the attraction of the nest to prevent any untimely
separation; in the second, there will be nothing in the external
environment to induce them to remain in any particular spot. Now if we
turn to any common species and observe the sequence of events in the
life of different pairs, we shall find that pairing is seldom followed
by an immediate attempt to build; that an interval of inactivity is the
rule rather than the exception, and that this interval varies in
different species, in different individuals, and in different seasons.
Our imaginary male and female will therefore be faced with considerable
difficulty; for with nothing in the external environment to attract them
and with no restriction imposed upon the direction or extent of their
flight, their union will continue to be, as it began by being,
fortuitous. Next, let us consider their position were a disposition to
establish a territory to form part of the inherited nature of the male.
Each one will then be free to seek food when and where it wills and to
associate with other individuals without the risk of permanent
separation from its mate; and, no matter how long an interval may elapse
between mating and nest-building, each one will be in a position to find
the other when the appropriate moment for doing so arrives. Hence, while
preserving freedom of movement for each individual, the territory will
render their future, as a pair, secure.
No doubt the course of behaviour, as we observe it to-day in the lives
of many species, is the outcome of, rather than the condition which has
led to, the evolution of the territory. Thus, in many cases, we find
that early mating is the rule rather than the exception; we find that
the sexes frequently separate to seek their food, and fly away
temporarily in different directions; and, under exceptional climatic
conditions, we find that they even revert to their winter routine and
form flocks; only, however, to return to their territories, as pairs,
under more congenial conditions. Yellow Buntings, for example, pair
comparatively early in the season--some in the latter part of February,
others in March, and others again in April; and some build their nests
in April, others in May. There is a gorse-covered common which I have in
mind, a favourite breeding resort of this species. Between this common
and the surrounding country, the birds constantly pass to and fro. If
you watch a particular male you will observe that it sings for a while
in its territory, that it then rises in the air and disappears from
view, and finally that it returns to the tree, bush, or mound which
constitutes its headquarters, where it again sings. Meanwhile the
female, with which there is every reason to believe that this male has
paired, behaves similarly; she, too, flies to the surrounding country
and in time returns with equal certainty. Sometimes male and female
accompany one another--that is, they leave simultaneously and likewise
return; at other times, though they depart together, the male returns
alone; or the male may disappear in one direction whilst the female does
so in another--and, on the whole, there is a sameness in the direction
of flight taken by the same pairs on different occasions. An interval of
nearly two months may thus elapse between mating and nest-building,
during which the sexes are not only often apart but often separated by a
considerable distance.
What does this species gain by the individuals belonging to it mating so
early in the season? If the appropriate condition which leads the
females to seek males were to arise in each individual at a late date,
the first stage in the process--mating--would not be completed before
the second--the discharge of the sexual function--were due to begin.
Thus, instead of having ample time, the females would have but a short
period in which to discover males; and this in some cases might lead to
delay, in others to failure, and in others again to needlessly severe
competition, entailing physical exhaustion at a critical moment in
their lives. Hence those females in which the appropriate organic
condition developed early in the season would not only be more likely to
find males, but would be in a position to rear more broods than those in
which it developed late; and they would have a better chance of leaving
offspring, which, in their turn, would reproduce the peculiarities of
their parents. Moreover, within certain limitations, the more these
successful females varied in the date of their development, the less
severe would be the competition, and the more uniformly successful would
the mating of all the individuals in a given district tend to become.
But all of this renders an interval of sexual inactivity unavoidable; an
interval which must constitute a danger unless there were something in
the external environment to prevent the male and female from drifting
apart. Inasmuch, then, as the occupation of a territory serves to remove
all possibility of permanent separation, I suggest that its evolution
has afforded the condition under which this beneficial procedure has
developed--free to mate when they will, free to seek food where they
will, free to pursue their normal routine of existence, and to meet all
exigencies as they arise in their ordinary daily life--whilst free to do
this, their future, as a pair, is nevertheless secure.
Thus far we have considered the territory in its relation to the
discharge of the sexual function. In many of the lower forms of life,
the success or the failure of reproduction, so far as the individual is
concerned, may be said to end with the completion of the sexual act--the
female has but to deposit her eggs in a suitable environment and then
her work is done, because in due course and under normal conditions of
temperature the young hatch out, and from the first are able to fend for
themselves. And so, when we come to consider the question of
reproduction in the higher forms of life, we are apt to focus attention
too much upon the sexual function and too little upon the contributory
factors, the failure of any one of which would mean failure of the
whole. For a bird, success in the attainment of reproduction does not
merely imply the successful discharge of the sexual function; much more
is demanded; it must find somewhere to build its nest and to lay its
eggs, it must shield its young from extremes of temperature and protect
them from enemies, and it must be in a position to supply them with food
at regular intervals. And, consequently, every situation is not equally
favourable for rearing young; there must be a plentiful supply of food
of the right kind in the immediate vicinity of the nest, and it must be
in greatest abundance just at the moment when it is most urgently
needed--that is to say, during the first few weeks after the birth of
the young. Success, therefore, depends upon manifold relationships which
centre in the station, and these relationships vary in intensity with
the conditions of existence.
First, then, let us examine the problem from the point of view of the
food-supply. There are many species whose success in rearing offspring
is largely dependent upon the rapidity with which they can obtain food;
and it makes but little difference which species we choose out of
many--Finch, Bunting, Warbler, or Chat. I shall choose the Buntings, as
their life-history in broad outline conforms to the general type, and,
moreover, their behaviour is fresh in my mind. The young are born in a
very helpless state; they are without covering--fragile organisms,
ill-fitted, one would think, to withstand extremes of temperature, and
wholly incapable of protecting themselves from enemies of any
description. For the first three days after they are hatched the female
spends much of her time in brooding them, and, when she is thus
occupied, the male sometimes brings food to her, which she proceeds to
distribute or swallows. But all the young cannot be fed, neither are
they ready to be fed, at the same moment; and the parents have besides
to find food for themselves, and the nest has to be cleaned--all of
which necessitates the young being exposed to the elements at frequent
intervals. Now it is impossible to observe the instinctive routine of
the parents, when the young need attention, without being impressed with
the conative aspect of their behaviour. Why, we ask, are the movements
of the female so brisk; why does she seek food and clean the nest so
hurriedly; why, if her instinctive routine is interrupted, do her
actions and her attitude betray such bewilderment? I take it that the
only answer we can give to these questions is that the part of her
inherited nature which predominates just at this particular time is to
brood. But why is brooding of such importance? Partly to maintain the
young at the proper temperature, and thereby to induce sleep--and sleep
for offspring newly hatched is as important as food--and partly to
protect them from the risk of exposure to extremes of temperature. This
latter danger is no imaginary one. Examine a young bird that has
recently left the egg; observe its nakedness; and consider what it has
to withstand--a temperature that may rise to 70° F. or may fall to 40°
F., the tropical rain of a thunderstorm or the persistent drizzle of
many hours' duration, the scorching effect of a summer sun or the
chilling effect of a cold north-easterly wind, and, constantly, the
sudden change of temperature each time that the parent leaves the nest.
One marvels that it ever does survive; one marvels at the evolution of a
constitution sufficiently elastic to withstand such changes. But,
however much the constitution may give us cause to wonder, it is clear
that much depends upon the parents. A slight inefficiency of the
instinctive response which the presence of the young evokes, a little
slowness in searching for food or sluggishness in returning to the nest,
might lead to exposure and prove fatal. And, however much is demanded of
the parents, it is clear that much also depends upon the relationships
in the external environment; for no matter how sensitive or how well
attuned the instinctive response of the parent may be, it will avail but
little in the presence of unfavourable conditions in the environment.
Everything turns upon the question of the effect of exposure. And in
order to ascertain how far extremes of temperature are injurious, I
removed the nests of various species containing newly hatched young,
and, placing them in surroundings that afforded the customary amount of
protection from the elements, I made a note of the temperature and the
atmospheric conditions and then observed the condition of the young at
frequent intervals. Details of these experiments will be found at the
end of the chapter.
The experiments with the Blackbirds and the Whitethroats gave the most
interesting results. Both broods of each species were respectively of
much the same age, yet one brood of Blackbirds survived for five, and
the other only for two and a half hours, and one brood of Whitethroats
lived for twelve hours whilst the other succumbed in a little over an
hour. This difference is rather remarkable; and it seems clear that the
power of resistance of the young diminishes rapidly when the temperature
falls below 52° F. It must be borne in mind, however, that the
conditions under which the experiments were made were, on the whole,
favourable--the weather was dry, the temperature was not unusually low,
nor was the wind exceptionally strong or cold; and even in those cases
in which the young succumbed so rapidly, the atmospheric conditions
could by no means be regarded as abnormal.
What, then, would happen in an unusually wet or cold breeding season?
For how long would the young then survive? In the spring and early
summer of the year 1916, I was fortunate in observing the effect of
exposure under natural but inclement conditions. I happened to be
watching the Yellow Buntings on Hartlebury Common--200 acres of Upper
Soft Red Sandstone, profusely overgrown with cross-leaved heath (_Erica
tetralix_), ling (_Calluna vulgaris_), and furze (_Ulex_)--in one corner
of which eight males had established adjoining territories covering some
fifteen acres of ground. The males obtained mates towards the end of
March or at the beginning of April; nests were built in the middle of
May, and the successful pairs hatched out their young in June. On the
10th June the weather became exceptionally cold, and during the next ten
days the temperature fell at times to 40° F. during the daytime. Slight
frosts were registered at night in the district, and the young bracken,
which covered the Common in places, had the appearance of having been
scorched and eventually withered away. At the coldest period of this
cold spell the young were hatched in two of the nests--in the first one
on the 10th June, and in the second a day or so later; and on the
morning of the 10th June, having found a suitable position near the
first nest, I began to watch the movements of the parents, with the
intention of keeping some record of their behaviour each day so long as
the young needed attention. An hour passed without their appearing, and
on examining the young I found that they were cold, feeble, and
unresponsive, but the female presently arrived and went to the nest.
Later in the day the young were lively and responded freely when the
nest was approached, but nevertheless I was impressed with the length of
time during which the parents were absent; for, judging by the
experience of previous experiment, there seemed to be every likelihood
of their losing their offspring in such abnormally cold weather, unless
they brooded them more persistently. On the 11th June at 5.50 A.M.
neither parent was to be seen and the young could scarcely be made to
respond; but shortly afterwards both male and female appeared, and,
after remaining a few minutes, again disappeared without even
approaching the nest. At 6.45 A.M. no attempt had been made to brood and
the young were then so feeble that they were scarcely able to open their
mouths, and at 6 P.M. one was still alive but the remaining three were
dead. Yet the parents returned and the female went to the nest; and,
from a distance of a few feet, I watched her brooding the living and the
dead. At 5.45 A.M. the following day the remaining young bird had
succumbed, the temperature then being 49° F.
At the second nest, I was unable to watch the behaviour of the parents
so closely. On the 15th June the nest contained three young from three
to four days old, and during the morning of that and the succeeding day
nothing unusual occurred, with the exception that the period of exposure
seemed, as in the former case, to be too long. On the 17th June at 3.10
A.M. the young had collapsed and were stiff, but the parents were in
their territory and anxious apparently to attend to their brood. At 9.15
A.M. only two of the young were left in the nest, and though I searched
amongst the undergrowth and in the gorse bush in which the nest was
placed, no trace of the third bird was to be found. Of the two remaining
young, one was alive and responsive but the other was dead, and though
the female attended assiduously to the sole surviving offspring, yet it
too had succumbed by the following morning.
In a third territory, there was a nest containing four eggs. These eggs
were due to hatch at much the same time as those in the two nests just
referred to, but they failed to do so, and an examination showed that
they contained well developed but dead chicks.
To what can the death of the young and of the chicks in the eggs be
attributed? Not to any failure in the instinctive response of the
females, for they fed their young, they brooded them, they even brooded
the dead as well as the living, and probably did all that racial
preparation had fitted them to do. Yet the fact that the young in the
second nest were lifeless and exposed at 3 A.M. seems to betoken absence
on the part of the parents during the night, and may be interpreted as a
failure of the parental instinctive response. Let us return for a moment
to the experiments. These showed, it will be remembered, that a rise or
fall in the temperature of but a few degrees was sufficient to make an
astonishing difference in the length of time that the young were able to
survive without their parents; that when the temperature reached 58° F.
the bodies of the young retained their warmth, and that under such
conditions even a night's exposure had little, if any, effect; so that
even supposing that the parents were absent during the night, the death
of the young cannot be said to have been due to a failure of the
parental instinct, because under normal conditions--and under such has
their instinctive routine been evolved--their absence would not have
prejudiced the existence of the offspring. I attribute the collapse of
the young solely to the exceptional cold that prevailed at just the most
critical time, and I base this conclusion partly on the experience
gained from experiment, but mainly on their condition observed at
different intervals; for during exposure they collapsed rapidly, their
flesh became cold and their movements sluggish, their response grew
weak, and gradually they became more and more feeble until they could
scarcely close their bills after the mandibles had been forced asunder.
Yet, even after having reached so acute a stage of collapse, the warmth
from the body of the brooding bird was sufficient to restore them
temporarily; once more they would become lively and responsive, only,
however, to revert to the previous condition soon after the parent had
again abandoned them. Doubtless their power of resistance grew less and
less during each successive period of exposure.
If the nestling Bunting is to be freed from the risk of exposure, it is
evident that there must be, in the vicinity of the nest, an adequate
supply of food upon which the parents can draw liberally. Hence those
pairs that exercise dominion over the few acres surrounding the nest,
and are thus able to obtain food rapidly, will stand a better chance of
rearing their offspring than others which have no certain supply to draw
upon--and this, I believe, is one of the biological ends for which the
territory has been evolved. But it must not be supposed that each pair
finds, or even attempts to find, the whole of the food within its
territory, or that it is necessary for the theory that it should do so;
all that is required is that such overcrowding as might lead to
prolonged absence on the part of the parents and inordinate exposure of
the young shall be avoided. So that the problem has to be considered not
merely from the point of view of the individual, but from the larger
point of view of all the pairs inhabiting a given area.
Now there were eight pairs of Yellow Buntings occupying the one corner
of Hartlebury Common, and their territories in the aggregate covered
some fifteen acres. The birds obtained part of their food-supply amongst
the gorse and in some young scattered oak-trees, and part in an
adjoining coppice and on the surrounding arable land. But they were not
the sole occupants of this corner of the Common; other insectivorous
species had territories there also--amongst which were Whitethroats,
Grasshopper-Warblers, Willow-Warblers, Whinchats, Stonechats,
Meadow-Pipits, Tree-Pipits, and Skylarks. Suppose then that there had
been sixteen pairs of Yellow Buntings instead of eight; that there had
been other pairs, which assuredly there were, inhabiting the locality;
that they had also resorted, which assuredly they did, to the coppice
and arable ground for the purpose of securing food; and that their
numbers had also been increased in a similar ratio--would a supply of
food for all have been forthcoming with the necessary regularity and
promptitude? Well, the parents might have had to travel a little
farther; but even if they had been compelled to do so, their absence
would only have been prolonged by so many minutes the more, and under
normal conditions what harmful result to the offspring could possibly
have followed? The question for us, however, is not what might have
occurred under normal conditions, but whether the life behaviour is so
adjusted as to meet the exigencies of diverse, and in this case of
abnormal, circumstances. Now the capacity of the young to resist
exposure diminishes very rapidly when the temperature falls below the
normal--the danger zone seems to be reached at approximately 52° F., and
the length of time during which they survive then becomes astonishingly
short--and moreover the fall in the temperature would tend to decrease
the supply of insect life upon which they depend, so that if the size of
the territories had been reduced by one half, and the parents in
consequence had been compelled to seek their food at a greater distance,
can it be doubted that the cumulative effect of even a few minutes of
additional exposure would have been detrimental, if not disastrous, to
the offspring?
We speak, however, of the parents extending their journeys a little
farther in this direction or a little farther in that, as though they
could do so with impunity except in so far as it affected themselves, or
their offspring, or the other Yellow Buntings inhabiting that particular
area. But, most certainly, any extension would have meant so much
encroachment upon the available means of support of other members of the
species inhabiting adjoining areas, whose young in turn would have been
liable to have been affected; and, with even greater certainty, the
Whitethroats, the Stonechats, the Tree-Pipits, and the Willow-Warblers
that had also established themselves in that one corner of the Common
would have been hard pressed to find sufficient food with sufficient
rapidity.
Let me give another illustration of a somewhat different kind. Lapwings,
as we saw in the previous chapters, establish territories and guard them
from intrusion with scrupulous care. The young are able to leave the
nest soon after they are hatched, and consequently the parents are not
necessarily obliged to bring food _to_ them--they can, if they so
choose, lead them _to_ the food. Whether each pair limits its search for
food to its territory, I do not know. But even supposing that all
ownership of territory were to lapse directly the young were hatched,
that the boundaries were to cease to exist, and that the birds were free
to wander at will without fear of molestation, the end for which the
territory had been evolved would none the less have been obtained; for
inasmuch as the parents are accompanied by their young, it matters not
in what part of the meadow they seek their food; all that matters is
that the number of families shall not exceed the available supply of
food. So far, then, as the Lapwing is concerned, the territory fulfils
its purpose when once it limits the number of males, since, by doing so,
it limits the number of families and prevents undue pressure upon the
means of support.
Nevertheless, there are many birds that seem to rely entirely upon the
territory to supply them with all that is necessary. Each Warbler seeks
its food within the precincts of its own particular domain, and, except
in occasional instances, neither resorts to neutral ground nor makes
excursions into the locality immediately surrounding the territory, as
does the Bunting. Probably it would be disastrous if it attempted to do
so, for since its young at birth are so delicate and so susceptible to
changes of temperature, it cannot afford to be absent from them for
long. Of the two experiments made with young Whitethroats, one was made
under favourable and the other under unfavourable conditions. In this
latter case the temperature was 50° F., and the young, it may be
remembered, only survived for a little over one hour. Now exposure at
that temperature is evidently dangerous, but it would be still more
dangerous if the weather were wet instead of dry, and the temperature
46° F. instead of 50° F.; and it is, I imagine, on this account that the
impulse to brood is so strongly implanted in the female. No sooner, it
seems, does she depart than she returns with a small quantity of food
which she hurriedly distributes and immediately settles down to brood;
and if forcibly prevented from returning, her attitude betrays symptoms
of what, humanly speaking, we should term great distress. If, then, the
conditions in the external environment were such as would make it
difficult for the female to obtain food rapidly, what advantage would
she derive from so strongly developed an impulse? Might it not be a
disadvantage? Might it not mean that she would abandon the search too
readily and be content to return with an insufficient supply, and might
not that be as injurious to the young as prolonged exposure? Manifestly
the impulse to brood could only have developed strength in so far as it
fitted in with all the other factors that make for survival; and the
principal factor in the external environment seems to be the territory.
How could the young have been freed from the risk of exposure if the
impulse to brood had not been so strongly implanted in the parent? How
could the impulse to brood have been free to develop if a supply of food
had not been first insured? How could the supply of food have been
insured if numbers of the same species had been allowed to breed in
close proximity?
From the foregoing facts it is clear that the young of many species are
at birth susceptible to cold and unable to withstand prolonged exposure.
The parents must therefore be in a position to obtain food rapidly, and
consequently it is important that there should be an ample supply in the
vicinity of the nest. This end the territory certainly serves to
promote; it roughly insures that the bird population of a given area is
in proportion to the available means of subsistence, and it thus reduces
the risk of prolonged exposure to which the young are always liable.
This leads on to a consideration of those cases in which the question of
securing food is subordinate to the question of securing a station
suitable for reproduction.
I take the Guillemot as an example. In principle its behaviour is
similar to that of the Bunting; the male repairs to a definite place,
isolates itself, and becomes pugnacious. But the Guillemot is generally
surrounded by other Guillemots, and the birds are often so densely
packed along the ledges that there is scarcely standing room, so it
seems, for all of them. Nevertheless the isolation of the individual is,
in a sense, just as complete as that of the individual Bunting, for each
one is just as vigilant in resisting intrusion upon its few square feet
as the Bunting is in guarding its many square yards, so that the
evidence seems to show that that part of the inherited nature which is
the basis of the territory is much the same in both species. What we
have then to consider is, What is the biological value to the Guillemot
of an inherited nature which, for the Bunting, has utility in relation
to the supply of food for the young? Up to a point, the act of securing
a territory has like value for each respective species, whether the area
occupied be large or small--that is to say, it enables the one sex to
discover the other with reasonable promptitude.
For the greater part of the year, Guillemots live at sea; singly, in
twos or threes, or in small parties, they move upon the face of the
waters, extending their wanderings far away from land, out into the
broad ocean, where for weeks together they face the gales and heavy seas
of the Atlantic. But in due course and in response to internal organic
changes, they return, like the Warbler, to their breeding grounds--rocky
headlands or islands appropriately situated and affording the
appropriate rock formation. During all these months of wandering, the
majority seem to ignore the land, to pass away from it altogether, and
to spread themselves over the surface of the ocean regardless of
mainland or island. Some useful observations, which throw some light on
the distance that Guillemots are accustomed to wander from land, were
made by Lieut. B. R. Stewart during a number of voyages between various
ports in Great Britain and Ireland and ports in North America,
principally New York and Quebec. Thus, on the 24th March, large numbers
were seen in lat. 55° N., long. 24° W., five hundred miles approximately
from land, though on the following day--four hundred miles off Tory
Island--they were not so plentiful. Again, on the 1st October, in lat.
53° N., long. 27° W., seven hundred miles or so from land, one bird was
seen, whilst on the following day, in lat. 52° N., long. 21° W., a
single individual was washed on board by the heavy seas and seemed
little the worse for the adventure. Within two hundred miles of the west
coast of Ireland, he found them plentiful on various occasions. From
this it is clear that the circumstances under which the bird lives for
many months in succession must impose a considerable strain upon its
constitution; and how it is able to withstand the buffeting of wind and
water, to secure its food, and to endure, is a mystery. It is important,
therefore, that the young bird should be properly nourished and
protected from anything that might harm its constitution, and important,
too, that the parents should be freed from any undue strain during the
course of reproduction.
The conditions which the breeding station has to fulfil are threefold:
in the first place, it must be in proximity to the food-supply;
secondly, it must provide the necessary shelter for the egg and for the
helpless offspring; and, in the third place, it must be so situated that
the young can reach the water in safety. We will examine these
conditions one by one.
The proximity to the food-supply is a consideration of some importance.
The life of the Guillemot during the winter is a strenuous one; we know
that large numbers succumb in stormy weather, and we can infer that
slight constitutional defects might make all the difference between
failure and success; and, therefore, the less severely the constitution
of the parent is taxed during reproduction, and the more securely the
constitution of the offspring is built up, the greater prospect will
both have of resisting the hardships of the winter successfully. Much,
then, will depend upon the distance the parents have to travel in order
to obtain food. The farther the breeding station is removed from the
feeding ground the greater the physical strain which will be imposed
upon the birds, and the greater the chance will there be of the
offspring being improperly nourished. Now the food consists of small
fish, largely of sand-eels, which are secured in deep water, and the
abundance of which varies, possibly according to the nature of the
currents. Hence cliffs which are situated away from the water, or from
which the water recedes at low tide, or which are surrounded by an area
of shallow water, and are thus not in proximity to the feeding ground,
even though they may fulfil the second and third condition, will not
answer the requirements of a breeding station.
Of no less importance is the type of rock-formation. Not every formation
affords the necessary ledges upon which the egg can be deposited with
safety--the face of the cliff may be too smooth, or too jagged, or the
shelves may run at too acute an angle. Many of the large assemblages of
Guillemots in the British Islands are found where the rock is quartzite,
mica-schist, limestone, or chalk. The reason of this is that such rocks
are weathered along the planes of stratification, of jointing, of
cleavage, or of foliation--the strata being probably of unequal
durability--with the result that innumerable shelves, ledges, and
caverns, which are taken advantage of by the birds, form a network over
the face of the cliff. But only those ledges can be made use of which
are placed at a considerable height above the water, because, when the
cliff faces the open sea, the lower ones are liable to be washed in
stormy weather by the incoming swell and thus become untenable. There is
a small cove in the midst of the most precipitous part of the breeding
station at Horn Head, wherein the shingly shore shelves rapidly to the
Atlantic and faces to the west. Here, towards the end of July, young
Kittiwake Gulls can sometimes be found washed up on the beach--some
living, but in every stage of exhaustion, others dead, and in every
stage of decomposition; here is the young bird, recently caught by the
swell and thrown upon the shore, lying side by side with the remains of
others that had previously succumbed to starvation--on every side
evidence of the devastation wrought by the Atlantic. May not some of
this destruction have been brought about by the nests having been placed
upon the lower ledges within reach of an exceptionally heavy sea? Hence
much depends upon the nature of the rock-formation, and many a mighty
precipice, even though it may fulfil the first and third condition, is
nevertheless valueless as a breeding station.
Finally, the young bird must occupy a ledge from which it can reach the
water in safety. There is much difference of opinion as to the manner in
which it leaves the ledge, but all agree that it does so before it is
capable of sustained flight. If, then, the face of the cliffs were made
up of a series of broken precipices, or if the rocks at the base
projected out into the water, or if detached rocks abounded in the
waters beneath, the mortality amongst the chicks would no doubt be
considerable.
The coast-line of Co. Donegal will illustrate the foregoing remarks. On
the southern and western side of the Slieve League promontory there is
no real Guillemot station; only on the northern side--the quartzite in
the vicinity of Tormore--are the birds to be found in large numbers.
Northwards from here, a wild and rugged coast is passed over before
other stations are reached--at the eastern end of Tory Island and on
Horn Head; and beyond this, to the east, there are none, not even on the
old rocks that form the promontory of Inishowen. Why, we ask, do
countless numbers crowd the ledges of Horn Head, whilst they are absent
from the precipices of Slieve League; why, too, are they absent from
the granite cliffs of Owey? The reason is not far to seek. Either the
face of the cliff is made up of a series of broken precipices, or the
face of the precipices is too smooth, or the otherwise suitable ledges
are situated too near the water, or the water recedes from the base of
the cliff at low tide. Many miles of rock-bound coast are thus useless
for the purpose of reproduction.
Now when we bear in mind how large an expanse of coast is formed of
blown sand or of rocks of low altitude, and how many miles of cliff fail
to supply the three essential conditions that we have been considering,
we can see that suitable breeding stations must be limited both in
number and extent. From a wide expanse of ocean hosts of individuals are
therefore obliged to converge at certain definite points; and hence,
each recurring season, there must arise a competition for positions at
the station, just as there is competition between individual Buntings
for positions in the marsh. And the ability to obtain a position upon a
suitable ledge involves, in the first place, an impulse to search for
it; in the second place, an impulse to dwell in it; and in the third
place, an impulse to resist intrusion upon it. It would be useless for
an individual to be pugnacious if it had no fixed abode; equally useless
for it to establish itself on a particular ledge if it had no power to
defend it--all of which implies an inherited nature similar to that of
the Bunting. But the proximate end to which the competition is directed
is not alike in the case of both species. In the case of the Guillemot
it has reference solely to the piece of rock whereon the egg is laid; in
the case of the Bunting to a piece of ground capable of furnishing an
adequate supply of food for the young; and the reason for the difference
is this, that there is always an abundance of food in the water beneath
the cliff, but breeding stations are scarce, whereas there is always an
abundance of situations in the marsh in which the Bunting can place its
nest, but the supply of food varies and at times can only be obtained
with difficulty.
If then the Guillemot were to behave after the manner of the Bunting and
assign to itself a portion of the face of the cliff, or if it were only
to occupy a few ledges, or an even lesser area--a single ledge--what
would be the result? That it would attain to reproduction is beyond
question; that the egg would be safely deposited there can be no manner
of doubt; neither is there any reason to suppose that the offspring
would not be successfully reared. But, indirectly, its behaviour would
affect the Guillemot race. For if it be true, as the crowded ledges
certainly seem to show, that there is a dearth of suitable breeding
ground, no greater calamity could befall the species than that some
members should exercise dominion over too large an area of the habitable
part of the cliff and thus prevent others from breeding. Under such
conditions the race could not endure, since in this, as in every case,
its survival must depend upon a close correspondence between the
behaviour of the individual and the circumstances in the external
environment.
Scarcity of suitable cliffs is the principal reason of the ledges being
so closely packed with Guillemots, just as it accounts for this part of
the precipice being crowded with Kittiwake Gulls, that part with
Herring-Gulls, and that part again with Razorbills and Puffins. Yet each
individual preserves its few square feet of rock or soil from
molestation, and the area each one occupies varies according to the
conditions of existence of the species. Thus the Herring-Gull occupies a
comparatively small area, although one many times larger than that of
the Guillemot. It requires more space than the latter, owing to the fact
that it not only builds a nest but rears four instead of a single
offspring, and it can be allowed this, because, since its young remain
in the nest until they are capable of sustained flight, it can make use
of many miles of cliff from which the tide recedes at the base, or which
have, at their base, rocks jutting out into the sea; but manifestly it
cannot be allowed so much space as the Bunting.
Martins build in close proximity to one another, owing probably to
shortage of accommodation, and, in their case, the nests have to be so
situated as to be sheltered from the wet. If water drips upon them for
any length of time, the mud, of which they are composed, crumbles and
large pieces fall away, with the result that the eggs or the young are
precipitated to the ground. Consequently, not every house or
perpendicular cliff will answer the purpose of a breeding station. A few
pairs build their nests beneath the eaves close against the walls of my
house, and year after year the result is much the same; after every
downfall of rain, the water collects into rivulets, trickles down over
the eaves, is absorbed by the mud and destroys the nests. Thereupon, the
birds set to work and rebuild; but again the nest is destroyed, and
again they rebuild, and so on throughout the summer, and only on rare
occasions do they succeed in rearing offspring at the proper season.
Similar conditions must prevail in many situations; but, clearly, the
more binding and plastic the building material, the longer the nest will
withstand the action of the dripping water and the greater chance will
there be of the young being reared in safety. Observe, therefore, how
far-reaching an effect so small a detail as the nature of the mud can
have upon the status of the species in any given locality. Where the
conditions are favourable, there the birds must congregate to breed,
and, like the Guillemot, if each individual exercised dominion over too
large an area, the species as a whole would suffer.
In all these examples, the fact of different individuals being in such
close proximity may afford some protection from enemies both as regards
the egg and the offspring, and in so far as there is a mutual advantage
such assemblages may be spoken of as communities. A community, however,
in the true sense of the word, is a collection of individuals brought
together, not primarily as a result of shortage of breeding ground, but
in consequence of advantages of communal ownership over individual
ownership. A rookery is an example of a true community. Neither shortage
of nesting accommodation nor scarcity of food can account for Rooks
assembling together to breed; for if the different pairs which go to
make up the rookery were to scatter throughout the surrounding
neighbourhood, they would, as a rule, find plenty of trees in which to
build their nests, and plenty of food.
How, then, can the theory apply to a species that breeds under such
conditions? What part can the territory play in furthering the life of
the individual when large numbers of nests are built closely together in
the same tree? There is much evidence to show that mutual protection is
a necessary condition of the Rook's existence; many cases are on record
of rookeries being destroyed by Carrion-Crows, Hooded Crows, and Ravens.
For instance, Mr. Ward Fowler records a case in which a pair of Crows
attacked a small rookery, ransacked the nests, and destroyed the eggs,
with the result that not a single pair of Rooks was left in the
settlement. Each Rook must therefore secure a position within the
precincts of the community if it is to have a chance of success in the
attainment of reproduction. But every locality cannot supply sufficient
trees of the right kind, appropriately situated and in suitable relation
to the food supply, in which numbers of nests can be built in close
proximity; so that if more than one community were to attempt to
establish itself in a limited area, the supply of food or the supply of
trees might become a pressing problem. Each community must therefore be
prepared to defend its own interests, and each must be regarded as one
unit and the area occupied as one territory within which are included a
number of lesser territories. The individual may fail to establish
itself within a community, but, even if it succeeds, the community may
fail to establish the rights of communal ownership; hence it has to face
a twofold possibility of failure, and if it lacked the inherited nature
which leads the Guillemot to secure a position upon the ledge, or the
Bunting to obtain a position in the marsh, the chances are that it would
fail in the attainment of reproduction.
The question now arises as to how it comes about that the area occupied
by each individual conforms in broad outline to that which has proved
beneficial for the welfare of the species as a whole. We shall find that
up to a point the answer is a simple one. No one could study the
behaviour of animals without observing the important part that habit
plays in the life of the individual; an action performed to-day is
liable to be repeated to-morrow and the following day until it becomes
ingrained in the life of the individual. This must not be taken to mean,
however, that a particular action has to be performed for many days in
succession before it becomes definitely fixed; if only it is repeated a
number of times, even within the space of a few hours, it will acquire
sufficient strength for its continuance; but continued repetition gives
increased fixity, and, as time goes by, it becomes increasingly
difficult for the creature to make a change unless the character of the
situation necessitates readjustment.
For example, when the organic condition which leads to nest-building
becomes active, the bird tentatively collects some of the necessary
material in its bill, flies round with it, and then drops it. After a
while it collects some more, and this time leaves it perhaps in a bush.
Later on it makes another attempt, and, meeting with a situation which
calls forth the appropriate response, it thereupon lays the foundation
of the structure. We will assume that the nest is placed in the midst of
a tangled bush. Well, the bird lays the first strands of the foundation
and then goes in search of more material. The next time it approaches
the nest from the opposite side of the bush, and presently it finds yet
a third entrance. But each entrance is not made use of in turn: one is
employed more frequently than the other two, and in the course of time
becomes the sole highway to and from the nest. Suppose now that, when
the young are hatched, I cut away the foliage from the bush on the
opposite side from that on which the bird customarily enters, and by so
doing leave the nest exposed, what is the result? The female arrives
with food, threads her way through the bush, and, when beside the nest,
pauses as if aware that some change had taken place, and then flies away
through the new opening. In a short time she returns, flits from twig to
twig on the outskirts of the bush, and comes upon the new opening--there
she hesitates. But though the nest is in full view and within a few
inches of her perch, and though the young stretch out their necks, yet
so strong is the former habit that she is compelled to return to the
opposite side and approach the nest by the usual circuitous route before
she distributes the food amongst her offspring.
Let us see how far this law of habit formation may have been effective
in defining the extent of the area occupied. When a male Warbler arrives
at its destination in the spring it seeks out a suitable environment,
and, having found a place unoccupied by any other male, settles in it
and remains there--its behaviour up to this point being determined by
racial preparation. After the fatigue of the journey its movements are
at first sluggish; hunger, however, asserts itself and a search is made
for food; wandering away from the position in which it first settled and
which acts as a headquarters, it hunts through certain trees here or
certain bushes there and returns, and presently it wanders away again,
perhaps in another direction, but, as before, works its way back again
to the headquarters. The journeys thus radiate outwards from the
headquarters, and according to the success with which the bird meets,
so, probably, it happens that some trees are searched more often than
others and certain directions are taken more frequently than others, and
by constant repetition a routine is established which limits the
direction and scope of its wanderings.
But in the case of the Guillemot the conditions of existence are
reversed: food can be had in abundance but suitable breeding stations
are scarce. The few square feet of ledge correspond to the tree or clump
of bushes which acts as a headquarters for the Warbler, and the
occupation of them is determined, as it is in the case of the Warbler,
by racial preparation. Since, however, the ledge is only made use of for
the immediate purpose of incubation and is in no way affected by
questions relating to food, there is no occasion for the bird to wander
along the ledge nor to encroach upon those adjoining. Hunger stimulates
the Warbler to search the surrounding trees, and so to extend its area;
but hunger takes the Guillemot down to the water, and hence the area
which it primarily occupied remains unmodified.
To sum up: the territory is useful in various ways, but not necessarily
in the same way for every species. Reproduction would always have
remained fortuitous, and the number of individuals that attained to it
would seldom have reached the possible maximum unless some provision had
been included in its system for insuring that the males and females
could meet at the proper moment and afterwards remain in touch with one
another, and that the number of pairs inhabiting a given area did not
exceed the available means of support. I have tried to show that the
inclusion of a disposition to secure a territory tends to remove these
difficulties. In the first place, the disposition which leads to its
occupation comes into functional activity (in the male) early in the
season; and so, by the time that the appropriate pairing condition
arises in the females, the process of acquiring territories is well
advanced, and the males being regularly distributed, each in its
respective position, are readily found by their prospective mates. The
behaviour of each sex is thus adjusted to further the end of mutual
discovery. Next, after mating has taken place, the position occupied by
the male acts as a headquarters to which the birds can always repair,
and becomes a bond of union which is serviceable in that it prevents any
possibility of their drifting apart. And in the third place, the males
become pugnacious and in this way secure for themselves areas which vary
in size according to the conditions of existence of the species, so that
there is no possibility of too many congregating in this locality, and
all the less likelihood of too few finding their way to that; and hence,
on the average, different pairs are distributed throughout all suitable
localities. Furthermore, owing to the fact of their having a
headquarters, the male and female are allowed a freedom of movement
which otherwise they would only possess when the construction of the
nest had actually begun; they can seek their food independently, and,
even though paired, they can if necessary continue their winter routine
without risk of separation. This means that the organic condition which
leads to pairing, is free to develop in the female earlier than would be
the case if there were nothing in the external environment to attract
the pair to a particular spot; and the longer the period over which the
process of pairing can be spread, the greater chance will females have
of discovering mates, the less severe will the competition tend to
become, and, consequently, the smaller the percentage of individuals
that fail to obtain suitable partners.
In these ways the territory has been serviceable alike to a number of
species. But much as the questions of mutual discovery and regular
distribution may have influenced the course of its development, there
can, I think, be little doubt that, on the one hand, the supply of the
necessary accommodation for rearing offspring, and on the other, the
necessity for an adequate supply of food in close proximity to the nest,
have been the main determining factors, and have led to a wide
divergence in its function. At the one extreme the function is to insure
a plentiful supply of food for the young; at the other, to insure a
station suitable for rearing offspring. I took the Bunting and the
Guillemot as types of the two extremes. The young of the former species
are born in a very helpless state. They are susceptible to cold and
unable to withstand prolonged exposure, and therefore it is essential
that there should be an ample supply of food, upon which the parents can
draw liberally, in the vicinity of the nest. But the nest is placed in a
variety of situations, and accommodation in this respect may be said to
be unlimited. The young of the latter species are not so susceptible to
exposure, and moreover there is always an abundance of food in the
waters beneath the cliff; but ledges of rock, upon which the egg can be
securely deposited and the young successfully reared, are limited both
in number and extent. The position then is as follows: there are
situations in plenty in which hosts of Buntings can build their nests
but the supply of food is a difficulty, and if the respective areas of
different individuals were insufficient to supply them with the
necessary food with the necessary rapidity, they would run the risk of
losing their offspring and the species would not endure; on the other
hand, cliffs upon which the Guillemot can rear its young are limited,
but the supply of food presents no difficulty, and consequently the
smaller the area over which each individual exercises dominion, the
greater the number that will attain to reproduction and the greater
prospect the species will have of survival. The emphasis in the one case
lies on the fact that the area occupied must be sufficiently large; on
the other, on its being just sufficient and no more to accommodate the
egg. Hence the difference in the function at the opposite extremes is
brought about, not by modifications of the instinctive behaviour which
leads to the establishment and defence of the territory, but solely by
modifications in the size of the area occupied, in accordance with the
conditions prevailing in the external environment. No doubt, if we had
the life-histories of a sufficient number of species worked out, we
should find that the gradations were complete from the one extreme to
the other. We are justified in thinking that this must be so because in
many directions we can not only observe differences in the size of the
area occupied, but can recognise a close correspondence between those
differences and the conditions of life of the species. Thus the
Herring-Gull occupies a comparatively small area, though one which is
many times larger than that of the Guillemot. It requires more space
because it not only builds a nest but rears four instead of a single
offspring, and it can be allowed more space because the young remain in
the nest until they are capable of sustained flight, and consequently it
can make use of many miles of cliff from which the tide recedes at the
base, and which on this account are denied to the Guillemot, but
manifestly it cannot be allowed so much space as the Bunting, for then
comparatively few individuals would attain to reproduction.
Again, the Reed-Warbler inhabits swamps overgrown with the common reed,
and in such places insect life is abundant just at the time when the
young are hatched. But these swamps cover a comparatively small acreage
in the breeding range of the bird, and if each pair were to attempt to
establish dominion over an area equal, let us say, to that of the
Willow-Warbler, the species would have but a poor chance in the struggle
for existence. So that, in a case of this description, the supply of
food and the comparative scarcity of breeding stations have been factors
of like importance in the evolution of the territory.
Finally we were led to inquire as to how it comes about that the extent
of the area occupied by each individual is adapted to the circumstances
in which the individual finds itself; and we came to the conclusion that
the movements of the bird, subsequent to the initial act of establishing
itself in a position, are regulated and defined by the law of habit
formation. For example, the Warbler, in response to its inherited
nature, takes up a position in an appropriate situation. It then
proceeds to search for food; it makes short journeys first in this
direction and then in that; it repeats these journeys, and gradually
forms a habit which compels it to remain within more or less
well-defined boundaries. But the actual distance that it traverses on
the occasion of its first attempt must be determined by the relative
abundance or scarcity of the particular kind of insect life which it
requires. So that, although habit defines and in some measure helps to
determine the boundaries of the territory, it is clear that in the last
resort they must depend upon the nature of the conditions in the
external environment.
We have, then, the congenital basis which leads to the occupation of a
position, and to the enmity shown by the owner of the position towards
other individuals; and this congenital basis is found alike in many
widely divergent forms, living under equally widely divergent
conditions; we have acquired accommodation; and we have relationships in
the organic and inorganic world--and the outcome of it all is a system
of behaviour which we, who can perceive the end to which such behaviour
is tending, are justified in speaking of as "a disposition to secure a
territory." In the development of this system a primary value must be
ascribed to the conditions in the external environment, for they
determine the direction of the variations of instinctive procedure and
of acquired habit which work towards the same goal--that of adjustment
to the conditions of life.
NOTE
The following are the experiments referred to on page 181:--
On the 14th May 1915, a nest of Blackbirds approximately four days old
was removed at 6.45 A.M. The temperature was considerably below the
normal, and snow lay on all the high ground in the neighbourhood. In a
short time the birds collapsed, and at 9.15 A.M. were dead. On the 29th
May, at 6 A.M., a second nest was removed, containing young of
approximately the same age, and although the conditions were more
normal, the temperature being 50° F., the birds collapsed at 8 A.M., and
an hour later one of the brood showed little signs of life. The wind,
however, then changed to the west, and the temperature rose one degree,
with the result that they were still living at 11 A.M. A further
experiment was made with Song-Thrushes on the 5th June. The wind was in
the south and the temperature 63° F. The young, approximately four days
old, were removed at 7.25 A.M., but as they showed no signs of collapse
at 1 P.M. I replaced the nest in the original site.
On the 30th May, a nest of Whitethroats three days old was removed at
7.15 A.M. The wind was northerly and the weather fine, but the
temperature low--50° F. At 8.15 A.M. the birds showed no sign of life. A
second experiment with this species was made on the 10th June under more
favourable circumstances, for although the sky was overcast and the wind
northerly, the temperature was 59° F. In this case the young survived
from 6.55 A.M. to 7 P.M.
On the 27th May 1915, a nest of Hedge-Sparrows hatched the previous day
was removed at 7 A.M. The temperature was below the normal, being 49° F.
At 8 A.M. the young were cold and in a state of collapse, but they
survived nevertheless until 3.20 P.M.
On the 7th June 1915, a nest of young Skylarks three days old was
removed at 7.15 A.M. The temperature was 62° F., and the birds survived
until 4 A.M. the next day.
On the 6th June 1916, a nest of Linnets just hatched was removed at 6.47
A.M. The temperature was 51° F. At 7.50 A.M. the birds were cold and in
a state of collapse, and only survived until 8.50 A.M.
CHAPTER VI
THE WARFARE BETWEEN DIFFERENT SPECIES AND ITS RELATION TO THE TERRITORY
We have now considered the various ways in which the territory is useful
in furthering the life of the individual. We have seen that, in some
cases, there is competition for stations where the egg or eggs can be
deposited and incubated in safety; that, in others, there is competition
for stations capable of furnishing an adequate supply of food for the
young; and that the establishment of "territories" not only renders the
attainment of reproduction for the individual secure, but serves so to
regulate the distribution of pairs that the maximum number can be
accommodated in the minimum area. This being so, the question arises as
to whether competition for territory is strictly limited to individuals
of the same species, or whether it may not occur also between different
kinds of birds, providing always that similar conditions of existence
are required. First of all I shall relate a number of facts which will
serve to show the nature and extent of the warfare, and I shall then
give the reasons which lead me to believe that the fighting not only
bears some relation to the "territory," but that it is an important
factor in contributing to the attainment of that which for biological
interpretation is the end for which the whole territorial system has
been evolved.
Those who have studied wild life on one of the rocky headlands, which
are so numerous round our coasts, will probably be familiar with the
rivalry that exists between the Raven and certain birds of prey. Where
the Raven finds shelter for its nest, there, too, the Peregrine has its
eyrie--and so it happens that these two species are continually at war.
Now the warfare occurs not only during the season of reproduction but
continues throughout the greater part of the year, and can even be
observed in the late summer or early autumn--the period when we should
expect to find the instinct least susceptible to appropriate
stimulation. But it is of a more determined kind early in the spring,
and it is then that we often witness those remarkable exhibitions of
flight, the skill of which excites our admiration. The Falcon rises
above the Raven, stoops at it, and when it seems no longer possible for
a collision to be avoided, or, one would imagine, for the Raven to
escape destruction, the Raven skilfully turns upon its back and
momentarily faces its opponent, and the Falcon with equal skill changes
its course, passing upwards and away. The attack, however, is soon
repeated, and though no collision may actually take place, yet the fact
that the Raven, when it turns to face its adversary, is obliged to drop
the stick which it carries, is not only an indication of the character
of the struggle, but it shows that a definite end is gained--that the
efforts of the Raven to build in that particular locality are hampered.
But the Falcon is not the only enemy that the Raven has to face;
Buzzards are just as intolerant of the presence of Ravens in their
neighbourhood as the Ravens are of them, and consequently there is
incessant quarrelling wherever the same locality is inhabited. As a
rule, the fighting occurs whilst the birds are on the wing; the Buzzard
rises to a considerable height, and, closing its wings, stoops at the
Raven below, and when within a short distance of its adversary, swerves
upwards and gains a position from which it can again attack. The
Buzzard, however, is by no means always the aggressor; I have watched
one so persistently harassed by a Raven that at length it left the rock
upon which it was resting and disappeared from view, still followed by
its rival. Thus it seems as if they were evenly matched, and, when they
occupy the same locality, it is interesting to notice how the initiative
passes from the one to the other according to the position occupied by
the birds in their respective territories.
[Illustration: Peregrine Falcon attacking a Raven
Emery Walker ph.sc.]
That there is constant warfare between the Green Woodpecker and the
Starling is well known, the purpose of the Starling being to gain
possession of the hole which the Woodpecker with much skill has drilled
for itself. As far as my experience goes, the Starling is always the
aggressor, and there is only too good reason to fear that, in the course
of time, the Green Woodpecker will disappear as a result of the greater
fertility and tenacity of its enemy. The Martin suffers a similar kind
of persecution from the House-Sparrow, and here again there is reason to
believe that the greater virility of the Sparrow will hasten the
extinction of its rival. In cases of this description the purpose of the
fighting is clear, and one can understand why such divergent species
should be hostile to one another; yet others, equally remote in the
scale of nature, are hostile when no such ostensible reason can be
assigned for their hostility. Few birds are more pugnacious than the
Moor-Hen, and the determined manner in which different individuals fight
with one another is notorious. But the intolerance it displays towards
other species is no less remarkable, and its pugnacious instinct seems
to be peculiarly susceptible to stimulation by different individuals
belonging to widely divergent forms. At one moment a Lapwing may be
attacked, at another a Thrush or a Starling, harmless strangers that
have approached the pool to drink; even a Water-Rail, as it threads its
way through the rushes, may fail to escape detection; and, which is
still more curious, a covey of Partridges will evoke response if they
approach the pool too closely.
Here is a curious instance of apparent waste of energy. A pair of
Magpies built their nest in an ilex tree. Early one morning there was a
commotion in the tree, much flapping of wings and a medley of sounds
which told of large birds engaged in a struggle--the Magpies were
attacking a pair of Wood-Pigeons. There was no question as to the
genuineness of the struggle, nor any doubt as to the proximate end for
which the Magpies were striving, for their efforts continued so long as
the Wood-Pigeons remained in the tree, and only ceased when they had
succeeded in driving them away.
Turning next to species which are less distantly related, we find that
instances of intolerance are more numerous and that a wider range of
species is involved. The hostility that the Lapwing displays towards the
Snipe calls for special remark. It often happens that the marshes or
water meadows, that are such favourite haunts of the Lapwing, are also
resorted to by Snipe for the purpose of securing food, or it may be even
for the purpose of reproduction. In such places both species are often
abundant; the meadow is divided up into Lapwings' territories, and early
in the season the Snipe wander over it in small parties, singly, or in
pairs. Now, if it were only on isolated occasions that the Lapwing paid
heed to the Snipe, one would not perhaps attach any peculiar
significance to the fact; but the pugnacious instinct of the bird
responds to the presence of this intruder almost as freely as it does to
that of another Lapwing. Again and again, day after day, the Snipe are
attacked and driven off in a manner which would be fittingly described
as persistent persecution, for the Snipe has neither the physical
capacity nor apparently any instinctive tendency to retaliate. Thus a
Lapwing may come suddenly upon a small party of Snipe hidden from view
in a dyke where they are probing the ground for food; the Snipe
immediately rise and fly away and there is momentary confusion as the
Lapwing darts first at this one, then at that; or, espying a Snipe at
rest at the opposite end of its territory, it will first of all run
rapidly towards it, and then fly after it, as, with twisting flight, it
darts hither and thither a few feet above the ground; or again, it will
attack and rapidly pursue solitary individuals as they skim across its
territory and attempt to settle. Is this intolerance merely an exuberant
expression of an instinct which is serviceable in another direction? The
behaviour of the Lapwing scarcely justifies such a conclusion, for all
its actions denote a striving towards some end which we can describe,
and it seems to gain satisfaction only when the ejection of the intruder
has been accomplished.
Many of the Warblers display irritation when approached by other birds
which we should scarcely expect would arouse their hostility. The
Hedge-Sparrow, for example, is frequently regarded with suspicion, and
it is by no means unusual to see it attacked by so small a bird as the
Chiffchaff. The Wood-Warbler is also pugnacious, and will even attack a
pair of Chaffinches. Between the Tit family and some of the smaller
Warblers there are constant exhibitions of hostility; even the Great Tit
is liable to be driven away, but the Blue Tit is especially marked out
for persecution, though doubtless it is well able to hold its own.
The following incident will show how real is the antagonism between
these two families. A Chiffchaff occupied the corner of a small osier
bed, and was particularly aggressive towards other closely-related forms
in its immediate neighbourhood. On two mornings in succession ten Blue
Tits invaded its ground, passing from end to end of it as they wended
their way from tree to tree in search of food. Their presence evoked the
usual hostile response, yet, withal, aroused the fear of the Chiffchaff,
which, at times, appeared to be swayed by conflicting impulses. Now, in
attempting to interpret the nature of the instinct which was evoked, one
has to be guided, in a case of this description, by the similarity of
the response to that which can be observed on other occasions and in
other situations when the intention of the bird is clear. And on this
occasion the Chiffchaff betrayed all the symptoms which normally precede
an attack; it spread its tail, quivered its wings, uttered its
high-pitched note rapidly, hopped from twig to twig, or flew restlessly
from tree to tree, and seemed to be prevented from attacking only by the
number of its opponents. This, indeed, was shown by its subsequent
behaviour, for whenever a Tit became temporarily detached from its
companions it hesitated no longer but forthwith attacked.
There are other species which are no less aggressive than the
Warblers--the Chats for example. The Stonechat regards with suspicion
almost any bird of its own size, and will even pursue a Tree-Pipit if it
approaches too closely. The same is true of the Whinchat, and one would
scarcely expect to find this bird attacking Buntings as it sometimes
does. A Whinchat that occupied some marshy ground was constantly at war
with a pair of Reed-Buntings; their territories were adjacent and in
some measure overlapped, and the Whinchat drove away either sex
indiscriminately, and was not only always the aggressor but seemed to
be master of the situation.
Coming now to kindred forms, those, that is to say, which belong to the
same family, we find that, both in intensity and extent, the warfare far
exceeds anything that we have thus far considered. So frequent, indeed,
are acts of intolerance, and so readily awakened into activity is the
pugnacious nature of the bird, that the fighting will almost bear
comparison in volume with that which occurs between individuals of the
same species. Between the Thrush and the Blackbird there are incessant
quarrels early in the year, and the initiative seems to pass from one
to the other according to the circumstances in which they are placed. If
the territory of a Thrush is invaded the Thrush is the aggressor, and,
conversely, if that of the Blackbird is threatened, the Blackbird
becomes the aggressor; and so, when the territories of the two birds are
adjacent or overlap, as frequently they do, there is constant friction,
resulting in quarrels which attract attention on account of the
noisiness of the birds.
All the Warblers are exceedingly pugnacious, the fighting being
especially severe between those that are very closely related. The
Blackcap and the Garden-Warbler are constant rivals, and the scenes
which can be witnessed when the two meet in competition are interesting
from many points of view. The birds not only pursue and fight with one
another, but their emotional behaviour reaches a high level of
intensity--excitable outbursts of song are indulged in, tails are
outspread, wings are slowly flapped, and feathers raised--in fact the
attitudes assumed are similar in all respects to those which occur
during the contests which are so frequent between the respective
individuals of each species; and it would be difficult to point to any
one item of behaviour which is not also manifest at one time or another
during the battles between these rivals, and still more difficult to
trace any difference in the intensity of the excitement. And if we are
satisfied that the fighting in the one case is purposive, so, too, must
we regard it as having some biological purpose to serve in the other.
But the Garden-Warbler is not the only bird that acts as a stimulus to
the instinct of the Blackcap; Whitethroats are often attacked, and the
Chiffchaff is a source of irritation. Even when a male Blackcap is
engaged in incubation, it will leave its nest on the approach of a
Chiffchaff, and, having driven away the intruder, proceed to sing
excitedly. At other times both male and female will combine to attack
this small intruder.
But this does not mean that the Chiffchaff suffers persecution; it is
itself most aggressive, as is shown by the fact that it will join in the
Blackcap quarrels and attack the combatants indiscriminately. Its
behaviour, however, requires further consideration, especially as
regards its relations with its nearest of kin--the Willow-Warbler; for
here we have a mutual intolerance which is somewhat remarkable, and
evidence of it can be found wherever the birds occupy the same ground.
Now it can be observed that the hostility is not limited merely to
occasional acts of intolerance, but that there is organised warfare
lasting, it may be, for many days in succession, and that the actions of
the birds bear the stamp of a persistent striving towards some end. On
one occasion the Willow-Warbler may be the aggressor, on another the
Chiffchaff, and at times it is difficult to say which of the two is
responsible for the quarrel. In size and in strength they are equal, and
the "will to fight" is as strong in the one as in the other, so that it
is seldom, if ever, possible to point to this one as the victor and that
one as the vanquished. Success or failure probably depends more upon the
cumulative effect of many combats entailing physical exhaustion, than
upon the issue of any one particular battle; and whilst observation
might quite well fail to distinguish any resultant change in the
relative positions of the birds, or any harmful effect upon their
constitutions, yet the area occupied by this one might be sufficiently
curtailed to prejudice the welfare of the young, or the vitality of that
one might be seriously impaired--and we should be none the wiser.
Neither the Marsh-Warbler nor the Reed-Warbler will tolerate strangers
within the small space of ground over which they exercise dominion. Of
the two, the Marsh-Warbler is perhaps the more pugnacious, and will
attack any other Warbler that approaches too closely; Whitethroats are
often pursued and driven away, and less frequently, Garden-Warblers. In
one case, a male occupied the same ground as a Sedge-Warbler, and there
was a constant feud between them; a willow-tree formed its headquarters,
and this same tree seemed to be the headquarters of the Sedge-Warbler,
so that they often met and whenever they did so they quarrelled. As a
rule the Marsh-Warbler was the aggressor and had the mastery over its
opponent, and when it attacked, it uttered a peculiar harsh scolding
note, raised the feathers on its back, spread out its wings, and
betrayed the usual symptoms of emotional excitement.
On the other hand, the Sedge-Warbler is most aggressive towards other
kindred species, and when a male happens to occupy the same ground as a
Reed-Warbler, there are frequent battles between them and incessant
commotion; they fly at one another and meet in the air with an audible
clicking of bills, or pursue one another amongst the reeds, each one
uttering its characteristic scolding note.
The Tits, as a family, are notoriously pugnacious. I have seen a pair of
Blue Tits attack a single Long-tailed Tit with great determination, and
not only did they pursue it, but, flying at it, struck it with
considerable force.
In giving an account of the domestic economy of the Carrion-Crow, Mr.
Edmund Selous refers to the hostility between this bird and the Magpie.
"About a week ago," he says,[6] "I saw a Crow busily engaged in chasing
away several Magpies, not only from three or four tall slender trees
close together, in one of which it had its nest, but also from various
other trees, not far off, round about. In this the Crow had a good deal
of trouble, as the Magpies were always returning. After a time it was
joined by another crow, which however did not take so active a part in
the drama, nor did I see either of the two actually go to the nest,
though I could only explain their action by supposing it was their own.
This morning I saw the same thing reversed, for a pair of Magpies, with
an undoubted nest, kept attacking a Crow that insisted on settling in
one of a row of trees--also tall and slender--in which it was placed.
Both were equally persevering--the Crow, though often chased away,
always returning, and settling generally in the last tree of the row,
where he would be left alone sometimes for a minute or two, but before
long one of the Magpies flew at him, and put him to flight. The Crow
defended itself, but not, it would seem, very successfully, and in the
last attack upon him, made, with great spirit, in the air, a large black
feather floated to the ground, which I made no doubt was his. Yet this
did not drive him from the trees, and it was only on my approaching
nearer that he finally left them. Thus we see that both species look
upon the approach of the other to within a moderate distance of their
nest as an intrusion."
That the Rook suffers persecution from the Carrion-Crow is a
well-established fact, and there is reason to believe that it has
another dangerous enemy in the Hooded Crow. According to the late
Mr. Ussher, Choughs will attack both Hooded Crows and Ravens. "I once
saw," he says, "two Choughs energetically attacking a pair of Ravens;
they shot up into the air and darted down on the latter, whose heavy
flight made them helpless against their agile tormentors."
Birds of prey are often hostile to one another. The Merlin is
exceptionally pugnacious, and its boldness in attacking intruders is
well known. When, for example, a Kestrel approaches its territory, it
leaves the tree, bush, or rock upon which it was resting, utters its
characteristic cry, and soars rapidly upwards; then, rising to a
considerable height, it swoops down upon the Kestrel, and by
alternately stooping at and chasing its opponent, drives it away from
the immediate neighbourhood.
What we have, then, to consider is, Do these battles between different
species contribute towards the attainment of the end for which the whole
territorial system has been evolved?
Let us take the individual and see whether we can establish any relation
between the hostility it displays towards members of other species and
its general disposition to secure a territory. We must remember that a
male can have no knowledge of the prospective value of its behaviour,
nor is it likely that it has any ulterior purpose in ejecting other
males, beyond the pleasure it derives from satisfying its impulse to do
so. The proximate end of its behaviour is to attack, nothing more, and
this, of course, it can only do just in so far as the intruder evokes
the appropriate instinct.
Now the arguments we shall employ will, on the whole, be similar to
those which we made use of in the second chapter, wherein we attempted
to ascertain the conditions under which a male becomes intolerant of
other males of its own species, and examined more especially the claims
of the "territory" as opposed to those of the "female." But here we
start on firmer ground, because the one factor which introduced an
element of uncertainty--the female--can be definitely excluded; at least
it seems so to me, for granting even that her presence is the condition
under which the pugnacious nature of the male is rendered susceptible to
stimulation, it is difficult to see why a male of a different species
should supply that stimulus, or what biological purpose could be served
by its doing so.
When dealing with the attitude of a male towards others of its kind, we
attached considerable significance to the fact that its pugnacious
nature gained or lost susceptibility according to the position which it
happened to occupy. We found, it will be remembered, that the same bird
that was pugnacious in its own territory took no further interest in its
opponent when the boundary was passed; and, moreover, that if it
happened to wander into an adjoining one, it made no real effort to
defend itself when attacked, but returned forthwith to its own
headquarters. It remains to be shown whether the rivalry between
different kinds of birds is similarly related to the position which the
opponents happen to occupy at the time.
First, then, there is the general consideration, namely, that the enmity
occurs for the most part just at the time when the territories are in
process of being established. During autumn and winter, many birds of
more or less close affinity assemble together in flocks, wherever the
supply of food is abundant, and are then not only sociable, but, so
there is reason to believe, are mutually helpful both in discovering the
necessary means of subsistence which are often none too plentiful, and
in affording protection from enemies, which, on the contrary, are often
numerous. That the different units of which these flocks are composed
should live on amicable terms is therefore as necessary for the welfare
of the whole community at this particular season as that the different
individuals of the same species should do so. But just as the sociable
relations, which obtain between these individuals throughout the winter,
undergo a marked change at the commencement of the breeding season, so,
too, do different species, which habitually associate together, suddenly
become hostile to one another. This change is coincident in time with
the rise of the organic condition which leads to the establishment of
territories; and the hostility continues, though in diminishing degree,
throughout the breeding season, and dies away the following autumn.
For example, different Warblers resort to the elders (_Sambucus nigra_)
in September, and there pass much time feeding on the fruit which is
then ripe and often abundant. In the same bush there may be Blackcaps,
Garden-Warblers, Whitethroats, and Lesser Whitethroats, some preening
their feathers, others searching for the berries, others again, with
feathers relaxed, making feeble attempts to sing. Occasionally there may
be a scuffle, perhaps between a Blackcap and a Lesser Whitethroat, or
between a Garden-Warbler and a Blackcap, but it is of short duration and
lacks vigour. Apart, however, from such temporary disturbances, there is
no real rupture in their relations, and certainly nothing to lead one to
suppose that the bickerings are determined by the functioning of any
specific instinct. Yet only a few months previously some of them were
constantly at war, and their quarrels betrayed symptoms of great
persistence; and if we remember how the observed behaviour of the birds
suggests the fact that they were striving to attain something definite,
we shall understand the nature and extent of the change, and shall, I
fancy, be in a better position to estimate its biological worth at its
true value.
We can find many similar examples--flocks are to be found on arable
ground, on the water meadows, and on the mud-flats; here different kinds
of Thrushes feed on the berries of the yew, there different kinds of
Tits travel together in parties; hosts of Finches collect in the hollies
to pass the night and Buntings roost together in the gorse; and, in
fact, in whatever direction we choose to look in the autumn and winter,
we find various birds assembled together and living on amicable terms.
All of this changes in the spring, and the relationship undergoes a
gradual but noticeable alteration; so much so that whereas the
outstanding feature of bird life in the winter is sociability, that of
the spring is hostility.
So much, then, for the seasonal change of relationship; let us now turn
to particular cases and attempt to trace the condition which accompanies
such change.
Many migrants in the spring seem to follow the course of the Severn
during their journey northwards through Worcestershire; and where the
river bends to the north-west at Lincombe Lock, there they leave it, or,
rather, continue in a north-easterly direction which takes them across
the southern end of Hartlebury Common. As I have already mentioned, this
Common is overgrown with gorse, heather, and ling, and scattered here
and there are a number of dwarf oak-trees and small elder-bushes. The
situation is therefore an ideal one for the smaller migrants to rest for
a brief time, and, from the point of view of the observer, very suitable
because it is open and the movements of the birds can be traced for some
distance. Turtle Doves pass over at a great height, or skim across a few
feet above the gorse; Redstarts settle for a few minutes and then
disappear; Tree-Pipits, Whinchats, and Willow-Warblers pass from tree to
tree or flit from bush to bush--and all in a north-easterly direction.
They do not sing, they are restless, and, judging by their behaviour,
they are anxious to conceal their presence, not to make it known. Yet we
know that when they reach their destination, as presently they will, all
this will change; that each of them will employ every means at its
disposal to make itself conspicuous; and that each, as far as it is
able, will resist intrusion on the part of other species.
Now the southern end of the Common is always inhabited by individuals
belonging to one of these species, or to others of close affinity; so
that wherever these travellers settle whilst passing across it, the
chances are that they will find the ground occupied--and their behaviour
under such circumstances is no less interesting than the behaviour of
the bird upon whose ground they are trespassing. We will take the case
of the Whinchat. It arrives from the south-west, and, flying from bush
to bush, works its way in a north-easterly direction. In doing so it
intrudes upon the territory of a Stonechat; and the Stonechat, becoming
excited, flies towards it, and it retires for a short distance in the
direction from whence it came. Here again it is followed and attacked
and again moves on, and then, flying in a circle as if to avoid the
territory which blocked the path, resumes its former line of flight,
though still followed by the Stonechat, which after continuing the
pursuit for perhaps a quarter of a mile, suddenly turns in the air and
returns to its headquarters.
It is difficult to put oneself in the place of the Stonechat or of the
Whinchat. But even after making due allowance for the danger inseparable
from any attempt to do so, there remains the unquestionable fact that
whereas the impulse to attack was strong in the one, the impulse to
defend itself was wholly lacking in the other. Yet a Whinchat, when it
has established itself, is most pugnacious; it not only attacks every
bird of a similar size that approaches its position, but its behaviour
under such circumstances bears the impress of unusual determination; and
if we were to take a male and place it in the position of the Stonechat,
we should find that its nature would change, that the presence of the
Stonechat would evoke a hostile response, and, conversely, that the
instinct of the Stonechat would not be susceptible to stimulation. Hence
it is clear that the nature of a bird when on migration is not quite the
same as it is when its destination is reached; that the positions
occupied from time to time during the journey carry no meaning, or,
rather, are not brought into relation with its life in quite the same
way as is the position which it finally occupies; and further, it is
clear that the interest it displays in other species undergoes a
somewhat remarkable transformation when at length its destination is
reached.
This altered nature of the migrant is a fact of some importance in
relation to our present subject, but it does not stand alone--the same
characteristic is observable in other phases of bird life. Some of the
residents, the Buntings and the Finches for example, occupy their
breeding ground very early in the year, and it often happens that the
situations which they select are not capable of supplying them with food
so early in the season, though at a later date food will be there in
abundance; so that they are compelled to resort to the surrounding
neighbourhood, and since, even there, the available supply is sometimes
scarce or, if plentiful, limited to certain areas, they are constrained
from time to time to join together again in flocks. Thus, for part of
the year, they may be said to lead a double existence; for just as the
Whinchat, that is sociable on migration, betrays a changed nature when
it reaches its destination, so too does the nature of these residents
change from hour to hour according to whether they are seeking food or
occupying the breeding ground.
In the newly-sown fields of grain the birds frequently find a supply of
food. Here Yellow Buntings, Greenfinches, and Chaffinches collect from
the surrounding neighbourhood. The majority are somewhere in possession
of territories, and not a few are paired. Between the territories and
the feeding ground a highway is formed by individuals passing to and
fro. Sometimes both members of the pair leave together in order to seek
food, at other times they separate and the male may be in his territory
whilst the female is with the flock. Apart from occasional
manifestations of sexual emotion on the part of a male, there is nothing
to disturb the harmony of the flock nor anything in the behaviour of the
birds which would lead one to suspect that, when they return, their
nature will change and that they will be no longer sociable; and, which
is still more remarkable, no matter how great the provocation which an
individual, when in company with the flock, may be called upon to
endure, its customary hostile response will fail to be elicited. An
incident which happened in the spring of 1917 will serve to make this
clear. A flock of some thirty Yellow Buntings, Greenfinches, and
Chaffinches were feeding in one corner of a field which had recently
been sown with barley. As they sought their food they wandered outwards
into the middle of the field, and in so doing, passed across the
territory of a Skylark. Whereupon the Skylark became excited, uttered
its call-note rapidly, and rising a few feet from the ground, attacked
those members of the flock that were nearest, which happened to be the
Yellow Buntings; and so determined were its onslaughts that the Yellow
Buntings were forced to retire. The Skylark showed no discrimination as
to sex, but attacked both males and females, and within a few minutes
succeeded in driving away at least two pairs. One would have expected
that the Yellow Buntings would have made some show of resistance; one
would have thought that the fact of being violently attacked would have
supplied a stimulus sufficiently strong to evoke a corresponding hostile
response: yet there was no mistaking the lack of interest that they
displayed in the contest--they made no effort to retaliate but seemed to
accept the situation as unalterable and left.
So far we have examined only those cases in which the pugnacious
instinct was stimulated in one of the adversaries, and in which
consequently the fighting seldom reached any high degree of severity. We
must now consider some others in which each of the opponents acts as a
stimulus to the pugnacious instinct of the other. It is here, of course,
that we find the most violently contested battles, and it is here, too,
that the purpose of the fighting seems clear. The persecution which the
Green Woodpecker suffers from the Starling is well known. The purpose of
the Starling's behaviour is clear, namely the possession of the hole
occupied by the Woodpecker. Bird for bird, the Woodpecker is more than
the equal of the Starling, but persistent endeavour ultimately wins the
day. The Starlings perch close beside the hole, and, whenever the
Woodpecker shows itself, attack with determination; and not only do they
do so but they are assisted, so there is reason to believe, by other
individuals or pairs in the attainment of their end, so that no matter
how stoutly the Woodpecker defends itself, in time it is almost certain
to be deprived of its ownership.
In like manner different kinds of Woodpeckers contend with one another
for the possession of a hole, and here the opponents are more equally
matched. I have seen a pair of Lesser Spotted Woodpeckers endeavouring
to drive away a Great Spotted Woodpecker. The excitement of all three
birds was exceptional. Each of the Lesser Spotted Woodpeckers kept
swooping in turn at their rival, sometimes in the air and sometimes when
it was settled on the topmost branches of a dead tree, and the sounds
produced reminded one of the piping of a flock of Oyster-Catchers in
flight.
A battle between a pair of Green Woodpeckers and a Great Spotted
Woodpecker is worth mentioning. It occurred on the 24th of April.
Passing through the middle of a wood, I noticed a Great Spotted
Woodpecker fly out of a hole in an oak-tree. Shortly afterwards, a pair
of Green Woodpeckers settled near the hole and then flew to some
oak-trees close at hand, where they were joined by their rival and signs
of hostility were soon apparent. Presently the Great Spotted Woodpecker
returned to the hole and entered. Both of the Green Woodpeckers then
flew into the tree; and one of them, settling upon the trunk, climbed
up to the level of the hole and, when it became aware of the Great
Spotted Woodpecker within, extended its wings fully and proceeded to
peck viciously at its opponent. Whereupon there was a scuffle at the
mouth of the hole and the Great Spotted Woodpecker hurriedly left. After
this, all was quiet and the Green Woodpecker eventually descended and
entered the hole. The Great Spotted Woodpecker, however, returned again,
but, after fluttering around the hole, disappeared, leaving the Green
Woodpeckers in possession.
In this varied field of hostile behaviour which we have explored, one
feature stands out prominently, namely, that the interest which a bird
displays in other species varies not only at different seasons but even
from hour to hour. I have used the word "nature" as equivalent to
"interest," and I have spoken of the bird's nature changing or altering
according to the circumstances in which it was placed. But its nature is
its inborn constitution, and its constitution cannot change from day to
day, still less from hour to hour. So that, in a sense, and having
regard to strict scientific accuracy, it is misleading in this
particular connotation to say that the bird's nature changes.
[Illustration: H. Gronvold dcl. Emery Walker ph.sc.
A battle between a pair of Green Woodpeckers and a pair of Great
Spotted Woodpeckers for the possession of a hole in an oak tree.]
What then does happen? The instinct of pugnacity must form just as much
a part of the hereditary make-up of the migrant, when on migration, as
when finally it reaches its destination; still more must it form part of
the constitution of the Bunting when it leaves its headquarters
temporarily and joins the flock. And, if it is there, the question
arises as to why it does not respond. Now every instinct requires for
its response a stimulus of an appropriate kind, and, therefore, a
reasonable view to take would be that the necessary stimulus was
lacking. But this is a view which we cannot uphold, because on all these
occasions an opposing male was present--and, so far as it is possible to
judge by observation, that is the stimulus which in the main evokes a
hostile response. We must therefore look elsewhere than in its
direction for a reason which will adequately explain the behaviour.
Though it be true that every instinct requires for its functioning a
stimulus of an appropriate kind, yet it is also true that the condition
which will render it responsive must be present. What we have then to
consider is whether the phenomena which we have explored give us any
clue as to the particular nature of that condition. In the first place,
we have the general fact that the hostility is not confined to a few
species belonging to a few families, but that it is of wide
application--birds of prey, Warblers, Woodpeckers, all supply us with
evidence which serves to show, in greater or less degree, its nature and
extent. Next, we found that the hostility was peculiar to a certain
season--and that one the season of reproduction. And if the question
were asked: What condition would then be most likely to render the
instinct susceptible, the answer that would most certainly be given
would be--the presence of a female. And in reply to a further question
as to the particular nature of the stimulus to which the instinct would
respond, we should be told--the presence of another male of the same
species. Now the possible influence of the female on the course of the
male's behaviour was the subject of inquiry in the second chapter,
wherein we endeavoured to explain the hostility between males of the
same species, and we came to the conclusion that it was not alone
sufficient to account for the facts disclosed. Still less likely,
therefore, is it that her presence can bear any direct relation to the
hostility between different species, the more so since the biological
end of securing a mate is definitely excluded. And we have something in
the nature of proof of the correctness of this view in the fact that she
accompanies her mate when he joins the flock, and that there his
instinct is not susceptible to stimulation. We then proceeded to examine
certain cases in which all the indications pointed to the fact that the
"will to fight" was present in only one of the opponents; and we
attached considerable importance to this circumstance, because we knew
from experience that the same bird which seemed to lack courage, could
at other times and in other situations be most aggressive. If then we
ask what condition was present on the one occasion that was absent on
the other, we have no difficulty in finding a reply--on every occasion
on which the opponents appeared to be unevenly matched, one was in
occupation of a territory and the other was not. And if we inquire
further as to which of the two was the aggressor, the answer is again
clear, namely, the bird that occupied a territory. Finally we considered
some particular instances in which the "will to fight" was present alike
in both opponents, and in which the battles were protracted and severe.
But the fact that a bird has established a territory is not in itself
sufficient to render its hostile nature susceptible; it must be actually
in occupation if a response is to be elicited. We reach this conclusion
step by step: the behaviour of the migrant, that lacks the "will to
fight" when on migration but is pugnacious when it has secured a
territory, shows it; the behaviour of the resident, which temporarily
joins the flock and is there sociable, shows it; and it is shown also by
the determination with which both opponents fight when the question of
ownership of a station is in dispute. And of all the facts we have
reviewed, this is perhaps the most important in relation to our present
subject, for it demonstrates that the change from sociability to
hostility is not merely an incident of the sexual season, not merely an
indirect result of the functioning of the general disposition which
leads to the establishment of a territory, but that it is intimately
associated with the whole process, and that the particular part of the
bird's nature which is concerned is so nicely balanced that it will
respond under one condition and one only.
Thus we are led to the only conclusion which seems consistent with the
facts, namely that there is a relationship between the "territory" and
the hostility.
If we are satisfied that all this warfare is not merely an expression of
an instinct which is serviceable in another direction, what part does it
play in the whole scheme of reproduction?
The young of many birds are delicate at birth and unable to withstand
exposure to cold, and in the previous chapter we came to the conclusion
that the territory was serviceable in that it provided an adequate
supply of food in the vicinity of the nest, and thus obviated the
necessity of the parents being absent from them for long. But manifestly
no matter how active a male may be in driving away members of its own
sex and kind, it will neither make its position secure, nor insure a
supply of food for its young, so long as any number of individuals of
different kinds are allowed to establish themselves in the same space of
ground. On the one hand, then, we have the fact that there is constant
strife between males of close affinity, whilst on the other, we know
that many species require like conditions of existence and are bound to
assemble wherever these conditions are suitable; and we can infer that
the territory would fail to serve its purpose if no restriction were
imposed upon the measure of such assemblies.
The question then arises: Does all this warfare contribute towards the
attainment of reproduction? Not far from my house there is a small water
meadow, three acres in extent, which for some years has been derelict
and is now overgrown with the common rush (_Juncus communis_) and small
alder trees. For three successive seasons I watched the bird life of
this meadow, and more especially the Reed-Buntings whose behaviour I was
studying at the time. In every respect the meadow was suitable for this
bird; there was an abundance of food and numberless situations in which
nests could be placed. Each year all the pairs were successful in
rearing one, if not two broods, yet the number of pairs never exceeded
five--the first year there were three; the second year five; and the
third year four. In addition to the four pairs of Reed-Buntings, there
were in the spring of 1915, six pairs of Whitethroats, one pair of
Lesser Whitethroats, four pairs of Willow-Warblers, one pair of
Sedge-Warblers, two pairs of Grasshopper-Warblers, one pair of
Chiffchaffs, three pairs of Hedge-Sparrows, two pairs of Tree-Pipits,
one pair of Skylarks, one pair of Whinchats, one pair of Flycatchers,
two pairs of Song-Thrushes, one pair of Blackbirds, one pair of
Redstarts, three pairs of Chaffinches, and one pair of Wrens--in all,
thirty-five pairs, whose young were mainly dependent for their living
upon the insect life of that meadow and the ground immediately
surrounding it. If we allow three young to each pair--and this would
take no account of second broods--we arrive at the following result,
namely, that one hundred and five young and seventy adults had to be
supplied with food from that locality, which would mean, if the search
for food were strictly limited to that meadow, that 83 square yards
would be allotted to each individual.
Suppose now that the four male Reed-Buntings had each admitted one other
male, and that they had secured mates, what would have been the effect
upon the whole community? The four additional pairs with their young
would have represented twenty individuals, which would have represented
a decrease of 8.5 square yards in the space allotted to each individual.
The pressure of the bird population upon the means of support would then
have been materially increased; and not only the Buntings, but the
Warblers, Pipits, and all the rest would have suffered. But the result
would have been the same if, instead of the four additional male
Reed-Buntings, four males of other kinds had been allowed to enter the
marsh, and we can multiply the number four until we arrive at a point
when the means of subsistence would no longer have been adequate for the
adults, still less for the young. If, then, there were nothing to
prevent this happening, many of the birds in that marsh would have no
chance of rearing their young successfully. Hence, if the territory is
adequately to serve the purpose for which we believe it has been
evolved, some provision must have been included in the system to meet
the difficulty.
There are three ways by which this may have been
accomplished--indirectly, by increasing the size of the area occupied by
each individual, and thereby reducing the relative number of each
species; or directly, by rendering the fighting instinct of the bird
susceptible to stimulation by individuals of other species; or,
possibly, by a combination of the two. There were four pairs of
Reed-Buntings in the marsh, and their territories covered the whole of
it. But inasmuch as other insectivorous birds were established there
also, and found sufficient food to maintain both themselves and their
families, it is clear that the area these Reed-Buntings occupied was in
excess of that which they would have required if they had been the sole
inhabitants. And such often appears to be the case. Many a Warbler
allocates to itself a space of ground more than sufficient to supply it
with all that it needs; so, too, does the Finch, or the Pipit, or the
Falcon--if we take no account of kindred species. Thus there is reason
to believe that, by limiting the number of individuals in a given
locality, this apparently wasteful expanse of territory is serviceable
in that it provides against the pressure of the bird population upon the
available means of support becoming too great. But though a reduction in
the numerical standing of the different species would certainly follow
from any increase in the area occupied by the respective individuals,
and with even greater certainty would place them in a more secure
position as regards their supply of food, yet, when we remember how
large a number are dependent upon a supply of insect life for their
young, we can understand that it would not alone be a sufficient
safeguard against the dangers attendant upon overcrowding. It is here, I
believe, that we shall find the true explanation of the hostility; it
roughly insures that the number of pairs in any given area does not
exceed the available means of support, and indeed it is difficult to
imagine how such uniformity of distribution as would free the young from
the risk of exposure could be obtained without some such control.
Some birds, however, have no difficulty in finding the necessary food
for their young, yet have great difficulty in finding a station where
they can rear their young in safety; and the area each one occupies has
been reduced to the smallest proportions in order that the maximum
number can be accommodated. Here, any increase in the size of the
territory would inevitably lead to the extinction of the race, so that
nothing stands between failure and success except the ability of the
bird to defend its territory. If we study the bird population at one of
the breeding stations on the coast, we find, generally speaking, that
each kind of bird inhabits a particular portion of the cliff; on the
lower ledges are the Guillemots and Kittiwake Gulls; higher up are
Razorbills and Fulmars, and at the top, where the cliff is broken and
the face of the rock covered with turf and soil, the Puffin finds
shelter for its egg. At the same time there is much overlapping; the
kind of ledge that suits a Razorbill is equally suitable for a Guillemot
or a Fulmar, and so, no matter how successful the Razorbill may be in
establishing a territory and preventing intrusion upon it by other
Razorbills, it will be all to no purpose if it allows itself to be
jostled out of its position by a Fulmar. Hence, inasmuch as breeding
stations are limited and competition for territory so severe, only those
forms in which the fighting instinct responds freely to a wide range of
stimuli will be in a position to maintain a footing upon the cliff.
In trying to estimate the importance of the hostility in its relation to
the territory, we must bear in mind that competition varies in different
seasons and in different localities. The surface of the land is
constantly undergoing modification, partly owing to human and partly to
physical agency--forests are cleared; marshes are drained; the face of
the sea-cliffs is altered by the erosion of the waves; here the coast
may be locally elevated, there locally depressed; and so forth. Many of
these changes are slow and imperceptible, many can be observed in our
own lifetime. The timber is felled and the undergrowth cleared in some
wood, and the following spring we notice a change in the character of
the bird population. Migrants which formerly found in it no suitable
accommodation now begin to appear, and as the seasons pass by and the
undergrowth affords more and more shelter for the nests and an
increasing supply of insect life, so their numbers increase until the
wood becomes an important breeding station, resonant with the song of
many individuals. But slowly the growth increases; the bushes pass into
saplings and the saplings into trees, and the undergrowth then
disappears just as surely as do the migrants which can no longer find
there the conditions which they require.
Or, as an illustration of the effect produced by natural agency, let me
describe a change which has taken place in a corner of Co. Donegal. The
promontory of Horn Head is bounded on the west by extensive sand-hills,
100 ft. or more in height. On the southern side it is divided from the
mainland by a channel, which narrows down to 100 yards or so in width
where it fringes the sand-hills, and then widens out again, covering an
area of approximately 270 acres. As far as is known in the memory of
man, this area has always been tidal. But in recent years a change has
taken place, and the blown sand has silted up the channel, with the
result that this tidal area has been transformed into a brackish lake.
What has brought about the change is not easy to determine. There is
evidence, however, of a slow alteration of the level of the shore-line;
for in the midst of the sand-hills, situated 150 yards or so from the
present sea-margin, and running parallel with it, there is an
accumulation of pebbles some 3 feet high by 4 feet deep. This raised
beach is now separated from the Atlantic by sand-drifts of considerable
height, and consequently there are some grounds for believing that
secular elevation is taking place, which, if it be the case, will
account for the change in progress. Now the effect on the bird
population can be seen even now, and will doubtless become more apparent
as the years pass by. Sand-Martins used to find plenty of places to
breed amongst the sand-drifts, and moreover do so still. But their
nesting sites are constantly changing and disappearing, and the
breeding-place of one colony, that was situated in the bank of a stream
twelve years ago, is now buried 10 feet or more below the surface of the
sand. The area that was once tidal, but is now a brackish lake, is fed
by mountain streams, and as the fresh water predominates, so in course
of time will it become fringed with vegetation; and instead of the
flocks of Curlew, Dunlin, and other waders that, at low water, resorted
there to feed, Coots will fight with one another for the possession of
territories, and the Wild Duck will teach her young to seek their food.
In whatever direction we turn, we find that many breeding grounds are
subject to incessant change. Ancient haunts disappear, new ones come
into being, a change which makes life impossible for this bird, as
likely as not benefits that one, and so on. There is no stability. Hence
in any given district each recurring season there must needs be a large
number of individuals which are obliged to seek new stations, and if
there were no control over their distribution, if each one were free to
establish itself wherever it chanced to alight, this locality might be
overcrowded and that one deserted; and, bearing in mind how many species
there are that require similar conditions of existence, we can infer
that the successful attainment of reproduction would become impossible
for many of those individuals so long as each species was indifferent
to the presence of the others. On the other hand, if there were no
control over the range of the intolerance, the smaller bird would have
no chance in competition with the larger, and it is doubtful whether the
larger would gain an advantage commensurate with the energy it would
expend in ridding its area of the smaller. I have described battles in
which the opponents were only distantly related; for instance, the
Moor-Hen will attack almost any bird--Partridge, Lapwing, or
Starling--that approaches its territory even temporarily. Nevertheless
the antagonism between kindred forms is more prevalent, and, as a rule,
characterised by more persistent effort; and thus it seems as if the
susceptibility of the fighting instinct has its limitations, the degree
of the responsiveness being dependent upon the affinity of the
opponents.
Suppose now that we take an area inhabited by a number of different
species requiring like conditions of existence, divide it into three
sections, and imagine that in one they were all sociable, that in
another they were all hostile, and that in a third those which were
closely related were intolerant of one another. Let us suppose further
that each one of them was represented by the full number of individuals
that the law of territory would allow. In the first section an
individual would establish itself, and, becoming intolerant of its own
kind, would exercise dominion over an area roughly sufficient, providing
conditions were normal, to insure an adequate supply of food for its
young. But it would take no account of other species, and since any
number might occupy the same ground, the fact of its having established
a territory would not alone suffice to render its supply of food
secure. Success in the attainment of reproduction would then become
largely a matter of chance, depending upon the number of individuals
that happened to settle in this place or in that. In the second section
there would be perpetual warfare; for whereas the appropriate organic
condition which leads to pairing arises in different species at
different times, fresh claimants to occupied ground would constantly be
appearing, and the efforts of the inhabitants to preserve their
boundaries intact would have to be maintained throughout the whole
period of reproduction; and while the stronger or more persistent forms
would be more likely to breed, they would do so at the expense of their
young, to which they would be unable to devote proper attention, and
with an expenditure of energy that would reflect itself upon the future
of the race. But the conditions of life in the third section would be
such as would be more likely to yield good results. The relations of the
different members of the community would be more evenly balanced, for a
male would only be called upon to compete with those of its own size and
strength. Thus, on the one hand, accommodation would be so divided as to
secure the breeding of the maximum number of individuals with the
minimum of expenditure of energy, whilst on the other, any undue
pressure upon the available means of subsistence would be prevented.
There can be no question that in the latter section a higher percentage
of individuals would succeed in rearing offspring. And so, by reason of
the fighting instinct being more susceptible or less susceptible
according to the affinity of the opponents, a control is established
which, while preventing unnecessary extension of warfare, allows for
sufficient extension to render the biological end secure.
These, then, are the facts--this the conclusion which can be drawn from
them. It may, however, be said of these facts, as it has been said, with
even less justification, of the battles between individuals of the same
species, that they do not afford evidence of genuine hostility. No doubt
there are many naturalists who could supplement these facts with others
in which the conflicts resulted in bodily injury, or terminated fatally,
or at least were of a more determined kind. But I have already drawn
attention to the fact that, so long as a definite result is attained,
the severity of the struggle and the amount of injury inflicted are
matters of small moment. Let us, however, run over the substance of the
argument, and then briefly refer again to this point of view.
After enumerating instances of hostility, sufficient in number, so it
seemed, to constitute reasonable ground for the belief that they had a
part to play in the life-history of the individual, the two questions we
set ourselves to examine in this chapter were: Is there any circumstance
in the life behaviour of the individual with which the hostility can be
definitely related; and, will the hostility lead to the securing of a
greater measure of success in the attainment of reproduction?
Many different species assemble together in winter and roam from place
to place in search of food. But in spring their behaviour undergoes a
remarkable transformation; they avoid one another and become
quarrelsome, so much so that whereas the outstanding feature of the
winter is sociability, that of the spring is hostility. With this
general fact before us, we proceeded to investigate this change of
behaviour still further. First of all we took the case of a migrant,
and, comparing its behaviour, as it journeyed, with that when finally it
reached its destination, we found that the bird which was notoriously
pugnacious when in occupation of a territory betrayed no interest in
other species as it travelled to the accustomed breeding ground. Not
only so, but even though it was attacked, we found that its pugnacious
instinct still failed to respond. Here, however, it may be contended,
and with reasonable justification, that in the interval which elapses
before the ultimate destination is reached, some change in the organic
condition of the bird may occur which will account for its altered
behaviour; or, it may be urged, with no less justification, that whereas
on migration the bird is unpaired, when the destination is reached it is
probably in possession of a mate and is therefore quarrelsome. Now, at
the most, the interval can only be a matter of a few days, and it is
unlikely that organic changes sufficient to bring about so important an
alteration of behaviour could occur in so short a time, still less
likely that they could be timed to come into functional activity just at
the moment when the bird reaches its breeding ground. And with regard to
the suggestion that the change can be accounted for by the presence of a
mate, we shall do well to remember not only that males as a rule precede
the females by some days, but that a male may even remain in its
territory, mateless, for some weeks, and yet display hostility.
Nevertheless the case of the migrant did not, by itself, afford
sufficient evidence upon which to base any conclusion. We therefore
inquired into the behaviour of some of the residents at a corresponding
period. The Bunting served as an illustration. Early in the season it
establishes a territory, and because food is then scarce it is forced to
seek it elsewhere than on the small plot of ground which it has
acquired; and so it makes its way to some spot where the supply is
abundant, and there, meeting with other species bent on a similar
errand, forms with them a flock. Part of its time is then spent in the
territory and part on the feeding ground, and between these two points a
highway is formed by the bird passing constantly to and fro. But the
attention which it pays to other species is very different on these two
occasions--when in the territory it is intolerant of strangers, but when
it accompanies the flock it displays no interest in their movements.
From hour to hour its nature seems to change. But, as we saw, the inborn
constitution of the bird cannot change, and therefore we came to the
conclusion that an explanation of the altered behaviour was to be found
in the fact that the pugnacious instinct is only rendered susceptible
under a certain condition. So that all the evidence tended to confirm
the impression which we had gained from the course of events in the life
of the migrant, namely, that the hostility bears a direct relation to
the occupation of a territory.
Finally we were led to inquire whether the hostility was serviceable in
promoting the welfare of the individuals. We saw that many different
species require similar conditions of existence, that ancient breeding
haunts disappear and that new ones come into being, and that in the
ordinary course of events such species must often assemble in the same
area for the purpose of reproduction. So that even though a male might
be successful in protecting its ground from intruders of its own kind,
yet it might still fail to rear offspring, just because it happened to
choose a position in which other kindred forms had gathered. Hence if
the territory is adequately to serve its purpose, some control over the
local distribution of species is of paramount importance. Nevertheless,
if all the different forms that require similar conditions of existence
were intolerant of one another in a like degree, the smaller bird would
have no chance in competition with the larger. This, however, is not
the case. Some, as we saw, arouse little or no animosity in others, in
fact the more closely related the rivals, the more responsive their
pugnacious nature seems to become.
To return now to the view that the fighting is not really serious, but,
on the contrary, that it is either vestigial and has no longer any part
to play in furthering the life of the individual, or that it is a
by-product of the seasonal sexual condition to which no meaning can be
attached. First, there is the relationship with the territory, and this,
it seems to me, is a fact of some importance; for if the fighting were
merely an exuberant manifestation of sexual emotion, one would expect to
find it occurring under all conditions, and not merely under one
particular condition in the life of the bird. The hostility is too
widespread, however, and too uniform in occurrence for us to suppose
that it has no root in the inherited constitution of the bird; and if it
served some useful purpose in the past, the instinct might still
persist, so long as it were not harmful. Thus the view that the
behaviour is vestigial is not perhaps unreasonable. But manifestly it
makes no difference whether it be vestigial or a by-product of sexual
emotion, whether the battle be fierce or so trivial as to appear to us
to be more in the nature of "play," so long as some change in the
relative prospects of the opponents is the result.
For us, then, the main consideration lies in the question: Is the
behaviour serviceable now in furthering the life of the individual?
Whether the evidence which we have examined affords sufficient ground
for the belief that the hostility is genuine and has a part to play in
the whole scheme of reproduction, each must judge for himself.
CHAPTER VII
THE RELATION OF THE TERRITORY TO MIGRATION
Coincident in time with the growth of appropriate conditions in the
environment, organic changes take place rendering certain instincts
susceptible to stimulation; and the stimulus being applied, the Warbler
leaves the country wherein it had passed the winter and finds its way
back, with apparently little difficulty, to the district in which it was
reared or had previously reared offspring. What is the nature of these
changes and of the impulse which is first brought into functional
activity; whence comes the stimulus; and what directs the bird on its
journey--these are all different aspects of one great problem, the
problem of migration. I do not propose to discuss all these various
aspects, for indeed I have no suggestions to offer which are in the
least likely to be helpful, but I seek rather to ascertain whether the
phenomena which we have explored bear any relation to the problem as a
whole; whether, that is to say, the competition for territory and all
that appertains to it can have supplied the conditions under which, in
the process of time, this complex and definite mode of behaviour has
evolved.
We are sometimes told that we must seek the origin of migration in the
physical changes that have occurred in the ancient history of the
earth--in glacial conditions which gradually forced birds to the south,
or in the "stability of the water and mobility of the land" which
brought about a gradual separation of the feeding area from the breeding
area--and which continued for a sufficient length of time to lead to the
formation of an instinct, and that the instinct persists because it is
serviceable in promoting the welfare of the race. But when we consider
the lapse of time, and the changes that must have occurred in the
character of the bird population--the appearance of new forms and the
disappearance of the old, the ebb and flow of a given species in a given
area--and bear in mind that, notwithstanding this, the migratory
instinct, if not stronger, is assuredly no less strong, and the volume
of migration, if not greater, is assuredly no less; in short, that the
whole phenomenon is progressive rather than retrogressive, we shall find
the view that the instinct owes its origin to conditions which no longer
exist, receives but little encouragement.
I doubt not that, throughout the ages, geological changes have been an
important factor in directing or limiting the scope of migration, and
moreover are so still; just as climatic changes and the relative
abundance or scarcity of enemies have influenced the course of its
evolution. These are all contributory factors operating in the external
environment. But there are, besides, internal factors which form part of
the inherited constitution of the bird, and, being passed on from
generation to generation, afford the conditions under which migration is
constantly being renewed. It is, I believe, in this field of organic
change and relationship that the conditions of origin must be sought.
Just as the moth in passing from the rudimentary to the perfect
condition runs through a series of changes, each one of which is marked
by a typical behaviour response adjusted to meet some particular
circumstance in the external environment, so the annual history of a
bird displays an ordered routine, each phase of which can be observed to
correspond with one of the successive changes in the environment. In
almost every direction, we find that this routine is characterised, in
broad outline, by great uniformity; so much so that, providing we know
the history of one species, we can forecast with no small degree of
certainty the general course of behaviour of other members of the
family. But only the _general_ course. There is endless variation in
just the particular way in which the behaviour is adapted to meet the
needs of particular species--the major details may be said to be
specific, the minor details varietal.
Now it is that part of the behaviour routine which has reference to the
relationship between one bird and another upon which, for the time
being, I wish to dwell; for the interest that A displays in B is by no
means always the same--it changes according to the season, and this
change can be observed to be uniform throughout a wide range of species.
In winter, in whatsoever direction we turn, we observe not only that
different individuals but that different species also collect together
in flocks. And since food at that season is not always easy to obtain,
and, moreover, is only to be found in certain situations, which are
limited both in number and extent, it would seem that such assemblages
are in the main determined by accident. No doubt the abundance or the
scarcity of food does determine the movements of birds, and hence to
that extent may be held to account for the flocks. But we shall but
deceive ourselves if we think that it is the sole or even the principal
reason, or that the situation is in no wise affected by internal
factors. The behaviour of the individual in relation to the flock bears
ample testimony to the presence of a gregarious impulse which derives
satisfaction from the fact of close association.
As an illustration, let us take a bird whose movements are easily
watched, and in whose hereditary constitution the impulse to which I
allude seems to be strongly implanted--the Curlew. When the breeding
season is over, Curlew leave the mountain and the moor and return to
the coast or tidal estuaries for the remainder of the year. Here, at low
water, they find an abundant supply of food--crustaceans amongst the
sea-weed upon the rocks, and lobworms (_Arenicola piscatorum_) in the
mud as the tide advances or recedes. But when the tide is full, they
retire to those parts of the shore that remain uncovered--to isolated
rocks, or to sand-dunes, or it may even be to pasture-land in the
neighbourhood. During this period of repose large numbers of individuals
gather together on a comparatively small space of ground. They are not
constrained to do so by any shortage of accommodation, nor by any
question relative to food, nor, for the matter of that, by any
circumstance in the external environment; they are brought together
solely, this at least is the impression that one gains, by some
inherited impulse working towards that end. And their subsequent course
of behaviour tends to confirm that impression. For if we watch the
gathering together of the different units of which the flock is
composed, and study more particularly the emotional manifestation which
accompanies their arrival and departure, we shall find that the coming
of a companion arouses some emotion which is expressed by a vocal
outburst that sweeps through the flock.
Now each call, and the Curlew has a great variety, is not only peculiar,
generally speaking, to certain occasions, but is accompanied by a
specific type of behaviour, whence we can infer in broad outline the
type of emotion which is aroused. Thus we come to recognise fear, anger,
or sexual emotion, by just the particular sound which is emitted. But
even if we are going too far in referring particular calls to particular
emotions, we can, without a doubt, divide them into two broad
categories--those which are pleasurable and those which are the reverse.
And we need have no hesitation in placing the particular call to which I
allude in the first of these two categories, not only on account of the
nature of the sound produced, but because the activities which are
aroused are not such as normally accompany irritation. This is well seen
if the behaviour of different individuals be closely observed. After
resting on one leg for some time, first one and then another is seized
with cramp, and running a few yards in an ungainly way, bumps up against
its companions as if it had not full control over its movements. Its
behaviour produces irritation which is expressed by a vocal outburst,
and followed by actions the meaning of which is clear. Moreover, the
call is taken up by other individuals and sweeps over part of the flock
as does the greeting. But the nature of the cry is entirely different
from that which greets the arrival of a companion--humanly speaking it
is a passionate and impatient utterance, the height of displeasure. The
arrival, then, acts as a stimulus to something in the inherited
constitution which is expressed in, and presumably is satisfied by, this
vocal outburst; and, since the bird that arrives joins also in the
chorus, there is reason to think that the impulse which determines its
movements is similar to that which is temporarily aroused in the flock.
Apart, however, from the evidence derived from the affective aspect of
the operation of the instinct, the general course of behaviour lends
support to the view that the assemblies are determined by internal
factors, and are not merely the outcome of circumstances in the external
environment. Observe, for example, the manner in which the flock is
built up. Single individuals are content to rest alone so long as no
assembly is in sight, but they are drawn towards their companions
directly the opportunity arises, just as surely as the smaller
aggregation is drawn towards the flock; and so, as the flock increases,
it gradually absorbs all the lesser flocks and smaller parties, for the
greater the flock the greater the attraction seems to be; and different
individuals appear to gain some satisfaction from being in close bodily
contact with one another.
When the Curlew flies to that part of the mud-flat which is first
exposed by the receding tide, and there associates with others, it does
not then do so because it has any interest in its fellows, nor because
they serve as an attraction, but because it is constrained by hunger--in
other words, the association is determined by accident. But when, during
periods of repose, it sees a flock, flies to it, and takes up a position
in the midst of it, it does so not because suitable accommodation is
lacking--not therefore because of external constraint--but because it
derives some pleasure from satisfying something in its organic complex.
We speak of this behaviour and of the emotion which characterises it as
the _gregarious instinct_: by which we mean that the inherited nature of
the Curlew, as a tribe, is so constituted that, given the appropriate
internal conditions and adequate external stimulation, every individual
will respond in a similar manner--that is, the behaviour is primarily
determined by racial preparation. This is what we mean by the
_gregarious instinct_ biologically considered. We may resolve our own
experience in relation to the crowd into its simplest constituents,
project our own primitive feelings into the Curlew, and say that the
bird feels uneasiness in isolation and satisfaction in being one of the
flock. But in truth we know nothing, save by analogy, of the correlated
psychical state. All the knowledge we possess is derived from a study of
the objective aspect of the behaviour, which in simple terms may be
expressed thus: the individual is drawn towards its companions; there is
a relation between the size of the flock and the strength of the
attraction; and all Curlew behave similarly under similar circumstances.
This instinct controls the movements of many birds from early autumn to
the commencement of the breeding season. And so powerful is the control
that the individual is suppressed and its activities subordinated to the
welfare of the community as a whole. Flocks of Waders roam about the
tidal estuaries in search of food, and different kinds of Gulls assemble
there and preen their feathers or sleep; Warblers alter their mode of
life, and in the osier bed, or amongst the elders, seek their food
together in peace; Finches, Buntings, Pipits, and Wagtails, though food
is everywhere abundant, gather themselves together respectively into
bands which, as winter approaches, grow into flocks and even into
composite flocks; and as the Warblers leave for the south, so their
places are filled by flocks of Thrushes and Finches from the north. In
whatever direction we turn, when the days begin to shorten, it is the
community, not the individual, that thrusts itself upon our attention;
and throughout the winter continues to be the outstanding feature of
bird life.
With the approach of the breeding season we witness that remarkable
change which I have endeavoured to make clear in the previous
chapters--the disintegration of the flock and the reinstatement of the
individual. Instead of continuing with the flock, the individual now
goes forth to seek the appropriate breeding ground; and having arrived
there, is not only content to remain in isolation, but so behaves that
isolation is insured. Intolerant of the approach of a stranger,
intolerant even of the approach of the very members of the community
whose companionship was previously welcomed, it not only fights to
maintain the position it has selected, but fights indeed for the
possession of ground already occupied, and, until reproduction is
completed, asserts its individuality and exercises dominion over its
territory. What, then, is the prospective value, biologically
considered, of the changing interest that A displays in B, and to what
will such changes lead? These are the questions to which we will now
direct inquiry.
The annual life-history of a bird is in broad outline conditioned by
two powerful and at first sight opposing impulses--the one to live in
society, the other to live solitary. But, manifestly, a bird cannot be
governed by opposing impulses. It has but one character, within which,
according to the season and the circumstances, different impulses
predominate. But these impulses, no matter how different they may appear
to be, have their respective parts to play in furthering the life of the
individual. Hence they cannot oppose, though they may conflict, if the
resultant behaviour contributes towards survival.
The majority of birds live to-day in constant danger from predatory
species, and that this danger was still greater in bygone ages there can
be but little doubt. A curious mode of behaviour of the Curlew,
Whimbrel, and Godwit demonstrates this, for it must be the outcome of
the necessity for constant watchfulness. Whilst resting with its head
turned back and its beak buried in the feathers of the mantle, the bird
constantly moves the axis of its body, so that an observer, if placed in
a direct line behind it, sees at one moment the right eye and at another
the left. No movement of the feet or of the legs is perceptible, and the
shifting of the body continues whether the eyes are open or closed. This
body movement enables the bird to survey a much larger area of ground
than it would otherwise be capable of doing, and thus adds to its
security. As far as my experience goes, the movement is less evident
amongst the members of a flock than when an individual is resting alone,
or even with a few companions, which may be due to the fact that since
some members are always awake and watchful, a bird of prey would have
more difficulty in approaching a flock unawares than it would have in
approaching a single individual. With the greatest ease a Sparrow-Hawk
can pick up a Thrush as it feeds on the meadow by itself, but if it
attempts to seize one of a flock, the chances are that its approach is
signalled and that its prey escapes. And not only do the different
members give warning one to another of the approach of danger, but they
also combine to harass or even to drive away an enemy. So that there can
be no doubt that the gregarious instinct is serviceable in promoting the
welfare of the race, and has, as its end, the preservation of the
individual in order that it may take its share at the appropriate time
in procreating its kind.
In winter, then, the individual loses its individuality and is
subordinated to the welfare of the community, whilst in spring it
regains its individuality, and all its inherited instincts which then
come into operation lead to its isolation from the flock. The impulse to
seek isolation is dependent upon internal organic conditions which are
peculiar to a certain season; whereas the gregarious impulse depends
upon internal organic conditions which inhere at all times, though its
functioning is inhibited by the functioning of the former impulse. The
evidence which leads to this conclusion is to be found in the fact that
a male often deserts its territory temporarily and joins the flock,
where it remains at peace with its companions--an aspect of behaviour
which we have discussed on various occasions. The former impulse becomes
dominant in the spring owing to its innately superior strength; the
latter becomes dominant in the autumn because the organic condition
which determines the functioning of the former then subsides. The
impulse to seek the appropriate breeding ground and to dwell there would
seem to be the strongest of all the impulses save one--the sexual.
When, however, I speak of the sexual, I refer to the actual discharge of
the sexual function, which is the consummation of the whole process. But
the territory and all that appertains to it is part of that process--the
search for the breeding ground, the dwelling there, and the intolerance
of intrusion are but different stages, each one of which must have an
impulse peculiar to it; and since the completion of the sexual act can
only be successfully accomplished providing that success is attained at
every stage, the probability is that, of the impulses concerned, one is
neither more powerful nor less powerful than another.
So that we have two impulses operating at different seasons and guiding
the behaviour into widely divergent channels. But though the proximate
end to which the behaviour is directed is apparently different, there
are not two biological ends in view, but one--the attainment of
reproduction; and the changes that we witness are not contrary but
complementary, and their prospective value lies in the circumstance that
they contribute towards the preservation of the race.
If, then, every male is driven by inherited impulse to seek the
appropriate breeding ground each recurring season; if, having arrived
there, it is driven to seek a position of its own; if, in order to
secure isolation it is obliged to attack other males or to ward off the
attacks of intruders; if, in short, success can only be attained
providing that the inherited nature is so adjusted that the bird can
accomplish all that is here demanded--what will be the general result?
That the individual will rear its offspring in safety and that they will
inherit the peculiarities of their parents, enabling them, in their
turn, to procreate their kind; all this will certainly follow. We are
not concerned, however, at the moment, with the direct effect upon the
individual, but with the consequences that will accrue to the species as
a whole.
Now certain facts are presented to observation which enable us not only
to understand the nature of the change that is wrought in the history of
the species, but to foreshadow, with no small degree of certainty, the
extent of that change. I suppose that it has come within the experience
of most of us to observe, at one time or another, the ebb and flow of a
given species in a given district. Some favourite haunt is deserted for
a year, or for a term of years, and is then revisited; or, if it is
always occupied, the number of inhabitants fluctuates--plenty of pairs
in this season, only a few in that. Many intricate relationships, both
external and internal, contribute towards this state of affairs.
Fluctuation in a downward direction, or temporary extinction, is brought
about by changes in the physical world, by changes in the available
supply of food, by the increase of enemies, or by adverse climatic
conditions; whilst fluctuation in an upward direction, though due
indirectly to a combination of circumstances in the external world
favourable to the survival of large numbers of individuals, is directly
determined by the impulse to seek isolation. As individuals of
different species establish themselves, and form kingdoms and lesser
kingdoms, we can watch the gradual quickening into life of moorland and
forest and we can observe the manner in which it all comes to pass.
Males that for weeks or months have lived in society, drifting from
locality to locality according to the abundance of food or its scarcity,
now set forth alone and settle first here and then there in search of
isolation. Lapwings settle in the water meadows, and, finding themselves
forestalled, pass on in search of other ground; Blackbirds arrive in a
coppice or in a hedgerow and, meeting with opposition, disappear; and
the Curlew, wandering with no fixed abode but apparently with a fixity
of purpose, searches out the moorland where it can find the particular
environmental conditions to which its inherited nature will respond. In
fact, wherever we choose to look, we can observe in a general way the
gradual appropriation of breeding ground; and if we fix our attention
upon particular males, we can watch the method by which success or
failure is achieved.
On more than one occasion I have watched the efforts of Reed-Buntings to
appropriate territories in a marsh that was already inhabited. Sometimes
their efforts met with success, at other times with failure. In the
former case, the males, whose ground was intruded upon, were severally
forced to yield part of their holding and were thus left in possession
of a smaller area. The success of the intruder seemed to depend upon
persistent determination, rather than upon superior skill in battle.
Recently I had an opportunity of observing the intrusion of a male
Willow-Warbler upon ground already occupied. By persistent effort it
succeeded in appropriating one half of the territory of its rival. The
intruder occupied some trees on the outskirts of the territory it was
invading, and used them as a base from which it made repeated efforts to
enter the ground of its rival. These efforts were time after time
frustrated. No sooner did it leave its base than it was seen and
intercepted, or else attacked; and no matter from which direction it
attempted to effect an entrance, its efforts, for a time, were all to no
purpose. The fighting was of a determined character, and after each
attack the owner of the territory showed signs of great excitement, and,
sitting upright upon a branch, spread and waved its wings, which is the
specific emotional manifestation during the period of sexual activity.
Eventually the intruding male succeeded by persistent effort in
appropriating part of the occupied ground.
Thus we can actually witness the efforts of the individual to isolate
itself from members of its own kind, and can observe the immediate
consequences that follow from success or from failure. And from these
consequences we can infer that, within a certain range but in accordance
with the relative abundance of the species that dwell in it, every
corner of the available breeding ground will be explored and every
situation that evokes the appropriate response will be occupied.
Moreover, since the annual dispersion is not merely a repetition in this
season of that which occurred in a previous one, a progressive increase
in the area occupied will follow. Yet, if the majority of species desert
their breeding ground so soon as reproduction is ended, how can this be?
An answer to the question will be found in the fact that a bird has an
innate capacity to return to the neighbourhood of its birthplace, or to
the place wherein it had previously reared offspring--which means that
the results of prior process persist as the basis and starting-point of
subsequent process.
Bearing then in mind that the seeming peace in bird life around us in
the spring is but the expression of transitory adjustments in the
distribution of individuals and of species; bearing in mind how
widespread is the search for isolation each recurring season, how
frequently the search leads to competition and competition to failure,
and how failure implies a renewal of the search; bearing in mind that
situations, which appear to be eminently suitable for breeding purposes,
are passed by year after year and remain unoccupied, just because, for
reasons which have yet to be ascertained, the environment fails to
supply some condition which is essential if the inherited nature of the
bird is to respond--can there be any doubt that the general result of
the functioning of the disposition will be expansion; or, since no limit
is placed upon it from within but only from without--that is, by
unfavourable circumstances in the external world, that the expansion
will not merely be in one direction but in every direction?
If now, when reproduction is ended, all the impulses relating to it die
away, and the gregarious instinct again predominates, what are the
consequences to which this change will lead? Just as the consequences
which flow from the functioning of the former impulse are accessible to
observation, so likewise can we observe the change that is wrought by
the latter impulse. The process is a gradual one. Less and less
attention is paid by the individual to intruders, more and more is it
disposed to pass beyond its accustomed limits. Little by little,
accompanied by its young or without them, as the case may be, the bird
deserts its territory and wanders out into the wilderness. Here it
associates with others, and finds in them a new interest and, I doubt
not, a new enjoyment. All this we can observe as it takes place. But
just as there is an innate capacity to seek, in the spring, the place
where the pleasures of breeding had formerly been enjoyed, so we are
bound to infer the existence in the adult of an innate capacity to
revisit the former area of association; and this capacity will
strengthen and confirm the gregarious instinct and set the direction of
the general course of movement.
We have seen, then, that the interest displayed by one bird in another
changes with the seasons; we have seen that it is so modified as to be
in useful relation to different environmental circumstances; as far as
possible we have traced out the consequences, and have reached the
conclusion that the change of behaviour must, on the one hand, lead to
expansion, and on the other, to contraction; and we have seen that this
conclusion is in accord with the facts of observation--that is the
general result of our inquiry into the functioning of the two powerful
impulses, the impulse associated with the disposition to secure a
territory and the gregarious impulse.
The phenomenon of migration embraces a number of separate problems, each
one of which presents features of great interest and of still greater
difficulty. On some of these problems I do not intend to touch; I seek
only to ascertain whether the impulses that are concerned in the
securing of a territory, and in the search for society, bear any
relation to the problem as a whole. I hold that the origin of migration
is not to be found merely in conditions peculiar to a remote past, but
that the conditions inhere in the organic complex of the bird, and are
thus handed down from generation to generation. Starting with this
assumption I examined the behaviour which normally accompanies the
seasonal life-history of the individual, and found, in that behaviour,
manifestations of cyclical change leading to definite biological
consequences. I now propose to inquire whether those consequences are
such as might, in the course of time, give rise to the seasonal change
of abode.
We are apt to think of migration in terms of the Warbler that enlivens
our hedgerows in the spring after travelling hundreds of miles from the
south, or of the Redwing that comes from the far north and seeks its
food during the winter on the meadows, or perhaps of the American Golden
Plover that each year covers a vast expanse of ocean in its journey from
its breeding ground. The length of the distance strikes the imagination
and constrains us to focus attention upon the extremes.
But migration is of much wider significance than is here represented. I
sit beside the River Severn in April and watch Swallows, Tree-Pipits,
and Yellow Wagtails passing in twos and threes, in small parties, or it
may be in small flocks; and I observe that while some establish
themselves in the neighbourhood, others pass on. Or I watch
Herring-Gulls returning to the breeding station at Bolt Head, an endless
stream of individuals coming from the east as far as eye can reach;
following them for some miles inland I see them still, first as specks
upon the horizon, then passing beside me as they beat their way slowly
against the strong south-westerly winds, and finally disappearing from
view in the direction of the cliffs. Or again, I watch Buntings and
Finches deserting the flock and seeking stations in the marsh, or
amongst the furze-bushes on the common, or in the spinneys. In each case
the proximate end of the behaviour is alike--wherein then lies the
difference? Only in the distance which separates the territory from the
area in which the birds formerly associated. And intermediate between
the extremes, I doubt not, if we had a sufficient body of observations,
that we should find numerous gradations, the lesser merging step by step
into the greater. Is the Swallow a migrant and the Herring-Gull not; is
the Tree-Pipit a migrant and the Bunting not; must a bird cross many
miles of sea or of land before it can be considered a migrant; is the
length of the distance traversed a criterion of migration? Surely not.
The distance traversed is merely a collateral consequence of the process
as a whole.
The annual life-history of a bird presents, as we have seen, two
distinct phases--the one in which the individual dominates the
situation, the other in which it is subordinated to the welfare of the
community. Let us take these two phases separately and endeavour to see
how they may have influenced the seasonal movements; and first let us
take the more important of the two, namely that one which is directly
concerned in the continuance of the race.
In this phase we must consider the three factors to which allusion has
already been made:--(1) the internal impulse, (2) the innate ability to
return to the former breeding ground, (3) the conditions in the external
environment. These three work in close relation and, as I shall
endeavour to show, lead to important results.
(1) If there were nothing in the inherited nature beyond an impulse to
seek the breeding ground, if, that is to say, when the appropriate
locality were reached, the bird took no further interest in the
developing situation, the attainment of reproduction would become
largely a matter of chance. A male in a congested district, having no
incentive to seek fresh ground, would remain inactive until a female
happened to cross its path and stimulate its sexual impulse, when its
activity would take another form. Hence some districts would be
over-populated, whilst others would remain unexplored. But the system of
reproduction does not consist merely of a search for the breeding
ground, and of the discharge of the sexual function; it is a much more
complex business, yet withal more complete. Nothing is left to chance;
the end is attained step by step; and each successive stage marks the
appearance of some specific factor which contributes towards the success
of the whole. We start with the appropriate organic condition under
which, when adequate stimulation is provided, the disposition to secure
a territory comes into functional activity. Within the field of this
disposition we can distinguish certain specific impulses. In sequential
order we have the impulse to seek the breeding ground; the appropriate
situation which gives rise to an impulse to dwell in it; and the act of
establishment which supplies the condition under which the impulse to
drive away intruders is rendered susceptible to stimulation. Grouping
these impulses, for the convenience of treatment, under one general
heading, I speak of an impulse to seek isolation. It implies some kind
of action with some kind of change as its correlated effect; and from it
there flows a ceaseless energy directed towards a definite end which for
us, who can perceive its prospective value, is isolation in an
appropriate environment. The emphasis here is on "isolation," for it
involves competition, and there cannot be competition without some
change in the relative positions occupied by different individuals; so
that in each recurring season there will be not only a re-arrangement of
ground formerly occupied but an arrangement of ground formerly deserted.
(2) That the older birds return to the locality wherein they had
formerly reared offspring, and the younger to the neighbourhood of their
birthplace, was always deemed probable. But in recent years evidence
which cannot be rebutted has been supplied by the marking of birds. This
evidence, details of which can be found in the summary of results
published annually by Mr. Witherby in _British Birds_, demonstrates that
the adult frequently returns not only to the same locality in which it
formerly bred, but even to the same station; that it does so year after
year; that this mode of behaviour is not peculiar to one sex; and that
many of the young breed in the locality in which they were reared. Such
being well-established facts, we can infer the existence of an innate
ability to revisit the place wherein the enjoyment of breeding, or of
birth, had formerly been experienced. Of its nature we know little or
nothing. It would almost seem as if there must be some recollection of
past enjoyment, but all that can be definitely asserted is--that past
experience somehow becomes ingrained in the life of the individual and
determines present behaviour. What, however, is of importance to us at
the moment is not the _ad hoc_ nature of the bird, but the biological
consequences to which the behaviour leads. For if, on the average,
individuals return to their former haunts, it follows that the annual
dispersion will not be merely a repetition in this season of that which
had occurred in a previous one, but that the little added this year will
become the basis for further additions in the next. The innate ability
is handed down from generation to generation, and, in so far as it
contributes to success, is fostered and developed by selection; and the
modifications of behaviour to which it leads, since the results of prior
process in the parent persist as the basis and starting-point of
subsequent process in the offspring may in a sense also be said to be
handed down.
(3) The conditions in the external world may be organic or inorganic. By
organic I mean the conditions which depend upon the number of
competitors or enemies by which a bird is surrounded. The competitors
may include other species which require a similar environment; and the
enemies, species which prey upon it, or animals which take its young or
its eggs. They vary in different seasons, in different districts, and
in nature and extent--the success of one species leads to the failure of
another, and the multiplication of the Jay or of the Magpie robs us of
many a songster.
By inorganic I refer to the changes in the climate and in the surface of
the earth. The nourishment of the young depends upon a regular supply of
food, and the supply of food depends upon the climate which alters in
different periods; in one decade the temperature falls below, whilst in
another it rises above, the normal, and, as the insect life fluctuates,
so there is fluctuation in the bird population. The changes in the
surface of the earth are manifold. Little by little the alder (_Alnus
glutinosa_) overspreads the marsh. Young shoots spring up here and
there, in a few years grow into bushes, and in a few more years are
trees; and the dense masses of rush which seemed to choke their growth,
yielding their position of importance, slowly disappear. And where
formerly the _Orchis latifolia_, _Orchis mascula_, and _Juncus communis_
grew in mingled confusion, nothing but water, moss, and the spreading
roots of alder cover the ground. As the rush disappears, many birds that
for generations have inhabited that marsh must seek accommodation
elsewhere. Ancient breeding haunts thus disappear, new ones come into
being, and even those which appear to be permanent are almost
imperceptibly changing.
Now the bird inherits a nervous system, which works under internal
excitation and external stimulation. Given the appropriate organic
condition and adequate stimulation, and the impulse to seek isolation
comes into functional activity. What the organic condition is and how it
arises we do not exactly know; all we know is that organic changes do
take place in the breeding season, that these changes profoundly modify
character, and that they correspond with the seasonal growth of the
sexual organs. And with regard to the question of stimulation, we have
again to confess to much ignorance, although certain facts are presented
to observation which seem to indicate the direction in which the
stimulus lies. For example, it is well known that abnormal climatic
conditions influence behaviour; we see migrants retracing their flight
along the very course they travelled a short time previously--driven
headlong by the blizzard, that at least is what we say. But if the wind,
instead of being cold and from the north, is warm and from the west, do
they retrace their flight? I have not found it so. And if there be no
wind and the temperature is low, are they still affected? Again, I have
not found it so. When, as we commonly say, they fly before the storm,
some change takes place in their organic complex, some new impulse
receives stimulation or the former one lacks it. If, after Lapwings have
established themselves in their territories, the weather becomes
exceptionally severe, the birds collect together again in flocks and
revert to their winter routine; and under similar circumstances,
Buntings fail to sing and temporarily desert their territories. In such
cases it is clear that the impulse to seek isolation ceases for a time
to dominate the situation. The inference, therefore, is that atmospheric
changes bear some relation to the functioning of the instinct; but
whether it be temperature, or humidity, or the direction and velocity of
the wind, or a combination of two or more of these factors that supplies
the stimulus, we cannot tell.
The appropriate organic condition and the stimulus have then still to be
determined, and we must pursue our inquiry from the point at which the
impulse comes into functional activity. We will take a simple case, and
one free from complication.
Let us suppose that there is an area bereft of bird life, if it can be
so imagined, but in proximity to other inhabited areas. Into this area,
whilst in search of isolation, let us imagine that a Yellow Bunting
finds its way. After the manner of its race it establishes a territory
and occupies, let us say at a low computation, half an acre of ground.
It then obtains a mate, breeds, and rears offspring, two of which we
will assume are males. Reproduction ended, the birds desert the area,
and in the following spring, when the impulse again asserts itself,
parents and offspring seek again their former haunts. We now have three
males, each of which occupies half an acre, and each of which rears two
offspring--that is the position at the close of the second year. In the
third year the number will have increased to nine and the area occupied
to 4½ acres; and so on in succeeding years, until by the beginning of
the eleventh year, we have 59,048 Yellow Buntings occupying 29,524½
acres or 46 square miles. This, then, will be the result of the
operation of the impulse, providing that all the individuals survive and
that no complications supervene.
But of course complications are numerous, some of which retard while
others accelerate the rate of expansion. These complications arise from
various sources--in the first place from natural enemies which prey upon
the birds or upon their eggs; in the next place from climate which, if
it happens to be unfavourable, may mean that food is scarce and that
only a small percentage of the young survive; and lastly from
rivals--and by rivals I mean closely related forms that require a
similar station and similar food--which, by occupying available ground,
may check expansion, or, by forcing a continuation of the search, may
widen it.
Now when individuals fail as many do fail in their initial attempt to
secure territory, the activity of the impulse still persists, and there
is no control over the direction in which the bird continues to wander
whilst in search of its end. Some therefore seek in this direction,
others in that; some wander inwards into inhabited areas and fail to
find accommodation, or, according to the relative strength of their
impulse, perhaps succeed and so set free a new competitor, others wander
outwards into country uninhabited by the species. These latter we will
call "pioneers." They may find accommodation within a comparatively
short distance of their base, or they may come into competition with
rivals and fail, not necessarily on account of any congenital weakness
of ability, but because being warned by an alien song, they may be
precluded from coming into contact with just the environing conditions
which can supply the stimulus and allow behaviour to run its further
course--and so be obliged to extend their search into remoter districts.
But it must not be overlooked that they will be placed in a most
advantageous position so far as the attainment of reproduction is
concerned. In their search for territory they will meet with little
opposition and will be free to select whatsoever ground they will; and
be free also from intrusion by neighbouring males, which is so frequent
in occurrence and continues for so long in congested areas. Moreover, in
thinly populated districts, the pressure upon the available means of
support will not be so great, neither will natural enemies be so
plentiful; and since the offspring, guided by prior experience, return
to the neighbourhood of their birthplace, the advantages thus gained
will be shared by the succeeding generation. It follows, then, that the
range of a species will not always be continuous, will not, that is to
say, proceed by a series of successive steps, but that sometimes in this
direction and at other times in that, the chain of territories will be
interrupted and different individuals separated by distances of greater
or lesser extent. New colonies will thus come into being; and as the
unlimited increase of the population over limited areas gradually
reintroduces into them the struggle for territory, new centres of
distribution, where the process will repeat itself and from which
expansion will proceed afresh, will be formed. Hence, though it is
clearly impossible for the progeny of one pair of Yellow Buntings to
overspread the whole of the 46 square miles, it is by no means
impossible for the limits of their range to exceed even those limits
within the eleven years.
To sum up our knowledge regarding this phase. Of the organic condition
which renders the impulse responsive to stimulation we know very little;
and though certain facts of observation seem to indicate the direction
in which the stimulus is to be found, we must here again confess to much
ignorance. So far as can be seen, however, the impulse to seek isolation
with its correlative territory, leads to constant modification in the
breeding range of most species. The occupation of the small space of
ground which each individual requires, the extent of which has been
gradually adjusted to suit the needs of different species, results in
expansion not only in one direction but in every direction, and not only
in one season but in every season. And if there were no complications in
the external world this expansion would proceed, as we have seen, with
astonishing rapidity. But complications, some of which are favourable
and others unfavourable, are numerous, and it is difficult to estimate
their importance or to indicate their precise effect; the former,
however, accelerate the rate of expansion, whilst the latter retard it.
Those individuals that wander outwards and seek territory on the
outskirts of the range we have called "pioneers." They will have
advantages over others that, wandering inwards, seek isolation in
congested districts, and will succeed where the latter fail; and since
there is in the young an innate ability to return to the district
wherein they were reared, the advantages so gained may be said to be
handed on from generation to generation.
Let us now turn to the contra-phase, and endeavour to ascertain whether
the gregarious instinct bears any relation to the seasonal desertion of
the breeding ground. The conclusion at which we have already arrived
regarding this instinct is that it forms part of the inherited nature of
most species; that its functioning is suppressed when a bird is actually
in occupation of a territory; and that it is serviceable in promoting
the welfare of the individual. We cannot of course observe the instinct.
What we observe, when reproduction is ended, is a change in the
relations of different individuals; instead of arousing mutual
hostility, they attract one another, from which we infer the existence
of something which determines their conduct, and this "something" we
speak of as an instinct.
To what does this change lead? Let us suppose that there is an area
inhabited by one species; that the number of inhabitants has reached the
maximum that the means of sustenance will allow; and that the season of
reproduction is drawing to a close. The position will then be as
follows. All the available breeding ground is divided into territories;
each territory is occupied by one unit, the family, and each individual
is able to fend for itself; changes both internal and external begin to
take place, the gregarious instinct comes into functional operation, and
the supply of food diminishes--that roughly is the position. The
internal factor operates so that the sight of this individual or the
call of that, instead of evoking hostility as heretofore and keeping
different units apart, proves now an irresistible attraction; so that in
place of a number of individuals evenly dispersed over the whole of this
area, a small number of flocks of various dimensions are stationed at
certain points, which points are determined partly by experience, partly
by the supply of food, and partly by accident. This implies for each
individual some movement in some direction. But since the population of
this imaginary area has reached the maximum, and the supply of food,
though limited in distribution, is nevertheless plentiful, such
movements will be irregular and will proceed in no definite direction.
Now let us suppose that the breeding range extends and that fresh ground
is occupied by pioneers. When reproduction and the rearing of broods are
ended and the gregarious instinct becomes dominant, these pioneers, or
at least some of them, will revisit the area wherein formerly they
associated with companions. Their offspring, however, though they will
have the inherited impulse and the innate tendency, will not have the
experience; how then will they behave? There can be no doubt that some
will accompany the older birds, and, being led by them, will share the
experience of a former generation; nor any question that others will
collect together in the neighbourhood of their birthplace and, if their
impulse is satisfied, will remain there so long as food is to be found.
Thus the gregarious instinct, working in close relation with acquired
experience, will on the one hand lead to the formation of organised
movements in certain directions, whilst on the other it will lead to the
formation of new areas of association which will follow in the wake of
the expansion.
We have assumed, in the imaginary case which we have just taken, that
the conditions in the external world are such as enable the birds to
endure throughout the year--in short, that there are no complications
regarding the supply of food. But we must bear in mind that so long as
conditions are favourable during the period of reproduction, which is of
short duration, the breeding range can continue to expand, and that
therefore, in the course of centuries, regions will come to be occupied
wherein, owing to alternations of climate or physical changes in the
surface of the earth, food will be impossible, or at any rate difficult
to obtain at certain seasons. Hence there will come a time when the area
of association ceases to follow in the wake of the expansion, and the
breeding area begins to diverge from the subsistence area.
How, then, is the gulf between these two areas to be bridged? We can of
course say that those individuals which, in virtue of some slight
variation of hereditary tendency, return to regions where food is
plentiful will survive; whilst others, less well endowed, will perish.
We can state the position in some such general terms, and doubtless
there would be truth in the statement, but it does not carry us far; we
wish to know more of the nature of the tendency, and of the manner in
which it has evolved. Well now, in this new situation which arises, two
things are apparent--that the struggle for existence becomes a struggle
for the means of subsistence, and that anything in the inherited
constitution of the bird which can be organised to subserve the
biological end in view becomes of selection value. So long as food can
always be procured in the new areas of association, the individuals that
behave in accordance with ancestral routine gain thereby no particular
advantage; but directly the breeding range extends into regions where
the supply fluctuates, traditional experience becomes a factor in
survival, and those individuals that come under its influence will, on
the average, be more likely to endure and so to procreate their kind and
maintain the tradition. Let it once be granted that there is an innate
capacity to retain in later phases of routine the experience gained in
earlier phases, and it is difficult to see how traditional guidance can
be refused recognition as a factor in the developing situation. But only
_a_ factor, and by no means the most important one; for observation has
shown that the young are capable of performing the return journey
without guidance. Something therefore _is_ inherited, some impulse
which comes into functional activity at a specified time, and leads the
bird to set forth in a given direction.
There are no grounds for supposing that the experience of one generation
forms any part of the hereditary equipment of subsequent generations. In
what direction then are we to look for the congenital factor? What is
given is an inherited tendency to co-operation and mutual help, and an
innate capacity to make use of the results of experience. The inherited
tendency, as we have seen, leads on the one hand to the formation of new
areas of association, whilst on the other, since it is the means of
bringing isolated individuals into contact, it leads to experience being
handed on from generation to generation, which, in its turn, results in
a certain amount of backward movement along the line of expansion. It
forms part of the hereditary equipment of many species, and is
serviceable in promoting the welfare of the individual. Moreover, there
is reason to believe that its origin dates back to an early period in
the evolution of the higher forms of life; and if in the subsequent
course of evolution it could have been so organised as to serve a double
purpose, so much the more reason would there have been for its survival.
In what does the instinct consist? Is it merely that the sight of this
individual or the call of that proves at some particular moment an
irresistible attraction, or does the appropriate organic condition give
rise, as is generally supposed, to some preceding state of uneasiness?
In the former case, the temporarily isolated individual or colony would
have but little chance of sharing in the benefits which mutual
association confers upon the associates; in the latter, the feeling of
discomfort would lead to restlessness, and would thus bring the bird
into touch with the environing circumstances under which instinctive
behaviour could run its further course. So that it is probable that the
movements of each individual, prior to its becoming a unit in the flock,
are not accidental but are determined in some measure by racial
preparation.
Now if the fundamental assumption of the doctrine of the struggle for
existence be true, the gregarious instinct will not be quite alike in
all the members of different broods, nor even in each member of the same
brood; that is, variation will occur in all possible directions. And we
shall not, I think, exceed the limits of probability if we assume that
different individuals vary in the persistency with which they strive to
attain their unknown end, and in the direction in which they travel in
pursuit of it. So that in each generation they will fall into three
classes: (1) those which are inert, (2) those which wander along the
line of expansion, (3) those which wander in other directions. If then
the struggle for life at this particular juncture in the evolution of
the breeding range is a struggle for the means of subsistence, the
members of these three classes will not be in a like satisfactory
position so far as the competition for food is concerned. Those in the
first class--_i.e._, those in which the activity feelings are weak--will
neither gain the benefits which arise from mutual help, nor will they
have much prospect of enduring through the season of scarcity. Those in
the third class will, it is true, derive some assistance one from
another, and so be in a better position to discover what food may be
available; but inasmuch as they will remain in regions where the climate
alternates and the supply of food is liable to fall below the minimum
required, the chances are that a high percentage will fail in the
struggle for existence. We come now to those in the second class, and it
is upon them that I wish more particularly to focus attention. The
initial movement in their case will be in the direction from which
outward expansion has all along taken place. Within a comparatively
short distance they will reach districts where the species is plentiful,
and here, associating with others that have some traditional experience,
they will be guided by them and will find themselves in regions where
food is plentiful. Hence in each generation those will survive that,
owing to some congenital variation of their instinct, seek satisfaction
for their impulse in a direction which brings them under the influence
of tradition. And though at first but slight and not in themselves of
survival value, such variations, since they coincide with modifications
of behaviour due to acquired experience, will be preserved and in the
process of time so accumulated as to be capable of determining the
direction and extent of the movement.
But the young Cuckoo deserts this country many weeks after its parents,
and there is no reason to suppose that it lives in society when
eventually its destination is reached; and the young Falcon passes to
the south, and is certainly not gregarious--how then can we explain
their behaviour in terms of something which they show no signs of
possessing? I do not wish to make light of a difficulty which
admittedly, at first sight, is a grave objection to the view that the
gregarious instinct has been operative in the manner here claimed for
it. It must, however, be borne in mind that this instinct, though
originally developed to serve the purpose of mutual protection, supplies
the material upon which evolution works when the extension of the
breeding range creates a situation requiring readjustment on the part of
the organism to new conditions of life; and that those variations which
can be so modified as to be in useful relation to the new environmental
circumstances are seized upon by natural selection and, being
transmitted, form the foundation of a specific inherited response, no
longer dependent upon, though operating in close relation with the
primitive response whence originally it sprang. Thus the primordial
instinct becomes so organised as to serve a secondary purpose, that of
rendering secure a means of access to a certain food supply. In the
course of evolution species were bound to arise which, owing to some
peculiar conditions, derived greater advantage from living solitary than
from living in society. Does it then follow, because such species
manifest no inclination to live in society, that the instinct never has
played any part in their lives? Or because the primary purpose has
lapsed, does it follow that the secondary no longer exists?
Let me recapitulate the principal considerations which I have discussed
in this chapter.
Though I have been advancing a theory, and though I have taken much for
granted, yet it will, I think, be admitted that both the theory and what
has been taken for granted rest on observational grounds. As our
starting-point we have a bird whose inherited nature alternates
according to the season, and in whose nature we can distinguish two
contra-phases--the one to live in society, the other to live solitary.
While both have their part to play in furthering the life of the
individual, for biological interpretation there is only one end, the
prospective value of which is the continuance of the race. We may say
that the latter phase is the more important of the two because it is
directly concerned with reproduction. But we shall make a great mistake
if we attach peculiar importance to one phase, or to one mode of
behaviour within that phase, or to one action within that mode of
behaviour; for if there is one thing certain it is that the whole is an
inter-related whole in which each part depends for its success upon that
which precedes it.
In that phase in which the territory is the central feature of the
situation, the struggle for existence is in operation in its acutest
form; all the congenital and acquired capacities of the bird--pugnacity,
song, capacity to utilise in later phases the experience gained in prior
phases, all these are organised to subserve an end--a proximate
end--which in its simplest terms may be described as "isolation."
Isolation is then the first step in the process of reproduction, and any
individual that fails to make it good, fails to procreate its kind. But
isolation implies separation, and the degree of separation varies in
different species, from the few square feet of cliff required by the
Guillemot to the few square miles of barren moor over which the
Peregrine exercises dominion. One species must occupy sufficient ground
to enable it to secure food for its young; another requires sufficient,
but no more, upon which to deposit its egg; and a third must secure a
position for its nest within the community. Hence it follows that the
degree of separation varies with the conditions of existence. Since,
however, the conditions in the external world are constantly changing
according to the relative abundance or scarcity of enemies, the rise or
fall of rivals, the physical changes in the earth's surface, and the
alterations of climate, it is clear that isolation can only be obtained
with difficulty, and that the competition for it must be severe. Some
individuals therefore fail to breed, whilst others, perhaps because
their impulse is stronger, persevere and seek stations elsewhere. What
are their prospects of finding them? By extending the field of their
activities, they will wander into districts remote from the scene of
competition, districts where not only food is plentiful but where
enemies and rivals are scarce; and to these pioneers, if to any, success
in reproduction will most certainly be assured. But not only is it they
who will benefit; their offspring also, when the time comes for them to
take their part in the maintenance of the race, will share in the
success of their parents, for even though they may not escape
competition from individuals of closely related forms, they will meet
with but little from those of their own kind. Now species which live
throughout the year in the vicinity of their territory are comparatively
few, the majority are obliged to wander in search of food so soon as
reproduction is ended, and their behaviour is determined not only by its
abundance or scarcity, but also by the powerful gregarious impulse which
waxes in proportion as the instincts connected with reproduction wane.
If, then, when the sexual instinct again becomes predominant, the
experience of the former season nowise affects their movements, little
or no progress will be made in the expansion of the range. But just as a
certain entrance into the bush and pathway through it, when once made
use of in the process of building, becomes so firmly established as to
form the sole highway to and from the nest, so likewise, when the
impulse to seek isolation repeats itself, the bird is constrained to
seek the neighbourhood wherein it had experienced the enjoyment of
breeding or of birth. Thus the little that is added one year becomes the
basis for further additions in the next, and new centres of distribution
are continually being formed from which expansion proceeds anew.
Now as the range gradually extends into regions where the climate
alternates and food at certain seasons is consequently scarce, the
distance between the customary area of association and that of
reproduction must perforce widen. The question then arises: How will the
young that have no experience find their way to regions wherein they can
endure? The forces which may have been organised to subserve the end in
view are three: (1) Acquired experience, (2) tradition, (3) the
gregarious instinct. The pioneer that carries the range a little further
forward starts from a base where it has associated with companions and
found food plentiful; and when the impulse to live in society again
asserts itself, it not only repeats its former experience but hands on
the habit thus acquired to those of the next generation that happen to
accompany it. Granting, however, that by successive increments in the
distance traversed, traditional guidance may in time accomplish much, it
cannot account for all the known facts, it cannot at any rate explain
the fact that in some cases the inexperienced offspring finds its way to
the food area without guidance. Something, therefore, _is_ inherited.
And my suggestion is this: That the gregarious instinct, the ancient
origin of which we can infer from its manifestation in so many and
diverse forms of life, supplies the material upon which evolution works;
that variations of the initial impulse, at first slight and not in
themselves of selection value, in so far as they coincide in direction
with modifications of procedure due to experience or tradition, are
preserved; and that, in the process of time, they are so accumulated as
to form a specific congenital endowment determining a definite mode of
behaviour.
FOOTNOTES:
[1] June 1915, R. M. Barrington.
[2] _Dictionary of Birds_, p. 556.
[3] _Social Psychology._
[4] _Manual of Psychology._
[5] _Ibis_, April 1918.
[6] _Zoologist_, 1912, p. 327.
INDEX
Acquired experience, 300
Adjustments, transitory, of distribution, 275
Alarm notes, 119
Arrival, advantages and disadvantages of late, 33-44
Assemblies in winter, 262, 263
Assembly grounds, 173
Attainment of reproduction, 171
Barrington, R. M., on the sex of migrants, 25
Battle between two male Cuckoos, 82
---- between two Moor-Hens, 86, 92, 93, 94
---- ---- Pied Wagtails, 86
---- ---- Raven and Buzzard, 217
---- ---- Raven and Peregrine, 216
law of, 13,19
Behaviour routine, 262
---- sexual, 3
Bickerings, 96
Birthplace, return to, 43, 50
Blackbird, 87, 182, 222, 244
Blackcap, 81, 156, 224, 230
Black Grouse, 63
Black-tailed Godwit, 53
Boundaries, 1, 5
---- conflicts for retention of, 7, 62
---- disputes as to, 1
Brambling, 124
Breeding ground, search for appropriate, 270, 271
---- range, extension of, 291-92
---- site, acquirement of, 3
---- stations, evolution of, 15-19
---- ---- repeatedly visited long before nesting-time, 64
---- territory, 2, 3, 7
---- ---- evolution of, 18
---- ---- foundation of, 7
---- ---- innate capacity to return to former, 279-81
Bridled Guillemot, 64
Brooding, 180
---- impulse, 191
Bunting, Cirl, 28, 140
---- Corn, 28
---- Reed, 28, 68, 69, 85, 104, 132, 158, 160, 244
---- Yellow, 28, 30, 47, 64, 140, 159, 162, 183, 187, 188, 189,
235, 236, 286
Buzzard, 217
Capacity, innate, to return to former breeding territory, 279-81
Carrion Crow, 226
Chaffinch, 28, 31, 32, 33, 45, 87, 103, 156, 159, 235, 236, 244
---- Donegal, 160
Change of breeding quarters owing to unsuitableness, 50
Chiffchaff, 49, 51, 80, 139, 140, 221, 224, 244
Cirl Bunting, 28, 140
Clarke, W. Eagle, _Studies in Bird Migration_, 24
Cleanliness of nest, 180
Cliff-breeding species, 63
Climatic changes, alteration of routine, due to, 284
---- changes, food dependent on, 283
---- conditions, influence of, 20
Communities, 202
---- birds after breeding-season remain in, 265-67
Competition, female, for males, 13
Complexity of strife, 84-85
Conflicts between males during the mating period, 74, 86
---- between males during the nesting period, 87
---- for areas, 10, 11, 13, 62
---- of Ruff, 54
---- sexual, 10
Congenital disposition, 135
Contests between males for possession of females, 80
Coot, 61
Corn-Bunting, 28
Corncrake, 39
Crow, Carrion, 226, 227
Crow, Hooded, 202
Cuckoo, 52, 82, 144, 296
---- restricted breeding area, 52
Curlew, 119, 138, 140, 250, 262, 263, 265, 273
Danger warnings, 269
Darwin, C., _Descent of Man_, 35
---- on the arrival of males before females, 35
Defence of territory, 6
Development, sexual, 6
Disposition, congenital, 135
---- functioning of, 74
---- inherited, 5
---- to defend the territory, 73-118
---- to mate, 27
---- to remain in a particular place in a particular environment, 6
---- to secure a territory, 6, 20-72
Distribution, adjustment of, 275
Dove, Turtle, 126, 232
Dunlin, 250
Emotional behaviour, 53, 82, 114
---- manifestation, 90, 283
---- response, 26
Enemies, 282
Energy, waste of, 219
Environment, 6
---- and food, 56
---- changes of, 283
---- external, conditions in, 279, 282
Equipment, hereditary, 6
Evolution of breeding stations or territory, 15, 19
---- of the territory, 176
---- of the voice, 163
Existence, struggle for, 294
Experience, acquired, 300
Experiments, removal of nests for, 181, 185, 190, 213, 214
Exposure, its effect on nestlings, 180
External environment, conditions in, 279, 282
Falcon, 48, 71
Feeding grounds, neutral, 125 in communities, 70
Females, fighting amongst, 109-118
---- sexual impulse of, 13
Fieldfare, 124
Fighting instinct, 79, 82
Flight, emotional behaviour of Godwit during, 53
Flocks, in winter, birds collect together in, 262
Flycatcher, 244
Food, procuring of, 5
---- abundance, or scarcity of, its relation to prosperity of young,
15, 16
---- its bearing, on the movement of flocks, 262
---- rearing of young dependent on rapid and regular, 179, 195
---- supply, proximity to, necessary for rearing young, 179, 195
Fortuitous mating, 174
Fowler, Ward, on the value of communities, Rooks, 202
Fulmar, 121, 247
Functional activity, 259
---- instinct of Reeve, 173
Functioning of the disposition, 275
---- of the primary dispositions, 100
Garden Warbler, 223, 225, 230
Gätke, H., _Birds of Heligoland_, 24
---- on the absence of song in birds on Heligoland, 124
---- on the early arrival of Guillemots on Heligoland, 64
Godwit, emotional behaviour of, during flight, 53
---- Black-tailed, 53
Grasshopper Warbler, 39, 131, 139, 153, 155, 187, 244
Greenfinch, 28, 33, 140, 235, 236
Gregarious instinct, 20, 61, 141, 265-66, 269, 276, 289, 290, 291,
296, 300
Grouse, Black, 63
Guillemot, Bridled, 64
---- Common, 63, 64, 121, 192, 195, 206, 211, 247
---- Ringed, 64
Gull, Common, 119
---- Herring, 210, 278
Habit formation, law of, 8, 62, 65, 66, 67, 205
Hawfinch, 28
Headquarters, 176, 206, 207, 274
---- restricted, 8, 9, 30, 50, 58, 64, 127
Hedge-Sparrow, 213, 221, 244
Hereditary equipment, 6
Herring-Gull, 210, 278
Hooded-Crow, 202
Hostility and territory, relationship between, 242
House-Sparrow, 218
Imitation, vocal, powers of, 156, 157, 161
Impulse, internal, 279
---- to brood, 191
Inherited disposition, 5
Instinct, fighting, 79-82
---- gregarious, 20, 61, 141, 265, 266, 269, 276, 289, 290, 291, 296,
300
---- migratory, 37
---- of song related to establishment of territory, 125
---- sexual, reawakening of, 4, 18
Instinctive response, 180
Instincts susceptible to stimulation, 259
Internal impulse, 279
Internal stimulation, 62, 123
Interpretation of battles, 75
Intolerance of other birds, 218, 219
Intrusion resented, 274
Isolation, impulse to seek, 288
---- of male, 12, 62, 65, 73, 81
---- of male during breeding season, 267, 272, 273, 275, 281
Jay, 87, 156, 283
Kestrel, 228
Kittiwake, 116, 200, 247
Lapwing, 58, 59, 61, 62, 64, 84, 103, 104, 126, 189, 190, 220, 251, 284
Lapwing, life-history of, 58-61
Late arrival, advantages and disadvantages of, 33-44
Law of battle, 74, 75, 86
Lesser Whitethroat, 230, 244
Linnet, 156
M'Dougall, Dr, _Social Psychology_, 77
Magpie, 219, 283
Males arrive before advent of females, 24
Marsh-Warbler, 39, 40, 52, 81, 132, 140, 153, 155, 156, 165, 225
Martin, 201, 218
Mating, difficulties of, 172
---- fortuitous, 174
Maximum number supportable in a given locality, 49
Meadow-Pipit, 188
Meeting places for antics, 54, 63
Mental Image, 77
Merlin, 227
Migration, 3-4
---- distance no criterion, 279
---- its relation to territory, 259
Migration, origin of, 260, 277
---- phenomenon of, 277
Migratory instinct, 37
---- species more highly specialised than resident species, 56
Missel-Thrush, 21
Mobility of the land and stability of the water, 260
Moor-hen, 61, 85, 103, 218, 250, 251
Morgan, Professor Lloyd, on instinctive behaviour, 74
---- on emotional behaviour, 114
Nest, cleaning of, 180
---- construction of, 3
Nests, removal of, for experiments, 181, 185, 190, 213, 214
Neutral feeding grounds, 62, 125
---- ground, 98
Newton, E., on the arrival at breeding stations of males before female,
24, 35
Nightingale, 39, 156
Notes of alarm, 119
---- of anger, 119
---- of recognition, 139
---- of warning, 119, 139, 141, 145, 151, 153
Offspring, rearing of, 3, 4
Organic change, sexual, 92, 123
---- changes, 65
---- condition of Reeve, 173
Owl, Wood, 156
Paired for life, 55-56
Parental instinctive response, failure of, 185
Partridge, 87, 218
Persecution, Carrion Crow and Magpie, 226
Persecution, Carrion Crow and Rook, 227
---- House Sparrow and Martin, 218
---- Lapwing and Snipe, 220
---- Raven and Buzzard, 217
---- ---- and Peregrine, 216
---- Starling and Woodpecker, 218, 237
Persistency to remain in territory, 68
Pied-Wagtail, 86, 155
Pigeon, Wood, 219
Pipit, Meadow, 188
---- Tree, 51, 188, 189, 222, 244, 278
Polyandrous females, 144
Predatory species, 268
Promiscuous pairing of Ruffs, 172
Proximity to food-supply necessary for rearing young, 179, 195
_Psychology, Manual of_, 1
Puffin, 63, 116, 200
Pugnacious instinct, 87-109
Pugnacity, 11, 62
---- of females to obtain mates, 109-118
---- of males, prior to mating-season, 77-81
---- of Moor-Hen, 218
Racial preparation, 41, 43, 46, 67, 205, 206, 266
Rail, Water, 218
Raven, 48, 202, 216
Razor-bill, 63, 64, 200, 247
Readjustment of territory, 146
Rearing of offspring, 3, 4
Red-backed Shrike, 39, 50, 51, 156
Redbreast, 47
Redshank, 139
Redwing, 124
Redstart, 230, 244
Reed-Bunting, 28, 68, 69, 85, 104, 132, 156, 158, 160, 244, 246, 273
Reed-Warbler, 49, 51, 68, 81, 132, 140, 152, 153, 211, 225
Reeve, 171
Relation of song to the territory, 119-68
---- of territory to migration, 259
---- of territory to the system of reproduction, 169-214
Relationship to a territory, 169
Reproduction, 14, 15
---- and territory, 169-214
---- attainment of, 2, 6, 37
---- goal of, 6
Ringed Guillemot, 64
Robbery of territory, 104-107
Rock-formation, suitability for Guillemots nesting on, 196
Rook, 202, 227
Routine behaviour, 262
Ruff, 54, 63, 172
---- meeting places for conflicts, 54
Ruffs, promiscuous pairing of, 172
Savi's Warbler, 139
Sedge-Warbler, 25, 44, 152, 226, 244
Selous, E., on the life-history of Ruffs and Reeves, 172
---- on meeting places for conflicts and antics, 54
---- on the meeting places of Black Grouse, 63
Service, Robert, on flocks of unmated Sedge-Warblers, 44, 45
Sexual behaviour, 3
---- conflicts, 10
---- development, 6
---- function, discharge of, 2, 3, 26
---- impulse of females, 13
Sexual instinct in the migratory male, 26
---- of Reeve, 173
---- ---- reawakening of, 4, 18
---- life of birds, 1
---- maturity, males arrive at, before females, 36
---- organic change, 92, 123
---- selection, 166
Shag, 121
Shrike, Red-backed, 39, 50, 51, 156
Skylark, 188, 236, 244
Snipe, 153, 156, 219, 220
Sociability when not paired, 125, 126
Song, as an aid in searching for a mate, 12
---- its influence on mating, 167
---- origin of, 138
---- relation to reproduction, 123
---- relation to territory, 119-168
---- volume of, influenced by age, seasonal sexual development,
or isolation, 166
Song-Thrush, 222, 244
Sparrow, House, 218
Sparrow-Hawk, 269
Spring, at approach of, birds lose their shyness, 138
Stability of the water and mobility of the land, 260
Starling, 217, 218, 237, 251
Stimulation, internal, 62, 123
---- question of, 284
Stonechat, 87 187, 188, 189, 222, 233, 234
Stout, Dr, _Manual of Psychology_, 1, 77
Struggle for existence, 294
Susceptibility to position, 96
Swallow, 21, 156, 278
Territory, 1, 5
---- adjustment of, 10
Territory and reproduction, 169-214
---- breeding, 2, 3, 7
---- dates of acquisition of, 33
---- defence of, 6
---- desertion of, after rearing young, 276
---- disposition to defend, 73-118
---- disposition to secure, 6, 20-72
---- establishment of, 74, 285
---- evolution of, 176
---- failure to secure, 286
---- fights for, 10, 11, 13, 62
---- ownership of, 189
---- possession of, a stimulus to song, 136
---- its relation to migration, 259
---- its relation to reproduction, 169-214
---- readjustment of, 147
---- restriction of, advantageous for mating, 172
---- restricted, 8, 9, 30, 50, 58, 64
---- separate for male and female Cuckoo, 144
---- song, its relation to the, 119-68
---- temporary desertion of, 28, 35, 58, 59
---- and hostility, relationship between, 242
Thrush, Song, 222, 244
Tit, Blue, 221, 226
---- Great, 221
---- Long-tailed, 226
Tradition, 300
Tree-Pipit, 51, 188, 189, 222, 232, 244, 278
Turtle-Dove, 126, 232
Union of sexes, 12
Ussher, H. B., on the hostility between Choughs and Hooded Crows
and Choughs and Ravens, 227
Vocal Imitation, 156, 157, 161
Voice calls of Curlew, 263
Wagtail, Pied, 86, 155
---- Yellow, 278
Wanderings from land, Guillemots, 193
Warbler, Garden, 223, 225, 230
---- Grasshopper, 39, 131, 139, 155, 187, 244
---- Marsh, 39, 40, 52, 81, 132, 140, 155, 156, 225
---- Reed, 49, 51, 68, 81, 132, 140, 152, 153, 211, 225
---- Savi's, 139
---- Sedge, 25, 44, 152, 226, 244
---- Willow, 25, 47, 50, 51, 80, 91, 140, 187, 211, 232, 244, 273
---- Wood, 50, 51, 132, 221
Warfare between different species and its relation to the territory,
215-58
Warning notes, 119
---- of danger, 269
Water Rail, 218
Wheatear, 25, 51
Whimbrel, 140
Whinchat, 39, 50, 51, 81, 222, 232, 233, 234, 244
Whitethroat, 25, 50, 68, 69, 124, 140, 182, 187, 189, 190, 213, 230,
244
---- Lesser, 230, 244
Wild Duck, 250
Will, the, to fight, 102
Willow-Warbler, 25, 47, 50, 51, 80, 91, 140, 187, 211, 232, 244, 273
Winter assemblies, 262-63
Witherby, H. F., in _British Birds_, on the return to former
breeding-ground, 281
Wood-Owl, 156
Wood-Pigeon, 219
Wood-Warbler, 50, 51, 132, 221
Woodpecker, Lesser Spotted, 237
---- Great Spotted, 237, 238
---- Green, 20, 71, 156, 208, 218, 237
Wren, 244
Yellow Bunting, 28, 30, 47, 64, 140, 159, 162, 183, 187, 188, 189,
235, 236, 286
Young die in nest from exposure, 184, 185
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