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Title: The Migration of Birds
Author: Coward, T. A.
Language: English
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*** Start of this LibraryBlog Digital Book "The Migration of Birds" ***
The Cambridge Manuals of Science and
Literature
THE MIGRATION OF BIRDS
* * * * *
CAMBRIDGE UNIVERSITY PRESS
London: FETTER LANE, E.C.
C. F. CLAY, MANAGER
[Illustration]
Edinburgh: 100 PRINCES STREET
Berlin: A. ASHER AND CO.
Leipzig: F. A. BROCKHAUS
New York: G. P. PUTNAM'S SONS
Bombay and Calcutta: MACMILLAN AND CO., LTD.
_All rights reserved_
* * * * *
[Illustration]
THE
MIGRATION
OF BIRDS
BY
T. A. COWARD
Cambridge:
at the University Press
New York:
G. P. Putnam's Sons
1912
* * * * *
_With the exception of the coat of arms at
the foot, the design on the title page is a
reproduction of one used by the earliest known
Cambridge printer, John Siberch, 1521_
* * * * *
PREFACE
Any attempt to elucidate the problems connected with the
Migration of Birds must, in the present state of knowledge, contain
some theory and speculation, but the diligent observations of an
army of careful workers yearly add facts, which though they may
appear insignificant when considered alone, tend in the aggregate
to confirm or repudiate the conclusions of past workers. I have
endeavoured to bring together some of the more important theories,
and to give prominence to ascertained facts; I have also striven
to check desire on my own part to wander into realms of pure
speculation, though conscious that I have not always evidence to
support my suggestions.
The numbers in brackets ( ) in the text refer to the books or
papers mentioned in the list at the end of the volume, which is in
no ways an attempt at a full bibliography. I have quoted freely
from the works of past and living ornithologists. To these I offer
apologies if I have misconstrued their arguments, and acknowledge
my indebtedness to those whose observations or writing have given
me light. In particular I tender thanks to Mr Wells W. Cooke for
his permission to reproduce the maps facing pp. 76, 78, 80. I have
found his writings and those of Herr Otto Herman and Mr W. Eagle
Clarke especially valuable. Mr Eagle Clarke's long looked-for book
on Migration is, as I write, still in the press; had mine been more
than a manual I should have hesitated to publish until his had
appeared.
T. A. COWARD.
BOWDON, CHESHIRE,
_4 November 1911_.
CONTENTS
CHAP. PAGE
I. MIGRATION OF BIRDS 1
Definition--Variation of migration.
II. CAUSE AND ORIGIN OF MIGRATION 13
Direction of passage--The potentiality of flight--Habit
of wandering--Memory--Extension of range--Influence
of Temperature--Desire for Light--Glacial Epoch--Food
Basis--Sexual Impulses--Competition.
III. ROUTES 33
Route or Broad
Front--Coasting--Fly-lines--Isepipteses
--Land-bridges--Coast Lights.
IV. THE HEIGHT AND SPEED OF MIGRATION FLIGHT 47
Altitude of Normal Migration--Variation in
Speed--Effect of Wind.
V. ORIENTATION AND ROUTE FINDING 56
Route Finding--Use of Memory--Eyesight--Errors--Guidance
of Young--Beam Winds--Homing of Terns.
VI. THE DISTANCES TRAVELLED BY BIRDS 65
The Swallow--Variation in Distances--Marking
Birds--Results--Routes of the Golden Plover--Evolution
of the Routes.
VII. MIGRATION AND WEATHER 83
Knowledge of Approaching Weather--Favourable and
Unfavourable Conditions--Importance of Winds--Cyclonic
and Anticyclonic Winds--Continental Migration.
VIII. THE PERILS OF MIGRATION 104
Contrary Winds--Lighthouses and Lightships--Leeward
Drift--Catastrophes.
IX. EARLY IDEAS OF MIGRATION 114
Literature--Hibernation--Carriage of Small by Large
Birds.
X. SUGGESTIONS AND GUESSES 119
Trans-Atlantic Migration--Ship-borne Wanderers
--Storm-blown Birds--Casual Wanderers--Swimming and
Walking.
XI. SUMMARY 126
BIBLIOGRAPHY 131
INDEX 135
LIST OF MAPS
FACING PAGE
Map showing the range of the American Golden Plover,
with its known migration route 76
(From _The National Geographic Magazine_.)
Map showing the evolution of the migration route of the
American Golden Plover 78
(From _The National Geographic Magazine_.)
Map showing the evolution of the migration route of the
Eastern or Pacific Golden Plover 80
(From _The National Geographic Magazine_.)
Map to show that a bird leaving Norway, near Aalsund,
might be carried round the British Islands in twenty-four
hours. The arrows indicate the actual directions
and force of wind at the times marked during a
slow-travelling circular storm in autumn 1901.
Speed of bird about twenty-five miles per hour 98
* * * * *
THE MIGRATION OF BIRDS
CHAPTER I
MIGRATION OF BIRDS
Migration is the act of changing an abode or resting place, the
wandering or movement from one place to another, but technically the
word is applied to the passage or movement of birds, fishes, insects
and a few mammals between the localities inhabited at different periods
of the year. The wandering of a nomadic tribe of men is migration;
the mollusc, wandering from feeding ground to feeding ground in the
bed of the ocean, migrates; the caterpillar migrates from branch to
branch, even from leaf to leaf; the rat leaves the ship in which it
has travelled and migrates to the granary; we pack our goods, hire a
removing van and migrate to a new abode. The word migration thus applied
may be literally correct but it fails to convey the generally accepted
meaning, and the expression Bird Migration suggests periodical and
regular movement, the passage as a rule between one country and another.
The popular application of a term does not do away with the need
of definition, especially as there are many complicated phases of
migration. The migration of birds is as a rule between the breeding
area or home and the winter quarters, but there are many migrants which
never reach breeding quarters in spring, and many others which leave the
regular breeding quarters or the place of residence in winter to perform
a very real migration under peculiar stress of circumstances. Again the
spasmodic movements of certain gregarious species, which at irregular
intervals change their location in large numbers to take up their abode
in another part of the range, is really migration, though it is now
usually described as irruption, incursion or invasion.
Newton says (38) that bird migration is "most strangely and
unaccountably confounded by many writers with the subject of
Distribution," but the very act of the bird which extends its range, the
first step in distribution, is migration. The histories of present-day
distribution and migration are irrevocably interwoven; as Mr P.A.
Taverner remarks (51), "migration is a dispersal, and conversely, this
dispersal, as it manifests itself, is migration," whilst distribution is
the outcome of dispersal.
Broadly speaking, all birds migrate, though the length of the journey
varies in different species, and in some cases in individuals of the
same or closely allied species, from the merest change of elevation to
a voyage almost as wide as the world itself. The sedentary red grouse
nests on the moors, often less than 1000 feet above the sea, but "when
snow-bright the moor expands" it feeds and resides in the cultivated
valley, and as shown by the committee appointed to study grouse disease,
not infrequently migrates from range to range across wide valleys. Many
tropical birds, usually considered non-migratory, are subject to short
movements, the origin and purpose of which is search for food and safe
nesting places.
The knot breeds in countless numbers in Arctic Greenland and America, so
far north that only a handful of ornithologists have traced its home;
it travels south in summer so far as Damara Land. The Arctic tern has
a northern breeding range extending perhaps as far north as that of
any bird, and it has been taken far to the south of South America in
the Antarctic regions; if the thesis that the further north the bird
goes in summer the further south it travels in winter is correct, as
it can be proved to be with some species, some of these terns must
annually travel about 22,000 miles (21). Between these extremes are an
endless variety of distances travelled and methods of migration, with
striking differences in the performances of individuals of the same
species. Take one instance, a song thrush reared in a nest in our own
garden. We may see and recognise this bird up to the middle of July,
but what trained ornithologist can, yet, say with certainty where that
bird will be by the end of the month or in three to four months time?
We know that all through the winter there are some song thrushes near
the house, and that they are the birds which not only begin to sing
early but actually nest with us; we know too that before there is any
marked immigration of northern thrushes there is a recorded emigration
from our southern coasts, presumably of thrushes which have nested with
us, beginning towards the end of July; further we know that there is
an autumn immigration of Scandinavian or other northern song thrushes,
sub-specifically distinct to the expert eye, and some, small and dark,
whose origin is by no means proved, as well as later emigrations of
birds to the Continent or Ireland, both regular and occasioned by
exceptional weather. Will our young July thrush remain in England or
will it join one of these streams, and if so which? We do not know yet.
I repeat "yet," for the study of races, sub-species or local variations
is commanding more and more attention; the patient work of the
"splitters," scorned by the old school of "lumpers," will eventually
solve many of the problems of to-day.
The ancients--a usefully ambiguous term--realised that birds migrated;
our immediate forefathers of two or three centuries ago realised that
certain birds vanished in winter and wondered how; and within modern
times the phenomena of migration, the "mystery of mysteries," has been
the subject of much study, speculation, and literary exposition. Indeed
a full bibliography of migration would be a considerable volume. Even
workers within the last few years have declared that certain phenomena
were beyond human understanding, only to be explained by instinct, a
word capable of most varied interpretation. In truth there is much to
learn, much to which we must still answer--we do not know; but the
speculative theory of yesterday is now either myth or fact, and the
theory of to-day may be proved true and add something to the data of
which knowledge is built. The wildest speculations, based on slender
locally ascertained facts or on no foundation whatever except the
fertility of the brain, have been offered as solutions of the mysteries;
the literature of migration is a jumble of contradictions. John Legg, in
1780, said "In relating so many instances of unparalleled credulity, I
confess I cannot suppress the irascible passion" (33), and Herr Otto
Herman, only a few years ago, pointing out the ingenious dogmas "void
of every firm foundation," says that "really it is a field in which
every thinking ornithologist may create new theses to any extent and
more or less incredible" (31).
Herr Herman's system of "ornithophænology," the accumulation of
substantiated observations and facts, will not prove everything, but
his work in Hungary, that of Dr Merriam and Mr Cooke in America, and
of Mr W. Eagle Clarke in Britain, each aided by a numerous band of
careful workers, are striking examples of what can be accomplished.
Whatever errors future enlightenment may show in their conclusions their
ascertained facts will remain positive knowledge; theirs is not what
Herr Herman himself described as "pretended authority."
In order to grasp the problems of migration it is necessary to get rid
of the puerile and insular aspect of the subject, namely that migrants
are merely those birds which come to us, like the swallow and cuckoo in
the spring, and those, like the fieldfare and brambling, which visit us
in winter but are not with us in summer. The complication of the subject
may be demonstrated by a rough classification of the migrants to be
observed in the British Islands.
Arbitrary grouping of the members of an avifauna is only for general
convenience; many species are represented in more than one group.
1. Permanent Residents: birds which remain in Britain all the year
round. These are comparatively few in number, and largely consist of
insular races of birds which perform regular and often long migration
journeys in other parts of their range. Most, if not all, perform short
migrations, in some cases only seasonal changes of altitude, spending
summer on the hills and winter in the lowlands; examples, the red grouse
and dipper. Others, like the tits and creepers are nomadic and more or
less gregarious in the colder months. Few appear to remain in the same
locality at all seasons, but possibly some of our British robins and
song thrushes, both sub-species of migratory Continental forms, may be
non-migratory.
2. Summer Residents: birds which nest in our islands, leaving in
autumn for countries to the south, and return in spring. In addition
to the regular summer visitors, which all leave in autumn, this group
includes a number of wagtails, pipits, finches and other birds which are
represented in winter in our islands by a proportion which remain.
3. Winter Residents: birds which nest to the north or east of our
islands and arrive in Britain in autumn, leaving in spring for their
breeding area. With birds like the fieldfare, brambling and jack snipe,
which do not nest in Britain, must be included many (for example the
robin, rook, song thrush and common snipe) which are also permanent
residents.
4. Birds of Passage or Spring and Autumn Migrants: birds which neither
nest with us nor normally remain for the winter, but merely use the
British Islands as feeding and resting places on their journey between
the northern breeding area and the southern or eastern winter quarters.
This group is an especially difficult one, for in it must be included
such birds as dunlins and curlews, which are represented as breeding
species in Britain, and also a number of birds which apparently go no
further south than our islands in winter, and others which, though not
breeding, go no further north in summer. The actual status of these
individual birds is uncertain. In this group too we have the Greenland
wheatear, so closely allied to our familiar early migrant that, unless
the bird can be measured, its identification is uncertain.
5. Irregular Migrants: birds which may be classed in other groups.
Some of these are really winter residents, but their visits are so
irregular that they may for convenience be classed with spasmodic or
occasional invaders, such as Pallas's sand-grouse, which arrive at
uncertain intervals in large numbers. Some of their number, during these
irruptions, usually breed and thus the bird becomes an irregular summer
resident or even, for the time, a permanent resident.
6. Stragglers or Wanderers: birds whose occurrence in our islands is
more or less accidental, due apparently to their having lost their way
or to their ordinary wandering habits having taken them far from the
normal range of their species. Some of the rarer petrels and other
oceanic birds certainly pertain to this group, but our knowledge of
the migration routes of others is still so slender that it is unwise
to declare dogmatically that they are lost. Some too of the so-called
stragglers may have been artificially or accidentally introduced; many
"records" prove on investigation to be the aimless wandering of escaped
captive birds, whilst others are known to have been aided in their
journey and carried out of their usual course when resting on shipboard.
When Mr Eagle Clarke was on the Kentish Knock Lightship, off the mouth
of the Thames, he found that in autumn there were continuing practically
simultaneously the following streams of migration. Immigration from
the Continent to England from east to west, and from south-east to
north-west, and passage along both lines; emigration from north to
south-south-west, and from north-west to south-east, with passage from
north to south-south-west. Birds of the same species actually crossed
paths, travelling in contrary directions (16).
The above grouping applies to the British avifauna, but a somewhat
similar arrangement might be made of the birds of any particular area,
large or small. The grouping of birds for the study of Geographical
Distribution is of little consequence in connection with migration,
but the mapping of the world into various ornithological rather than
zoogeographical regions is of considerable importance, both for
convenience in tracing the ranges of migrants, and in the discussion
of the history of migration, which almost certainly began in the
form of short wanderings from the centres of distribution. It is of
comparatively small importance what boundaries we take for the various
regions; these depend largely upon the view of certain ornithologists as
to which groups of birds shall be considered as typical of the regions
in question. Sclater's six regions are perhaps the most universally
used. They are as follows:--
1. Palæarctic, embracing the whole of Europe and northern Asia.
2. Ethiopian--Africa, Arabia, Madagascar and roughly half of the
Atlantic and Indian Oceans.
3. Indian, including India, Further India, Southern China, the western
portion of the Malay Archipelago and the Chinese Seas.
4. Australian, embracing Australia, New Guinea, New Zealand and the
southern Pacific.
5. Nearctic, roughly America north of the Gulf of Mexico.
6. Neotropical, America south of the Gulf.
Newton suggested an alteration, a continuous northern region to be
called the Holarctic Region, which embraces almost the whole of
the Northern Hemisphere, and the division of the Australian into
Australian and New Zealand Regions. Each of these southern regions is
the winter home of some of the Holarctic birds, and it is a matter of
dispute whether many of these originated in the northern or southern
hemispheres. The value of these artificial divisions of the world is
rather in the consideration of the conditions their varied climates and
physical features present as attractions to birds in search of suitable
nesting places and food supplies.
The study of Migration involves reference to the work of ornithologists
of the past and present, the mass of contradictory literature already
referred to, and we are repeatedly faced with the difficulty that some
particular theory about the vexed questions of the cause or origin of
migration, the height and speed at which birds travel, whether they
do or do not follow routes, how they find their way, in what order
they migrate, how and why they do or do not avoid dangers, or any
similar problem, which seems to give finality so far as certain cases
are concerned, is met by an absolute negation in other instances. The
truth seems clear; more than one factor has influence on most birds,
and different species in different places are influenced by different
factors. Elliott Coues' sweeping statement, though I strongly disagree
with the article in which it occurs, expresses much that is true.
"Isepipteses and magnetic meridians, coast-lines and river channels,
food-supply and sex-impulses, hunger and love, homing instincts and
inherited or acquired memory, thermometer, barometer and hygrometer, may
all be factors in the problem, good as far as they function; but none of
them, and not all such together, can satisfy the whole equation."
Some of the theses may be laws or rules, but there are no rules without
exceptions, and these exceptions may become local rules. Laws regulating
migration in one area, whether it be the great continent of America, the
British Islands or the islet of Heligoland, may have little application
in other parts of the world: local evidence alone can never solve the
great problems.
CHAPTER II
CAUSE AND ORIGIN OF MIGRATION
The question--What makes Birds Migrate? or what causes them to remove
from one zone to another at certain seasons, has been answered, no doubt
to the satisfaction of the respondents, in many varied ways. Closely
connected with the question of immediate impulse is the deeper, and less
easy to prove problem as to how migration originated.
It has been dogmatically asserted repeatedly that birds invariably
breed in the most northerly part of their range, and winter in the
most southerly. Winter, when speaking of Holarctic birds, only applies
to the season in the northern hemisphere; the birds which pass south
of the equator winter in summer. Whilst accepting this as a rule, two
reservations must be made. First, that it only applies to birds of the
northern hemisphere, and secondly that it is a rule with exceptions. It
seems probable that the breeding area of some of the birds which reach
the British Islands in autumn by the so-called east and west route is
in more southerly latitudes than our islands, and certainly it seems
evident that the temperature of the winter refuge has more effect upon
the birds than its geographical position. Perhaps the statement that a
bird always nests in the coldest part of its range is more universally
correct. Even this may not be invariably the habit, but in acknowledging
it as a rule we must clearly understand that this cold district is
resorted to at the period of the year when its temperature is at its
highest. There are certain birds which breed in Australia and winter in
Oceanic islands where the temperature is cooler than in their breeding
area.
When considering the migration of birds which summer in the extreme
north or breed in the extreme south--alas, but little is known about
the migratory habits of many southern breeders--it is comparatively
simple to offer an explanation; in the long winter months this home,
so desirable in the short weeks of daylight, is dark, ice-bound, and
foodless; it is wholly unsuited to the requirements of birds, which,
in spite of many assertions to the contrary, have never been proved to
hibernate, the only way in which animals can survive for any lengthened
period when food supply is entirely cut off.
Birds are structurally provided with the means of escaping from the
disastrous effects of adverse circumstances; the power of flight, though
not the only way in which animals can migrate, is at the root of the
migration of birds. The advantages of the power of flight, to which also
it owes its development, include the ability to avoid active and passive
enemies, and to remove from one feeding ground to another undeterred by
the barriers which restrict the terrestrial animal. A natural sequence
of this ability to take advantage of aerial locomotion is the habit of
wandering in search of food, more or less noticeable in all birds. The
habit of wandering led to the discovery of feeding grounds and suitable
nesting places; where these nesting places, probably at first, only
removed a short distance from the parents' nesting site, were suitable,
dispersal and an extension of the distributional area or range of the
species followed; but where the feeding area was unsuited or not so
well suited to the needs of the species, hereditary attachment to the
original home and memory of the direction of this home, or even in some
cases accidental wandering back to the more suitable locality, would
originate a migration. Coupled with this are two important factors which
would tend to make the habit periodical and regular both as regards
time and locality. The memory of the bird, call it instinctive memory
if we like, would limit the wanderings in search of food to a certain
number of places where food was most abundantly found, and the passage
between feeding area and breeding area become regular journeys, at the
seasons of the year when an increasing number of young birds in the
breeding area drove the overgrown population to seek food further from
the base, and again when the sexual impulses urged the birds to seek
secure nesting sites. The other factor is the weeding-out influence of
mistaken effort, the natural selection which leads to the survival of
the fittest. The young wanderer which reached unsuitable lands must
either wander further or perish. Judging by the juvenile mortality
amongst young birds the failures would be many, and only the successful
competitors would return to leave progeny.
Great stress has been laid on the attachment of birds to certain nesting
sites, an undoubted fact, and it has been argued that because, in some
cases, for hundreds of years certain sites have been occupied by the
same species, it is evident that after the death of parents the young
will return to and occupy the home. This has even been put forward as
evidence that birds do not wander in search of fresh nesting sites. The
argument is not sound. It is improbable that in most cases both parents
perish in the same year. Birds of prey, and many of the cited instances
of long tenancy refer to raptorial birds, have a wonderful power of
finding a mate, male or female, to complete the hatching and rearing of
the young, when one of a pair has been destroyed. The survivor of any
pair might have the home attachment and by bringing a fresh mate create
an attachment which would be passed on from mate to mate indefinitely.
Again it must not be overlooked that certain sites present advantages
to particular species which must be evident to all in search of those
advantages; it by no means follows that the occupiers of a nesting site
are in any way related, except specifically, to those which occupied it
in previous years.
The answer to the argument that birds do not seek fresh nesting places
and thus extend their distributional area, is evident when we consider
those species which, at the present time, are extending their range.
Within the last few years, for instance, the turtle dove and tufted duck
have begun to nest regularly in many parts of England in which they were
entirely unknown twenty or thirty years ago. The starling has spread and
in some parts is spreading still, and many other similar cases might be
cited.
In this manner migration, as we know it to-day, may have originated, and
as Mr P. A. Taverner expressed it, "however instinctive their habit may
now be, there must have been a time when migrations were intelligent
movements, intended to escape some danger or secure some advantage"
(51). Granting this, however, as the first cause, we are only on
the threshold; the question still remains unanswered, what actually
impels the birds to seek fresh food supplies or to look for safe nesting
places? The natural answer, the cravings of nature and sexual impulses
fails to give satisfaction in every case. Wanderings in search of food
might lead in any direction, and probably did in the first place, but
now birds in the main travel south in search of food and north in search
of home, and many of them perform immense journeys, passing over or
through lands which are capable of supporting a wealth of bird-life even
in the winter months.
The majority of Arctic birds or those nesting in high latitudes leave
before the great harvest of autumn fruits, and even our common swift
begins to depart--for all do not go at once--towards the end of July,
when insects are more abundant than at any other time of the year. Food
supply has not failed when most birds start their journey in search
of food! Again in spring, when it is claimed that the powerful sexual
impulses are sufficient reason to account for the northward journey,
hosts of sexually immature birds and of others which are apparently
mature but do not breed that spring, migrate northwards, some even
arriving before the mature birds of their own species.
The earlier students of migration insisted that temperature was the sole
cause of change of abode; that the northern lands became unsuitable
through their falling temperature, and that the birds deserted them for
warmer climes, returning when the lands they wintered in became too hot.
As a variant of this notion, which cannot be lightly cast aside, the
suggestion was mooted that it was not cold but the lack of food during
the cold months which drove them south, and that in the Tropics, where
at one time it was thought that all migratory birds wintered, food was
scarce during the months of extreme heat. Dr. Wallace went further and
stated that the incentive to northern migration was the inability to
find sufficient soft bodied insects suitable for the nestlings in the
Tropics during summer (54). Yet there are birds which do find food
enough for their young, and some of them are insect eaters.
Seebohm, arguing with reason that the first home of the _Charadriidae_,
was the Polar Basin (44), suggests that the desire for light
originated the idea or the action, and though this was only applied
by him to Arctic birds, others have striven to show that the longer
hours of daylight would be an advantage to all birds, even though the
difference of dark and light in the zone retired from and in that
arrived at might be inconsiderable (41). Against this must be taken
into consideration the fact that many waders and ducks, northern
breeders, feed by night or day, according to the state of the tide.
Light is not an absolute necessity to them.
The suggestion that migration owes its origin to the Glacial Epoch,
"that supposed solution of so many difficulties," to quote Mr Gadow
(28), has had many exponents. Some take for granted that the Polar
Regions were the original home, the centre of dispersal, of all northern
birds, and consequently that migration originated in the gradual pushing
back of avian life as the ice gained more and more land each year.
During the summer, the birds, urged by an irresistible love of home,
travelled as far north as the ice allowed them, but gradually they were
driven to nest further and further south until they found refuge in the
unglaciated parts of the earth. The individuals and the species, if not
the whole families of birds, which failed to retreat, went the way of
the "thousand types." On the retreat of the ice, the birds, impelled by
a mysterious hereditary memory of home and of the good times enjoyed by
their remote ancestors, for very very many generations must have been
born under more or less sedentary conditions during the Ice Age, began
the same pushing forward each year to the limits allowed them. In this
case they travelled nearer and nearer to the original home instead of
constantly being driven further from it.
Surely the question of original home, at any rate of the home in
pre-Glacial days, may be entirely left out of the question. No one can
ever prove that this wonderful memory did or could exist. Post-Glacial
dispersal northwards, and the foundation of migratory habits of
advancing to the new food-producing areas, suitable also for the rearing
of young, was doubtless a fact, but would have taken place in any case.
The congestion due to the increased numbers driven to a restricted area,
would involve a rebound outwards, and the uninhabited areas northward
of the refuge would be the natural bourn towards which the birds would
travel. The seasonal return of cold would drive them southwards in
winter, and the periodical migration habit would thus be originated.
The intense love of home during the spread of glacial conditions would
tend rather towards extinction than the formation of any new habits.
The birds which possessed the greatest attachment to the particular
district would be less likely to fly from adverse conditions, and the
reduction of their numbers through the ordinary physiological changes
in habit--reduction of the number of young produced, and possibly
disinclination to pair--would inevitably end in extinction. The stronger
the attachment to home the more likely the bird to remain to the bitter
end, and if driven away by increasingly severe winters, to return
and attempt to nest in the locality which had become unsuitable for
nesting. The spread of glaciation would be gradual and so would be the
annihilation of the species, but the end would be sure.
Birds which are cited as species which have shown this remarkable
attachment to home, have disappeared before adverse circumstances--the
great auk and the Labrador duck.
From what little we do know about the behaviour of our summer birds in
their winter home, we may safely conclude that their habits are similar
to those of winter visitors to Britain. Only in a few species are there
two restricted areas, two abiding places or homes. The necessity of
retaining a secure home for the young and the care of these young during
their more helpless age keeps the individual birds within a certain
area during the breeding season, but at all other times the bird is
more or less of a wanderer. The variation, however, of the wanderings
is remarkable. For instance the flocks of fieldfares, redwings, and
some of the finches which come to winter in the British Islands wander
continually from feeding ground to feeding ground, remaining in one
place only so long as the food supply is plentiful. When there is a
plentiful harvest of beech-mast, chaffinches and bramblings will linger
near one clump or avenue of beeches for many weeks, but when, as often
happens, the mast crop fails, they become nomadic, and pass from place
to place in their hunt for food. They visit fields top-dressed with
manure, glean the refuse of the harvest, frequent the farm-yards, and
in early spring, visit the budding larches to prey upon their insect
pests. On the other hand golden plovers and lapwings are remarkably
local in their winter habits, and so long as the weather remains open
will frequent the same fields throughout the winter. Severe weather,
especially snow, which effectually closes their chance of obtaining
food, at once drives them away. They will migrate to the unfrozen
mud-flats of the coast, or to those parts of England, generally the
south-west, and Ireland, where the climate is normally milder, or they
will even leave our islands altogether under great stress.
The wandering habit, except during the breeding season, is confirmed in
most birds, and experience shows that the same species of birds visit
the same districts again and again when there is some particular food
supply to attract them. Memory and experience guide them from place
to place. This regular visitation of certain food bases, being of the
greatest importance to birds which have a long period of travel or
wandering before them, tends to originate the so-called route by which
they travel. The fact that as a rule these stages are in consecutive
steps southward is surely due to the fact that the temperature is
falling in the north more rapidly than in the south. That they are not
always due south is certain. The American golden plover, as Mr Wells
W. Cooke so lucidly demonstrates, at first travels eastwards from its
home in western Arctic America to the fruit-laden lands of Labrador
and Nova Scotia, where it feeds for some time, stoking up for its long
oversea journey due south. Mr Cooke says, "It can also be said that food
supplies _en route_ have been the determining factor in the choice of
one course in preference to another, and not the distance from one food
base to the next. The location of plenty of suitable provender having
been ascertained, the birds pay no attention to the length of the single
flight required to reach it" (21). During the evolution of the route
many bases would be found which were superior to others, and skipping
and the gradual shortening of the journey from one to another would
result. The final goal, the food base which in any weather or season
provides the safe sufficiency of food, having been reached by the birds,
this becomes the winter quarters. They return to this secure retreat
each winter, instead of aimlessly wandering in search of a better, and
thus the long-distance migratory habit is formed. Heredity tends to
confirm this and it becomes an instinct.
Any observer may verify the assertion that birds regularly visit certain
favourable food-bases by paying attention to the occurrences of birds of
passage. The study of a county, for instance, shows that certain species
show partiality for particular localities. Thus in Cheshire goldeneyes
pass through every spring and autumn, and may be met with occasionally
on any of the meres; but at Oakmere, in the Delamere district, one may
be almost certain of seeing parties of this species any time during the
periods of passage. The curlew may be heard or seen passing over any
part of the county, but only in the Delamere fields do we frequently
meet with flocks feeding in inland Cheshire. Before the winter resident
golden plovers have arrived in autumn and after they have departed in
spring, the favourite fields are regularly visited by passing flocks,
and the lower reaches of the Mersey, where the common sandpiper is rare
as a summer resident, are visited every autumn by parties of birds on
passage. Chance may lead a casual wanderer to a good food-supplying
spot, but the regularity of appearance suggests habit and memory.
A fact which supports the theory that birds ramble far in search of
food in their winter quarters, is that in many species the winter range
is more extensive than the breeding area. Thus Mr Cooke shows that the
known breeding area of the Pacific golden plover has an east and west
extension of some 1700 miles, but in winter it ranges over an area with
an east and west extension of about 10,000 miles. The scarlet tanager,
however, has a breeding range extending for some 1900 miles across
eastern Canada and a winter home in north-western South America of only
some 700 miles in extent.
The winter quarters, or the outermost limits of the individual but not
necessarily the specific range, having been reached, the bird spends
its time in seeking food, remaining in one place if food is plentiful,
or wandering, according to necessity or the habit of the species.
The assertion that some birds have a second breeding season in their
southern home is either unsupported by any direct evidence or is the
result of a mistake in identification; the bird which has been found
breeding has in several instances been shown to be a southern form or a
related species of the one it was thought to be.
As the northern spring approaches, the strongest of all animal
instincts, on which reproduction and the very existence of the species
depend, overcomes all other desires, and the bird grows restless. The
hereditary instinct, the origin of which we have endeavoured to show,
urges the bird to seek the breeding area which has by degrees become so
far removed from the winter quarters. The bird returns home.
But here is a serious difficulty urged by some writers as a powerful
argument against the sexual impulse as the great factor in the return
journey. Many of the birds which migrate northwards or homewards are
sexually immature, and others of them are undoubtedly to be classed as
"non-breeders," which means that during that particular summer they will
not be engaged in the work of reproduction; why, then, should young
birds or non-breeders migrate from the winter base. Possibly in the
early days of migration only the mature birds did return; that we cannot
state one way or the other. But it is reasonable to argue that once a
regular migration habit has become not only confirmed by heredity but a
very true advantage to the species, its influence will be felt by each
and every individual. Again it is clear that the sexual impulses, in an
undeveloped form, are appreciated by the adolescent, and in many animals
by even the most juvenile. The play of all young animals is either an
imitation or reflection of the search for food--the hunting instinct--or
the love-making and sexual quarrels pertaining to reproduction, the
pretended competition by the young for the favours of the opposite sex.
They may play at and actually perform a migration which is so closely
bound up with the life of the species. That this impulse has not always
sufficient strength to force them to perform the whole journey is
apparent from the fact that many non-breeders, young or sexually mature,
on their northward journey through our islands or along our coasts,
never reach the breeding area; the food supply on the way attracts them
more than the memory of home; they linger with us until the breeding
season is over and the return journey has begun. Knots, sanderlings,
turnstones and many other waders may be seen on passage late in June,
and some remain on our mud-flats throughout the summer; in July the tide
of migration has turned.
It has been suggested that some of the sexually mature non-breeders may
be actually enjoying their winter during our summer; in other words that
they have bred in southern breeding-stations whilst their congeners
wintered in the same zone. This means a double breeding-area for certain
species--a possible explanation, but one hardly supported by known
facts. When a bird had so cosmopolitan a range that in the course of
its dispersal its breeding areas were separated, we almost invariably
find that the birds inhabiting these two areas are distinguishable
geographical forms or sub-species. Mr W. H. Hudson, in his "Naturalist
in La Plata" refers to the godwit, _Limosa haemastica_, which spends
the southern summer in La Plata and breeds in the north, and to birds
of the same species which winter in La Plata, arriving from supposed
breeding places to the south when the northern birds leave. Captain R.
Crawshay, author of "The Birds of Tierra del Fuego," found it in this
little known land, but speaks somewhat doubtfully of its identity; we
shall probably learn that the southern form is sub-specifically distinct
from the northern. There are other wide-ranging waders which are
suspected of having a southern nesting area, but we still await proof.
The lack of sufficient or suitable food in the winter home during our
northern summer may also cause the exodus, but this is a difficult point
to prove when it is remembered that the winter home of every bird is
not the parched tropical land or the waterless desert. From some zones
removal must be a necessity, but in others there is food for all, so far
as man can tell.
Dr J. A. Allen, a severe but discriminating critic of migration
theorists, says--"Migration is the only manner in which a zoological
vacuum in a country whose life-supporting capacity is a regular
fluctuating quantity, can be filled by non-hibernating animals" (51).
When in the early days of migration this periodically-supplied northern
zoological vacuum was filled to overflowing by the increased numbers of
avian inhabitants at the close of the breeding season, the natural food
supply would be taxed to its limits; the falling temperature drove some
and finally all to seek food further south, and their short migration
to lands already filled with old and young birds, caused pressure and
overcrowding further south. Further outward and usually southward
movement was necessary and the zone of stress was gradually extended,
though probably in those early days no particular species took long
passages. The winter passed and the vacuum was again provided, and the
rebound to fill it would create a slackening force all along the line;
birds would spread from congested districts so soon as food supplying
areas opened to receive them.
Mr Taverner, arguing on these lines (51), shows that competition
would be originated in areas containing the earliest breeders, and be
severest in the most productive districts. Weaker and later breeders
would be driven out or prevented from colonizing by the stronger and
earlier species, and the evicted ones would encroach on others, forcing
them in turn to trespass on a wider circle of species. He then argues
how the gradual recession of the glacial ice would increase the possible
northward breeding area, and cause longer migration, and that this
migration would delay breeding and conversely delayed breeding would
assist the evolution of migration.
But the lengthening of the journey might surely be occasioned in
another way, and the evolution of migration assisted apart from any
glacial influences. Each successive increase of the length of the
journey taken by the stronger and more go-ahead individuals, leading
them in advance of the bulk of southward moving and competing birds,
would be a distinct advantage to the individual and consequently to the
species. The pioneer would arrive, like the slower movers, in a land
already peopled with an avian population, but it would not have its own
fellows to add to the stress of competition; it would be ahead of the
greatest struggle. So the fittest would mould for the species the most
suitable journey both in distance and route, and the laggards would
gradually fall out of the competition.
Dr Wallace, without destroying these arguments, has shown that the
survival of the fittest has a powerful influence. Those birds which
do not leave the breeding area at the proper season will suffer and
ultimately become extinct, and the same will happen to those which fail
to leave the winter quarters when it would be a distinct advantage to
the species to move into lands better suited for reproduction.
It has been put forward as a serious objection to many arguments that
migration, instead of being advantageous to birds, is a danger to the
race; that the perils of the journey are greater than those occasioned
by more sedentary habits. It has even been suggested that migration is
a habit specially created to thin down the surplus bird population. Dr
W. K. Brooks, however, puts this idea, which is not entirely devoid of
truths, in rather a different way. "Adaptations of nature are primarily
for the good of the species--beneficial to individuals only so far as
these individuals are essential to the welfare of the species" (9).
The destruction of overabundant young, the thinning down of superfluous
numbers, may be an economic advantage. It is one thing to say that
migration has been caused to kill off a surplus, and another to show
that, once a habit has been originated and become an advantage, it will
be conducive to a greater prolificness, and that the natural sequence
of an increased birthrate, when food supply and other conditions remain
unchanged, must be an increased mortality. Thus the perils of migration
may become a boon to the species.
The theories of C. L. Brehm (7) and Marek that birds are living
barometers, foretelling by intuition the changes of barometric pressure,
may be dismissed as purely speculative. That birds begin their journeys
during particular barometric conditions is certain, but what they know
of forth-coming weather conditions is guess-work.
* * * * *
CHAPTER III
ROUTES
The migrating bird, when passing between the breeding home and the
winter quarters, travels by what is termed its Route. The definition of
the route has caused more controversy than perhaps any other incident
of migration; the chief point at issue is whether the bird uses a
particular high road, along which all its fellows from the same area
travel, or if all birds move in what has been called a "Broad Front."
Ornithologists have been, and to some extent still are, divided into two
camps, one upholding defined routes and the other the extended or broad
front movement.
After all the difference is merely one of degree. Even the widest
notion of the broad front, that of Gätke, who insisted, as dogmatically
as he did on most points, that the width or breadth of the migrating
host corresponded with the extent of the breeding range (29), is of
a route, bounded on the one hand by the northern or eastern and on the
other by the southern or western lines of latitude or longitude which
marked the limits of the range. The idea of a route may be narrowed down
to the extent of a wide river valley, or to a fly-line represented on
a map by a ruled line, which passes over certain ascertained places.
The absurdity of Gätke's arguments are proved by the study of his truly
remarkable book. According to him the island of Heligoland was only
remarkable in that it possessed an observer, himself, who saw marvels
unobserved elsewhere, though the same number of birds were every year
passing over any particular spot in an area which, for many species,
must have been many degrees in extent.
Had not so much weight been placed upon, and so many arguments based
on Gätke's extraordinary statements by, unfortunately, many of our
leading British ornithologists, his theories might have been ignored.
Unfortunately he is looked upon as an authority, even an oracle,
whereas, as Dr Allen pointed out, on many points which he treats with
great positiveness his knowledge is obviously as limited as the little
field which was the scene of his life-long labours (2). Glibly he
tells of hooded crows "in never-ending swarms of hundreds of thousands"
passing across and for many miles on either side of the island; of
"every square foot of the island" teeming with goldcrests, and of "dark
autumn nights" when "the sky is often completely obscured" by the
migrants, which pass thousands of feet overhead. How did he observe the
obscured sky? Indeed he again and again declares that migration passes
unseen yet calculates the numbers observed on the darkest nights; the
illumination of the lighthouse could not be sufficient to enable him to
even guess at the numbers he mentions. After stating that "the whole
vault of heaven was literally filled to a height of several thousand
feet with these visitors from the regions of the far North," and that
a certain east to west passage extended from the Faroes to Hanover, he
concludes that "the view--that migrants follow the direction of ocean
coasts, the drainage areas of rivers, or depressions of valleys as fixed
routes of migration can hardly be maintained."
As emphatically he maintains that most observable migration over
Heligoland is due east to west or west to east, though the birdstuffer
Aeuckens, who supplied him with much of his information, told Seebohm
that it was north-east in spring and south-west in autumn (45). Is
it not perfectly evident that the geographical position of Heligoland
makes it a convenient resting place for large numbers of migrants, for
it is certainly true that large numbers are observed there, which pass
southward and westward along the Baltic, crossing Schleswig-Holstein and
the mouth of the Elbe, or coast south along Denmark, and cross the Elbe
diagonally, _en route_ for the Dutch and French coasts and to a lesser
extent the south-east coasts of Britain?
Coasting undoubtedly exists; birds, day migrants especially, may be
observed following coast-lines in steady flight, though a mile or less
inland no passage is visible. On the North Norfolk coast I have seen
little parties of swallows passing along the shore in spring, coasting
slowly but steadily from east to west. All day long and almost every day
for more than a week this steady flight was continued, though I never
saw any passing within sight more than a few yards out at sea, nor any
at all more than a few hundred yards inland. Evidence which cannot be
refuted shows that this habit of coasting is general, though a deeply
indented bay, an estuary or strait, is usually crossed, and by no means
always at the narrowest point. The same careful observations prove that
both narrow and wide river valleys are followed by migrating birds in
greater numbers than are ever observed passing beyond the limits of
these valleys.
Seebohm's experience in Siberia led him to doubt the existence of
routes, but his later studies of migration in autumn at Arcachon and in
spring at Biarritz, caused him to modify his ideas. He found a gentle
but continuous stream of migrants following the coast of the Bay of
Biscay, arriving from over the Pyrenees on their northward journey, but
moving "only within a mile or two of the coast." He contrasts island
and coastwise migration; in the latter the travellers can rest at night
or take short journeys during bad weather, but in the former they must
await favourable conditions before attempting a perilous passage (45).
On the other hand many birds undoubtedly pass over inland localities
independently of any river valley or mountain range which might
indicate a route. Even such typical coast-lovers as the maritime waders
constantly cross or pass through inland England. They are heard at
night, or met with resting or feeding on inland waters, or their bodies
are found when, on a dark night, they have collided with telegraph or
telephone wires.
So long ago as 1886 Mr W. Brewster maintained that the breadth of the
fly-line varied according to the character of the country which was
being crossed. The migrating column, he said, might be hundreds of
miles in length, "a continuous but straggling army," which only became
a "solid stream" when travelling through some narrow pass (8). This
solid stream or army passage is, however, seldom observed when the
birds are crossing continents, especially if they are traversing a wide
area in which food is equally plentiful for miles on either side of the
direction of flight. The consolidation of their numbers appears only to
take place when, either on account of the indifferent food-supply or of
unsuitable weather conditions, the speed is accelerated.
In America Mr Cooke proves that the Mississippi Valley is undoubtedly
utilised as a fly-line by a large number of species, but by no means
all, and his evidence, though proving the use of routes, is that
these are seldom constricted pathways but broad areas crossed in a
generally coincident direction by the birds which make use of them.
This main fly-line is however formed in America as in other places by
the convergence of subsidiary streams, and it is these tributaries,
as Herr Herman points out, which have in many instances led to error;
they have been mistaken for main routes. The main route may be compared
to the trunk of a tree, the birds following the roots from the area
in which they have been nesting or wintering, and at the end of the
journey splitting off in various directions, like the branches, to their
temporary winter or summer homes.
The contrast in the method of travelling of different species or of
the same species under different conditions, may be realised by taking
two examples. Firstly, Mr Eagle Clarke's experiences at the Eddystone
and Kentish Knock Lightship, when birds passed during the daytime at
varying elevations, sometimes close to the waves, in twos or threes or
scores, and at night in large numbers. The other is an observation of a
"bird wave" by Mr P. Cox, during a snow storm in 1885 at Newcastle, New
Brunswick. The birds passed eastward in a column about twenty-five yards
wide, some just above the trees, others hardly visible, but the bulk in
a massed column directly over the margin of the shore, and not over the
river or meadow on either side. The movement was continuous for about
two hours.
Dr I. A. Palmén was the great upholder of routes in the Old World, but
his routes were largely speculative; they were founded on a considerable
knowledge of migratory birds, but not sufficient to cover the vast
area mapped out (39). Until a very large band of workers, working
on similar lines all the world over, accumulate a sufficient mass of
evidence as to which birds do or do not pass their various stations,
with the times at which they appear, accurate knowledge of the routes of
birds is impossible.
Von Middendorf collected statistics of the passage of birds in the
Russian Empire, and by reckoning the average date of arrival of a few
species at certain points of observation, worked out a number of curves
or lines which he calls "isepipteses," or lines of simultaneous arrival
(35). The result was, according to his argument, a general convergence
northwards; the birds passing through Central Siberia travelled roughly
in spring from south to north, in Eastern Siberia from south-east to
north-west, and in Europe from south-west to north-east; they converged,
in fact, upon the Taimyr Peninsula. This to some extent is doubtless
true, but Middendorf goes on to prove that the magnetic pole is situated
in this Peninsula and that the birds are drawn thither by magnetic
influence, "in spite of wind, weather, night or cloud." He calls them
"sailors of the air," possessed of an internal magnetic influence.
He supports his argument by the statement that there is a similar
convergence in North America towards the magnetic pole of the western
hemisphere.
But all birds do not go in the direction of the magnetic poles, and many
of those which do, stop short at suitable breeding places long before
they have travelled so far north. The Taimyr Peninsula, a vast area in
the extreme north of Siberia, is each spring a "zoological vacuum";
towards this desirable spot migrants will stream.
Herr Otto Herman cleverly shows the absurdity of many of the reputed
routes by cartography; his map is crossed in all directions by the
routes upheld by various theorists. Birds could not possibly follow all
the directions "which authors invented for them," most of which he adds
are founded on mere supposition (31). Dr Palmén, he shows, usually
managed to avoid districts where there were no observers, but Mr Dixon
and M. Quinet made their routes follow rivers and coast lines, whether
there was evidence to support this idea or not.
Only to a certain extent can it be safely contended that the present
route of a species is an indication of its earlier journeys, or that
the direction of original dispersal is recapitulated in the present
line of migration. Heredity, experience, and imitation would certainly
tend to preserve and confirm the general direction; the shortest and
easiest passage from food-base to food-base would become an hereditary
route, unless circumstances arose which caused a change. Mr Cooke
shows how there has probably been evolution of the route as well as
of everything else concerned with a mutable animal. The fly-line
across an arm of the sea may be lengthened if this lengthening means a
corresponding advantage in reaching the desired haven. Thus the birds
which now cross the Gulf of Mexico at its widest part, at one time
probably coasted round the Gulf, as many do still, by the land-bridge
of Mexico and Central America. The gradual straightening of this curve
would shorten the journey both in time and distance, though lengthening
the actual single flight across a portion of the sea. We can imagine
a bird arriving in autumn at the mouth of the Mississippi, at first
passing from Louisiana to Mexico, so as to save the time of travel
through Texas. Generations later the shortening of the journey, through
lengthening of the short cut, would lead the birds to Vera Cruz and
later still to Yucatan. It may be questioned, what object could the
birds have in risking an oversea voyage, away from chance of food
and hope of rest, when the land-bridge remained open for them? Each
individual or group of individuals which arrived at any particular place
a little in advance of the migrating multitudes of its own species,
or others which fed upon the same kind of food, would certainly gain
advantage, and would be the most likely to develop strong flight and the
power of endurance in its descendants; it would indeed be a winner in
life's race.
Great weight has been placed upon the use of land-bridges and the
hereditary habit of crossing seas where these land-bridges once existed
but have been submerged during the great geological changes in the
earth's surface. Many have insisted that wherever migrants cross the sea
they do so along submerged coast-lines or over submerged land-bridges,
arguing that the gradual evolution which has made the advantageous
adoption of a habit of migration possible was unable to eliminate
the hereditary tendency to follow the exact route by which their
ancestors passed from place to place. That there have been considerable
alterations in coast-lines and in the general distribution of land
and water since the time when birds began to be migratory is indeed
probable, but unless crossing the sea means a distinct advantage it
implies the retention of a habit which would not only be useless but
might be a positive danger to the species.
In the Gulf of Mexico and the Caribbean Sea there is evidence of perhaps
the most recent land-bridge in the chain of islands from Florida to
Venezuela, collectively known as the West Indies. Although vast numbers
of North American birds winter in South America only a few of the
species which annually pass from one continent to the other make use of
this comparatively easy passage. One might naturally conclude that the
final severance of England from the Continent was in the neighbourhood
of the Straits of Dover, yet this short passage is only used by a
comparatively small number of our migrants.
Mr Dixon indeed argued that there is no greater barrier to migration
than even a narrow arm of the sea (26). He refers to many Continental
species which are common breeders in France but are unknown as nesting
species in the British Islands, and others which are found in England
but not in Ireland. But this is surely but an incident of distribution;
the narrow strait or even river may for a time mark the limit of
expansion of a species, just as at the present time the westward and
northward unseen barrier prevents the range of the nightingale from
spreading to districts apparently well suited for its home, and until
recently the turtle dove and great crested grebe were checked in their
northward advance.
In the evolution of some routes land-bridges certainty appear to have
played their part, but once those bridges have ceased to influence
direction the shortening of the time occupied by the lengthening of
the single oversea flight is only a question of generations when an
advantage to a species is to be gained.
This subject will be further dealt with in connection with the actual
passages performed by certain birds.
The study of migration, based on observations at our lighthouses and
lightships, shows that even in the comparatively small area of the
British Islands there are certain routes followed with regularity. The
birds which pass along our western coasts of England and Wales do not as
a rule follow the shores round Cardigan Bay or along the eastward tidal
scoop of the Irish Sea towards the coasts of Lancashire; the main body
passes from Pembroke to the Lleyn Peninsula, and thence to Anglesey and
the Isle of Man, on its way to the southern Scottish shores.
A source of possible error in the method of deduction from these results
must be taken into consideration. The observations at lightships and
lighthouses are mostly made when untoward circumstances bring the birds
within range of vision, and on dark and foggy nights cause them to
strike the light in great numbers. What is their normal course when
no great migration wave or "rush" is observed? Are the few passing
stragglers noted all that go by this route in fair weather? The same
uncertainty must be applied to the observation of passing birds in
inland localities. The immense numbers which do pass is shown by the
observation of large movements, when as occasionally happens some
check to normal migration leads an army of birds to a dangerously low
altitude, or when high winds hold up a portion of the host on our
coasts; but even these multitudes must be small compared with the
millions of birds which annually pass from zone to zone unseen. The
few or many birds we meet with, either on the coast or inland, resting
on passage, may represent a lost or wandering party of stragglers or
weaklings from a vast army which has passed over; they may or may
not be on the route or course normally followed by the majority. The
cartography of bird migration is a study in itself.
Mr Abel Chapman, describing his experiences in the Mediterranean,
says--"For forty hours we were passing across (or beneath) the lines
of an army of migrants--say 500 miles in width; yet not a sign did we
see, save only the wreckage--the feeble that fell out by the way." On
April 10th a sudden bitter northerly gale sprang up, and two hours later
the steamer was the goal of hundreds of birds, no longer able to face
the adverse wind. These were blue-headed wagtails, swallows, martins,
pipits, wheatears, nightjars, and lesser kestrels. He thinks that the
strong ones may have passed on but all the others perished (12).
* * * * *
CHAPTER IV
THE HEIGHT AND SPEED OF MIGRATION FLIGHT
In the last chapter reference was made to the great height at which
birds may fly on migration. Certain species, even comparatively
weak-winged ones, appear normally to fly high, whilst others, often
birds with pointed wings and great aerial powers, usually proceed at low
elevations; but there is still much conjecture as to the actual altitude
reached by any migrants.
Gätke was of opinion that we do not see much of real migration, which
is certainly correct, but there is no reason for his statement that
it normally takes place at anything like the altitudes he mentions,
30,000 feet or more, or that birds at the time of migration undergo
physiological changes which enable them to fly at immense heights and
speeds and to see clearly in the dark (29). Nor need much weight be
placed on the speculation of Lucanus, who contends that the height
of travel must be less than 1000 metres, for above that elevation
aeronautical observations show that perspective lessens. There are
actual observations which, though liable to a margin of error, are proof
of migratory flight at very high altitudes.
Most of these observations were at first made by accident; birds were
seen through astronomical telescopes passing across the face of the moon
or sun; but recently this method of observation and several ingenious
plans of measuring height and speed have been made use of expressly to
study migration. W. E. D. Scott, at Princetown in 1880, thought that
by shape and size he could even recognise two species, _Chrysomitris
tristris_ and _Quiscalus purpureus_, which passed across his field of
vision at a height of at least half a mile above the earth (43). In
1888 Mr Frank M. Chapman published an account of similar observations;
he calculated that the birds passed at distances varying from one to
five miles, and that their altitudes must therefore have been between
600 to 1000 feet, and 3000 to 15,000 feet. He adds an important note:
"A number of birds were seen flying upwards, crossing the moon,
therefore, diagonally, these evidently being birds which had arisen in
our immediate neighbourhood, and were seeking the proper elevation at
which to continue their flight," but the direction of flight of most of
the birds which were observed was parallel to the earth's surface and
southerly. The average height was certainly far above the inferior limit
(13).
Mr F. W. Carpenter, reviewing these astronomical calculations, says
that Verey compared the apparent size of birds with lunar features, and
considering that the small birds noticed would average 6 inches in
length, calculated a height of 2000 feet above sea-level. At Detroit,
Winkenwerde obtained estimates of a little over half a mile, and R. A.
Bray, in England, saw birds crossing the sun in September 1894 which
were invisible to the naked eye and must have been 2 or 3 miles away
(11).
In December 1896 Mr H. H. Clayton, at Blue Hill Observatory, saw ducks
flying at a height of 958 feet. He was at the time engaged in measuring
the height and velocity of clouds, and was able to estimate the speed
of the birds at nearly 48 miles per hour. In March geese passed at over
900 feet at 44.3 miles per hour. In 1905 Prof. Stebbins and Mr Carpenter
worked out a scheme for ascertaining heights by simultaneous observation
from different points. They based their calculations only on birds which
were observed by them both, and found that these passed at various
altitudes, ranging from 1200 to 2300 feet; in the following October the
lowest altitude observed was 1400 feet, and the highest 5400. Allowing
the 25 per cent possible error, these results are of great value.
Mr Chapman's remarks about the upward flight of some of the birds are
enlightening, for when birds start on oversea journeys they frequently
ascend to a great height. Dr Allen and others think that the ascent is
to increase the visible distance, but it may also be to reach a zone or
stratum of atmosphere in which flight may be more easily accomplished.
Robert Service's account of the departure of migrants from the Solway
shores, gives suggestion of high flight. They arrive often one by one
and "seem to drop literally from the clouds," but when they actually
departed it was easy to see their method. They "fly upwards and onwards,
then they hesitate, fly sideways once or twice, again attempt an upward
and onward flight, hesitate again, and down they come once more to
earth." After repeating this manoeuvre several times, "away they go
over the sea." One morning he counted sixty blackbirds in one hedge, and
others kept arriving, but, however closely he watched, he failed to see
whence they came. "They came down from the upper air, becoming suddenly
visible, sometimes three at a time." "I saw about a dozen birds thus
drop into view, but I quite failed to see any indication of the point of
the compass from whence they had come" (46).
Gätke frequently mentions birds raining down from the sky, appearing
first as mere specks, and dropping vertically to the island, and others
when departing "with breasts directed upwards and rapid powerful strokes
of the wings, fly almost perpendicularly upwards."
On May 24th, 1911, I watched the departure of a spoonbill from Easton
Broad on the Suffolk coast. The bird rose and soared in ever-widening
circles until it was a mere speck, even when seen through powerful
prismatic glasses; the great stretch of its wings alone enabled me to
watch it for so long. When at a great height--I will not guess what
elevation--it ceased its circling flight and made straight for the north.
In October I saw several flocks of redwings leave the Spurn. They rose
to a great height before directing their flight southwards, although the
Lincolnshire coast was plainly visible.
Great differences of opinion have been expressed about the speed of
migrating birds, and here again Gätke's estimates, on account of the
weakness of his arguments and his immense presumption, cannot be
seriously considered. There are but few measured speeds, and most of
these, except perhaps the ducks and geese referred to already, are of
birds travelling at low elevations.
Many birds, especially day-migrating swallows, hooded crows and other
birds, frequently travel at slight elevations; it is not unusual to
see birds at sea flying a few feet only above the waves. Mr W. Eagle
Clarke, whose systematic observations demand the profoundest respect,
again and again urges that the direction of the wind has little effect
upon migration, but that the force of the wind may make migration
impossible. At the Eddystone, where he spent a month in the autumn of
1901, he noticed birds passing at heights varying from 20 to 200 feet,
all flying southwards. He concluded that "the wind is certainly the main
factor in migration meteorology--I am convinced that the _direction_ of
the wind is, in itself, of no moment to the emigrants, for they flitted
across the Channel southwards with winds from all quarters" (16). When
the velocity of the wind, however, was above 28 miles per hour (a fresh
breeze, force 5 on the Beaufort scale), no migration was observed.
Allowing this, and also taking into consideration the undoubted fact
that birds are frequently held up by strong winds on the shore before
starting oversea journeys, is there any proof that they do not actually
avail themselves of fairly steady strong winds when they reach the upper
air?
Mr F. J. Stubbs makes some useful suggestions (50). He points out that
Gätke and others imagine that a bird flying with the wind would suffer
inconvenience through the wind ruffling up its feathers. It is surely
evident that a bird supported in a moving medium could not progress at
any slower rate than that medium; the bird is not flying on one stratum
of air with another moving with a different speed immediately above it;
it is actually in a current, not on it. If the bird flies at 20 miles an
hour, and the wind or moving air is progressing at 10 miles an hour,
the bird will cover a distance of 30 miles in one hour, though the force
exerted by the bird is the force necessary for 20 miles in a calm.
Conversely, if a 10-mile air current meets it, it will unconsciously
be carried only 10 miles. If the speed of the bird is the same as the
opposing force of the wind, it will remain stationary; I have seen ducks
in a blizzard rise head to wind and fly rapidly, making no progress but
maintaining their position over the water, to which they dropped again
when the storm passed. Some black-headed gulls on the same water did not
attempt this manoeuvre, and in a few seconds had vanished down wind.
The swimmer, in a swiftly-flowing river, may hold himself in position
so long as he can swim at the same rate as the stream he is contending
with, but he cannot make headway if the speed of the water exceeds his.
He may, however swiftly the stream moves, swim in any direction, but his
actual progress will be down stream; if he aims to swim directly across,
his real course will be diagonal.
The fact that birds fly in any direction in a wind, and when at low
elevation pay little heed to the direction of the wind, when the breeze
is light, simply means that they can fly faster than the medium they are
in; if the medium travels faster than they do, they will be carried in
it to their advantage or disadvantage.
Even in these days of upper-air investigation we really know very little
about the speed, direction and steadiness of upper-air currents, but
we do know that at a moderate elevation--some two or three thousand
feet--the strength is usually greater than nearer the earth.
Mr Abel Chapman, in "Wild Norway," makes this pertinent remark--"Except
by aid derived from the operation of physical laws, the nature
and extent of which are unknown to me, and by taking advantage of
'Trade-wind' circulation in the upper air, I believe that migration is
impossible for short-winged forms of sedentary habits--but that aid,
and those advantages, may facilitate, and perhaps vastly accelerate, a
process which is otherwise impossible."
In "Bird Life of the Borders" he goes further. "Birds are warmer-blooded
than ourselves or other mammalia, and are capable of sustaining life
in rarified atmospheres where these could not. By a simple mechanical
ascent, they can reach, within a league or two, regions and conditions
quite beyond human knowledge: where, selecting favouring air-strata,
they may be able to rest without exertion; or find meteorological or
atmospheric forces that mitigate or abolish the labours of ordinary
flight, or possibly assist their progress.... It is in the upper regions
of open space where, I suggest, the final clue will be found" (12).
A warbler flying leisurely, say at 10 miles per hour, in a current of
air which was travelling at 20 or more miles an hour, could accomplish
the journey across the North Sea--say 300 miles, in ten hours. Allowing
much higher rates of speed for strong-winged species, and greater force
of wind, some of the marvellous distances covered by migrating birds
cease to be mysterious. Prof. J. Stebbins and Mr E. A. Fath made careful
calculations from observations with the telescope, and found that birds
passed at rates varying from 80 to 130 miles per hour, and these were
the minimums, for if the birds were not flying absolutely at right
angles with the line of observation, they must have travelled a greater
distance in the time occupied between their passage of the observation
points (47).
* * * * *
CHAPTER V
ORIENTATION AND ROUTE FINDING
The question, How do birds find their way? is answered by many ingenious
and often purely speculative theories, some of which have been already
referred to in connection with the points discussed.
Each theory, though it may apparently explain certain phases of
migration, can be answered by some exceptional difficulty which makes
it fail as a full explanation; we are driven to the conclusion that
birds possess a sense of direction, which is often, very incorrectly,
called Orientation. Biologically this term does not imply any connection
with the East, but is simply used to describe the power of finding
the way back to a certain base, or of returning home. It is a power
or sense which undoubtedly exists in many vertebrate animals and in
some invertebrates, though it is hard, in many cases, to separate or
distinguish it from memory and impression gained through eyesight. Mr
John Burroughs, one of the strongest opponents of what he calls the
"Sentimental School of Nature Study," gives in his "Ways of Nature" a
striking instance of this faculty which may serve as an example, though
the cases of dogs and cats amongst domestic animals, and of many wild
creatures finding their way, could fill many volumes. His son brought
a drake home in a bag from a farm 2 miles away, and shut it up in a
barn with two ducks for a day and a night. As soon as it was released
it turned its head homewards, but for three or four days its efforts
were frustrated. Then Mr Burroughs decided to see what the drake would
do, and to go with it to give it "fair play." The "homesick mallard
started up through the currant patch, then through the vineyard towards
the highway which he had never seen," and Mr Burroughs followed 50
yards behind. A dog scared the bird and turned it up a lane, but after
a detour it reached the road again; it stopped to bathe in a roadside
pool, then started off again refreshed. A lane, leading in the right
direction off the main road, puzzled it, and it took a wrong turning,
but, discovering its mistake, made for the road again, but not by
actually retracing its steps. The false move seemed to put it out, for
after hesitating at the next and right turning, it actually overshot the
mark. Its companion, unable to spare time to continue the experiment,
then headed it back, and when it reached the turning again it seemed to
recognise something familiar and raced home with evident signs of joy.
The duck was a domesticated bird, but the incident is not without
interest; the homing faculty was clearly exhibited, but it was not
infallible; the bird made a mistake. So, inexperienced young birds,
travelling instinctively by orientation may, and do, make mistakes.
Human beings, in varying degree, possess a sense of direction, and some
a wonderful power of finding their way in strange places; it is most
marked amongst those men we choose to call uncivilised, who, indeed,
live in closer touch with nature than those of us who depend so much on
compass, map, road, train and tram; we, as path-finders, are degenerate.
Middendorf marvelled at the powers of the Samoyeds, but when he
questioned them was met by blank surprise, and the cross-question--"How
does the little Arctic fox find its way aright on the great Tundra?"
In addition to this instinctive power, the bird has eyes and brain. We
can afford to put aside as purely speculative Middendorf's suggestion
that the bird is impelled or dragged by magnetic force, but we cannot
deny that it uses its eyes and that it has a wonderful memory; its
second journey will be easier than the first, for it will recognise
landmarks, just as the drake recognised something familiar when it
neared home. Sight, however, cannot be always necessary, for, at the
Kentish Knock Lightship, Mr Eagle Clarke noticed that birds flew so low
along the water that they could not possibly have seen their way.
It is purely speculative to say that the young bird which travels alone,
for it seems certain that many young do travel unguided on their first
journey, has an inherited memory of the actual route to be followed, but
that it has an hereditary sense of direction, or an hereditary impulse
to travel in a certain direction, is quite another matter. The sea, to
the young bird, may be a barrier; it may wander coastwise and be lost,
or, if this is the best way, find itself at the desired haven. If the
shortest and quickest way is across the ocean, the young bird may brave
the perils and succeed, or on this trackless waste it may wander till it
sinks to the waves and be added to the long list of failures.
Certain species summer in Greenland either in the same areas or in
areas comparatively near to one another; some of these travel in autumn
south-east, and winter in Europe or Africa, and others go south-west
into the States or South America. Most of these are distinctly eastern
or western forms, but occasionally American birds are met with in
Europe, and European in America. Probably at the start these stragglers
joined the wrong band, and travelled for company and unconsciously by
the wrong route. Birds drifted to leeward may find companions of quite
an alien tribe; others, wind-swept in this way, may travel alone to new
lands. A few pelagic South Pacific and South Atlantic petrels and other
birds have reached Europe from time to time, but theirs is an error of
too wide nomadic wandering. The wonder is not that these stragglers do
turn up, but that so few are noticed; probably the frequent mistakes
made by inexperienced or even experienced birds are speedily rectified
by nature; failure to find food or exhaustion spells death.
Mr C. Dixon emphatically declares that each party of young birds is
accompanied by one or more older ones to act as guides--"The many
winter'd crow that leads the clanging rookery home." This is as
emphatically denied by other observers. Probably there is no regular
rule for many species, as there certainly is not for swallows, in
which the old and young birds can be easily distinguished. I have seen
bands trailing south in autumn in which all the birds were immature,
and others in which a number of young were accompanied by a few mature
birds, though, certainly, these old birds did not appear to lead.
Another assertion, that the old and young do not even travel by the
same route rather supports the idea that the young birds find their way
simply by a sense of direction, a sense liable to blunder, whilst the
old birds travel by the perfected or best route which their experience
has taught them. Martorelli wisely asserts that orientation is not
infallible, but develops with age.
Mach-Bruer attempted to localise this sense of direction in the
semicircular canals of the ear, which are so highly developed in birds,
but Dr Allen contends that the theory has been refuted by experiments
on pigeons. Möbius urged that birds were guided in their journey by the
direction of the roll of the waves; Newton replies that though this may
be a constant direction in certain parts of the Pacific, it is most
inconstant in the stormy North Atlantic (37).
There is a very generally accepted idea that birds prefer to travel with
the wind striking them diagonally--the "beam-wind theory," a theory,
which so far as I can see, has absolutely no sound foundation. When on
the Kentish Knock Lightship, Mr Eagle Clarke noticed, on a bad day, east
to west migrants hurrying past "as if to avoid as much as possible the
effects of the high-beam wind."
Mr A. H. Clark worked out the long oversea and overland course followed
by the American golden plover, and showed to his own satisfaction that
the birds always travelled at right angles to the prevailing winds;
therefore, he argued, they were guided by the beam-winds; always
keeping the wind on their flanks led them aright (14). He says that if
they fly at 100 miles per hour, with a beam-wind of 30 miles per hour,
they will reach a spot 100 miles from whence they started, but 30 miles
to leeward of a line drawn at right angles to the wind. Thus, if they
rely upon the wind, their course is more or less diagonally across it
according to its strength. He maps out the supposed route according to
prevailing winds, but fails to notice that the very route he maps may
be caused simply by the leeward drift when flying on winds which are
not with them. One portion of the journey is enough to illustrate what
I mean. From Labrador to the east of Bermuda the birds fly south-east,
so, he argues, as to cross the south-west wind at right angles. But
supposing the birds headed due south, meeting the south-west wind on
their right front, they would of necessity, if the wind was strong,
drift away to the east. It is improbable that they actually aim to
strike the Bermudas, for it is only during certain weather conditions
that they visit these islands. In favourable weather the birds do not
touch the Bermudas, but continue their flight direct to South America.
The leeward drift of birds in a strong beam-wind may be noticed during
ordinary flight; it has occasioned one of the most remarkable of
Gätke's statements. Referring to hooded crows, he says--"To escape the
disagreeable experience of having the wind (south-east) blowing through
their plumage obliquely from behind, they turn their body southward,
and appear to be flying in this direction. This, however, is not the
case. They do not make the least forward progress to the south, but
their flight is continued in as exactly a westerly course, and with the
same speed, as though the birds were moving under favourable conditions
straight forwards, _i.e._, in the direction of the long axis of the
bodies. This is shown in the most convincing manner by such bands as
happen to pass immediately over the head of the observer.
"Besides hooded crows, many other, indeed perhaps all species, are
capable of executing a laterally-directed movement of flight of this
nature, not only under such compulsory conditions as they may encounter
during the flight of migration, but also during the ordinary activities
of their daily life" (29). He admits that he once thought it was a
drift to leeward, but that he is now convinced that it is intentional,
and is sure proof of his East to West flight. In the face of such absurd
statements as these, how can anyone quote Gätke as an authority on
migration! Yet, in recently-published books, this east to west flight
across Heligoland to Yorkshire is stated to be a proved fact, though
Mr Eagle Clarke, so long ago as 1896, showed it to be unsupported by
British evidence.
Dr Allen, reviewing Dr H. E. Walter's "Theories of Bird Migration"
(3), cites the following experiments as strong arguments in favour of
orientation. Dr J. B. Watson took fifteen sooty and noddy terns from
Bird Key, Tortugas, and liberated them at intervals after they had been
marked. The shortest distance was 20 miles from the Key, the farthest,
Cape Hatteras, 850 miles; thirteen returned to the Key. Neither sooty
nor noddy terns range, as a rule, north of the Florida Keys, so that
it is unlikely that any of the birds had been over the route before.
They could have gained no experience, or hereditary knowledge, and as
they were released during the breeding season, there would be no marked
movement southward which they might follow, nor would they at that time
be impelled by any desire to migrate. The change of direction from the
Florida Keys, westward, to the Tortugas, occasioned by the water course
which feeding habits would force them to follow, "removes the direction
of the wind as a guiding agency, whilst the absence of landmarks over
the greater portion of the journey makes it improbable that sight was of
service in finding the way."
* * * * *
CHAPTER VI
THE DISTANCES TRAVELLED BY BIRDS
Not only do the distances of the migration paths of different species
vary considerably, from a trip of a few miles to a voyage from the
Arctic to the Antarctic, but the individuals of one and the same species
do not all travel to the same degree.
The familiar swallow, _Hirundo rustica_, though subject to certain
geographical variations, is found throughout the Palæarctic and Nearctic
regions, nesting throughout Europe to between 63° and 70° north and in
Africa north of the Sahara, where, however, Canon Tristram found it
also wintering in the oases. South of the Sahara to the Cape it is a
winter visitor. In Asia it breeds, according to Seebohm, in Asia Minor,
Persia, Afghanistan, and western Siberia, and winters in Scinde and
western India. One form breeds in and north of the Himalayas, eastward
to China and Japan, and winters in India and Burma, and another ranges
from eastern Siberia across Behring Strait throughout North America,
so far south as Mexico. This form winters in Burma, in Central America
and Brazil, but the Mexican birds are more or less stationary at all
seasons.
Our swallow and its congeners have an almost cosmopolitan range,
summering in the Northern and wintering in the Southern Hemisphere or
comparatively near to the Equator in the Northern. Towards the centre of
its range its migrations are either short or the bird is non-migratory.
Mr W. L. Sclater, addressing the South African Ornithologists' Union
(42), stated that the swallow arrives at Cape Town at the end of
October, and is common from November to March; practically all have
left by the middle of April. Swallows begin to arrive from the south
in Africa north of the Sahara in the latter half of February; early
in March they reach southern Europe, later in the same month they are
in Central Europe and by the middle of April large numbers arrive in
England. Thus swallows leave South Africa actually after they have
arrived in England; the South African birds cannot be the same which are
in North Africa a month earlier! The swallow supports Seebohm's thesis
that the individuals which go farthest to the south in winter, breed
farthest north. A day-migrant and by no means a rapid one, the swallow
may be timed from place to place, and it is not presumption to suggest
that the birds which reach Britain to nest came from lands little south
of the Sahara and well north of the Equator, and that those which pass
through England and along our shores in May and even June are on their
way from Southern Africa to the northernmost limits of their range.
Mr Sclater points out one very interesting fact; when the swallow
reaches South Africa it is in ragged worn plumage, before it begins its
northward journey it passes through its one annual moult.
Waders and shore birds which reach South Africa in autumn--the spring of
the Cape--are moulting into winter dress; before they leave they have
often assumed or partially assumed the breeding dress. When they arrive
the native South African birds are breeding, but though Mr Sclater
thinks that some nest a second time in the south, no satisfactory
evidence has ever been brought forward to support the suggestion.
These long-distance travellers not only move from a zone of moderate
temperature to a warmer one, but many of them pass through the hotter
zone to a country having a similar temperature to the one in which they
bred, thus enjoying summer but not torrid heat all the year round.
There are birds in which the northern and southern forms are distinct.
The wheatear, _Saxicola oenanthe oenanthe_, reaches us early, sometimes
during the second week in March, and speedily settles down to nest.
Towards the middle or end of April a brighter larger bird appears, the
Greenland wheatear, _Saxicola oenanthe leucorrhoa_, which was recognised
in Greenland, Iceland and eastern North America before it was seen
that both forms occurred in Britain. This larger bird loiters through
Britain, for its northern home is not ready for it until the Arctic
spring. We know it breeds farther north than our wheatear, but its
winter range is not fully worked out. The smaller bird is found in north
and north-western Africa, and the larger form farther east, even south
of the equator on the eastern seaboard, and probably, when we know more
about the range of the two we shall find that the form breeding farther
north, winters farther south.
The folly of laying down the law on the strength of the knowledge of
the habits of a few species is shown by the study of the movements
of American birds. Mr Cooke shows that as a rule "the migration is a
synchronous southward movement of the whole species" in autumn, "the
different groups of individuals or colonies retaining in general their
relative position." The black and white creeper _Mnistitta varia_ breeds
from South Carolina to New Brunswick, nesting in the south in April and
reaching the northern limits in the middle of May. In the middle of July
old and young birds have been seen at Key West, 500 miles south of the
breeding range, and towards the end of August they have reached the
north coast of South America. The New Brunswick birds cannot be ready
to leave before the middle of July, and Mr Cooke allows them fifty days
for the trip, bringing them to the Gulf States in September; he argues
that this is proof that the earlier migrants must have been birds from
the southern part of the range. Black-throated blue warblers, _Dendroica
coerulescens_, reach Cuba at about the time that others of the same
species are arriving in North Carolina; the first, he concludes, are
birds from the southern Alleghanies and the others from northern New
England or beyond (20). Other species illustrate the same order which
he calls "normal," but show that it is not an invariable rule.
Southern-bred Maryland yellow-throats, _Geothlypis trichas_, reside
throughout the year in Florida; those in the middle districts of the
range migrate for a short distance only, whilst the Newfoundland
birds pass over the winter home of their southern relatives to the
West Indies. The palm-warblers of the interior of Canada travel 3000
miles to Cuba, passing through the Gulf States early in October; those
from north-eastern Canada travel later and slowly and settle in the
Gulf States, after a journey of only half the distance. He sums up
wisely--"No invariable rule, law, or custom exists in regard to the
direction or distance of migration.... Each species presents a separate
problem, to be solved for the most part only by patient, pains-taking
observation and by the recognition of sub-species."
The order in spring is yet unproved. "With many birds ... the first
individuals to appear in spring at a given locality are supposed to
be old birds that nested there the previous year." These are followed
by those which nested a little farther north, followed later by those
whose homes are in the most northerly part of the range. "If, then,
for any species, the southern nesting birds lead the van in both fall
and spring migration, and the near guard in each case is composed of
northern breeding birds, it follows that some time between October
and April a transposal of their relative positions occurs; and that
the more southern birds pass over the more northern ones, which delay
their migration, knowing that winter still holds sway in their summer
dominions." It is not known where this transposal takes place, nor
whether the northern birds remain in winter quarters till the southern
birds have passed, or start a slow migration, during which the southern
birds pass over them. Later another transposal occurs; the northern
birds cross the southern part of the range, passing birds which are
already nesting. "Spring migration seems to be therefore for some
species a game of leapfrog--the southern birds first passing the
northern, and the northern passing them in turn" (20).
The custom, now fortunately becoming widespread, of marking birds
by affixing a numbered metal ring to one leg, may help to elucidate
this and many other problems, but until a large number of results are
collected it is unwise to draw conclusions. Almost every month the
recovery of some of these marked birds is noted in the scientific
journals, but so far, beyond indicating the minimum distance travelled
by individuals, little can be proved. It is, however, plain that birds
do not invariably act as they ought to do if they obeyed all the laws
which have been invented for them. A few records or results may be
quoted, but any suggestions from these must be treated as suggestions
only; many more must be forth-coming before we can say, proved.
The white stork, _Ciconia alba_, has been systematically ringed in
Rossitten in East Prussia, in Denmark and in Hungary for some years, and
Mr A. L. Thomson gives a brief summary of the interesting results up to
date, in "British Birds" for May, 1911. Ten young birds, taken during
their first autumn journey, show a general south-easterly trend through
Europe. Three east Prussian storks were obtained in Syria, one in the
April after it was marked, the other two in April and July of the second
year; another was taken in Palestine, and one Hungarian stork in Syria.
In the May of the year following that in which it was marked in Prussia
one was obtained at Alexandria. In their first autumn Prussian storks
have been recorded from near Lake Chad in October, from Rosaires on the
Blue Nile in the same month, and from the Victoria Nyanza at the end of
November. A ringed bird is reported from German East Africa, but full
details are wanting, but one shot at Fort Jameson in north-east Rhodesia
in December 1907 had only been hatched in Pomerania a few months before;
it left the nest on August 19th and began its journey south on or about
the 26th. In its first winter a Prussian bird was shot in the Kalahari
Desert.
Seven Prussian and about a dozen Hungarian birds have been obtained
in winter quarters in the Transvaal, Natal, and other parts of south
Africa, and one in German south-west Africa; one, recovered in the July
following the summer in which it was marked, was possibly a weakling
bird which had failed to make the return journey. Storks which had
returned are recorded in their first, second and third summer; most of
them having been found within a few miles of their birthplace. One bird,
marked as a nestling near Brunswick in 1906 was reported in June 1908
from Sorquitten in East Prussia, over 430 miles away. Mr Thomson, from
his reference to this being roughly in the same latitude but reached
by a different "line of flight," suggests that it is an exception;
this will be shown when some hundreds more cases have been collected.
It may, however, be found that some birds deliberately launch forth in
search of new homes, though at present it looks more like a bird which
on approaching the breeding area had banded itself with the wrong local
body of travellers.
A bird marked at Cassel in western Germany has been recorded from
Barcelona in Spain; this so far single record may indicate that storks
get lost, that there is no regular direction, or that there is more than
one, a south-westerly as well as a south-easterly route. That too, we
hope, will be shown in the future.
That much will be learnt from storks is evident, but the lessons will
be only general; each migratory species must be treated separately, and
to some extent this is being done. In the opening chapter I mentioned
the uncertainty about the behaviour of any individual song thrush,
merely as an example. So far, the few records of marked song thrushes
add to rather than solve this problem. Years ago the song thrush was
looked upon as a permanent resident so far as Britain was concerned.
Then it was found to be migratory even in Britain, and it was suggested
that each song thrush performed a short migration, southern British
birds leaving the country, northern British retiring to winter in the
south of England, and northern European birds replacing these as autumn
immigrants. The study of local races quickly altered this opinion; it
was guessed that in Britain there was a sedentary insular race and a
migratory passage race; others, however, saw that some of our home-bred
birds left in autumn. What do we find? A song thrush, marked as a
nestling in July in Northumberland, is found in November in Durham;
another one, marked in Berkshire travels to Norwich and is recorded in
November, but a third, born in Aberdeen takes an autumnal flight of at
least 1500 miles and is found in Portugal. Evidently we cannot yet frame
any rule for our British-bred birds.
It is said that home-bred lapwings are somewhat sedentary, and that the
large winter flocks are composed of Continental immigrants. The frequent
westward migration of lapwings during exceptionally severe winter
weather has led to the supposition that these birds fly for refuge,
under these circumstances, to Ireland. This is true, so far as it goes,
but a lapwing marked as a nestling near Stirling has been found in the
south of France, and two others in Portugal, whilst five have been
recovered in Ireland.
The results of marking sea-birds are interesting, showing that the
young birds often wander northward in search of food before there is
any marked autumnal southward migration. Terns and black-headed gulls
have been found a month or more after they have left the nest to the
north of their breeding colonies in Cumberland and mid-Wales. A bird
from Ravenglass was taken in its first January in Brittany. Rossitten
black-heads have been shot in the Isle of Wight and in Breydon in
Norfolk.
This may only mean that the young blackhead is a confirmed wanderer
in search of food, but the few results with woodcocks, marked as
British-bred nestlings, are puzzling. They have been known to linger in
the neighbourhood of their home until November, and have been found in
Portugal only a month later. Birds marked at Tyrone have been found so
far apart as Cornwall, Harrow and Inverness; what route for the Irish
birds can be guessed at?
Birds marked as adults present further problems, but also provide
interesting evidence. Hooded crows, captured on migration in spring at
Rossitten and then released, have been recovered in autumn actually
in the same place and in other localities in Germany, and one marked
in October was taken two years later, in spring, in Finland. The sum
of these records of crows proves one thing conclusively--the fallacy
of Gätke's due east to west and west to east flight, and supports a
coastwise migration for this species.
Adult teal, captured in decoys, ringed and released in South Denmark
in September and October, were taken in November and December in
Hampshire, Suffolk and the Moray Firth, whilst others from the same
place were recorded from other parts of England and Ireland, from
western France, Holland, the south of Spain and the north of Italy.
Fly-lines, if followed, are divergent and complicated. Four young herons
were marked in one nest in Denmark; one was recorded in Holstein in
June, and another in Mecklenburg in July; the third was killed near
Salisbury in Wiltshire in October, and in the following February the
last was obtained in the north-west of France. Two from another nest
were recovered in Denmark, one in July and the other in February, twelve
months after birth. Another heron reached Andalusia by August. In each
case where there was indication of a direction it was south-westerly.
Many more records might be mentioned, but these are sufficient to show
the value of the method and the present insufficiency of results.
Many of these records show that the speed of the migrating birds, even
in spring, is not great. Mr Cooke proves that most species in North
America travel slowly through the districts where food is plentiful
and during the earlier part of the journey northwards only a few miles
are covered per day; they travel with the slowly advancing vernal
wave, but, as we shall show in the next chapter, many species actually
outstrip it, and travel from warmer to colder climates.
[Illustration: Map showing the range of the American Golden Plover,
with its known migration route.
(From _The National Geographic Magazine._)]
By the kind permission of Mr Cooke I am able to reproduce three of his
maps, illustrating the longest known distance travelled by any bird in a
single flight, and the probable evolution of this extraordinary oversea
voyage (21). This long journey, roughly 2500 miles at a flight, is
used in autumn by several species of American shore birds, and the
particular species most easily recognised, is the American golden
plover, _Charadrius dominicus_, which differs but little from our _C.
pluvialis_. An important point to notice is that the route followed in
the fall is not the one used by the bird in spring, an undoubted proof
that all routes are not identical with the original line of dispersal
of the species. Nor is the route directly from the north to the south,
though there is plenty of evidence to show the fallacy of the notion
that all birds move in this one direction.
The golden plover nests along the Arctic coasts of North America from
Alaska to Hudson Bay. So soon as the young are able to take care of
themselves the birds migrate south-east to Labrador, where for some
weeks they fatten on the autumn harvest of fruits. A short journey
across the Gulf of St Lawrence brings them to Nova Scotia, where they
gather before starting on their oversea flight. The eastward trip to
the food-supplying districts is support of the idea that a route is
originated by passage from food-base to food-base, rather than by any
hasty rush from the dangers of approaching winter. The birds start south
from Nova Scotia for South America!
During this long oversea journey, which Mr G. H. Mackay thinks, with
reason, may be undertaken under favourable conditions at a speed of from
150 to 200 miles an hour by birds with such magnificent power of flight,
the plovers may meet with many different winds. The Cape Cod sportsmen
look for them if the wind is strong from the north-east; the Barbados
gunners expect them when there is squally weather from the south-east,
but when westerly breezes are blowing they will pass so far as 400 miles
east of the Bermudas. Only when the wind is adverse and strong do the
plovers visit the Bermudas or even stop at any of the northern Lesser
Antilles, 600 miles from the coast of South America. In favourable
weather they neglect any of these "emergency stop-overs" and hasten on.
In the Guianas the birds rest and feed, but they soon move on. Across
the Brazils their actual route is uncertain, but they have been met
with in Amazonia, and are known to winter in Argentina, and, it is
suspected, in eastern Patagonia.
[Illustration: Map showing the evolution of the migration route
of the American Golden Plover.
(From _The National Geographic Magazine._)]
The return migration is, so far as it is known, in a steady northerly
direction, rather north-west across Bolivia towards Central America.
From Yucatan they cross the Gulf to Texas, then slowly travel up the
great Mississippi highway and across Canada to their northern breeding
grounds. "Its round trip has taken the form of an enormous ellipse with
a minor axis of 2000 miles and a major axis stretching 8000 miles from
Arctic America to Argentina."
The following is Mr Cooke's suggestion of the origin of this great
ellipse. Towards the close of the glacial era, when the ice began to
recede, the Florida peninsula was submerged and only a small area in the
south-east of the States was free from ice. Plover attempting to follow
up the retreating ice were confined to an all-land route from Central
America through Mexico to the western part of the Mississippi Valley.
As the east gradually became uncovered the route would be extended to
the north-east, until the area stretching to the Great Lakes was fit
for bird-habitation. As the route lengthened and the power of flight
developed, there would be a tendency to shorten the line by cutting off
some of the great curve (No. 1) through Mexico and Texas, and a short
flight across the Gulf (No. 2) would be gradually lengthened, until the
present spring route, then also the autumn route (No. 3), was attained.
As Canada opened out, the routes in spring and autumn diverged; in
autumn the fruits of Labrador were an attraction, but the Chinook winds
made the country east of the Rockies more suitable for spring migration;
the fall route tended eastward (No. 4), the spring route remained
unchanged. When the fall route had worked eastward to the Gulf of St
Lawrence (No. 5), shortening took place in the same way from the great
westward curve, culminating in an ocean flight, short at first (No. 6)
and later extended, the total distance shortened, until the present
route was attained (No. 7).
This reasoning, sound enough, helps to a more difficult problem--how
the Pacific golden plover, _Charadrius fulvus_, found its way to the
Hawaiian Islands, where numbers of the birds winter annually. Roughly
the islands are 2000 miles from California, 2400 from Alaska, whence
the birds fly, and 3700 miles from Japan. Mr Cooke scouts the idea that
any bird flies aimlessly out to sea to find a new winter home, and the
chance colonisation by a storm-swept party is as improbable; if this did
occur it is hardly likely that they would at once depart, in a single
season, from ancient habits and carve out an entirely new migration
route. Probably the origin of the route is as follows. The bird breeds
on the northern shores of eastern Siberia from the Liakof Islands to
Behring Strait, and on the Alaskan side south to the northern base of
the Alaska peninsula. It winters on the mainland of south-eastern Asia,
in eastern Australia, and throughout the Oceanic Islands from Formosa
and the Liu Kiu Islands on the north-west to the Low Archipelago in the
south-east.
[Illustration: Map showing the evolution of the migration route
of the Eastern or Pacific Golden Plover.
(From _The National Geographic Magazine._)]
It is fairly certain that the original route would be roughly north and
south, between Siberia and southern Asia. In time the species spread
eastward in winter, to Australia and to islands farther east, whilst the
breeding area extended to Alaska. If these extensions took place before
any cutting off of corners in the route, Alaska birds would travel
11,000 miles to reach the Low Archipelago, only 5000 miles in a direct
air-route (No. 1). Probably shortening began early among the Pacific
islands, from the northern islands to the Asiatic coast, and finally to
Japan (No. 2). From Palmyra the flight to the nearest of the Marshall
Islands is 2000 miles; thence a journey, provided with several possible
rests, of 3000 miles would bring them to Japan. A thousand-mile drift
through strong winds might cause the birds to reach Hawaii, whence they
would find a chain of islands which would help them, and render the
last flight to Japan no longer than the one they had been accustomed to.
Having once reached the Midway Islands the shortening of the route would
be carried on again by lengthening the oversea journey northwards until
the Aleutian Islands were discovered (No. 4). The present route, now
followed in spring and autumn (No. 5), would be the natural climax of
this long evolution. The two golden plovers, sub-specifically distinct,
nest little more than a hundred miles apart; their migrations and winter
homes are as different as they could be in any two widely divergent
species. It is one of the most striking of the ascertained facts in the
distribution and habits of birds.
* * * * *
CHAPTER VII
MIGRATION AND WEATHER
In previous chapters it has been necessary to refer repeatedly to the
connection between migration and meteorology; either the relation of
periodic movements to the rotation of seasons, or the influence directly
or indirectly of weather conditions upon normal and abnormal migration.
That there is an overruling relation between the advance of spring and
the passage to northern breeding quarters, and the gradual cooling in
autumn and the retreat to winter quarters is, of course, evident, but
it must not be held, as contended by the early students of migration,
that this is the sole factor which regulates migration. The actual
relationship between the weather and the movement of birds is far more
complicated than one would imagine, and the stimuli of continental or
overland travelling differ from those of a cross-sea flight.
In the British Islands most of our larger movements are at their start
or their finish, or both (so far as our area is concerned), oversea
passages, and unless the weather be absolutely favourable, birds do not
undertake these voyages. No one has added more to our knowledge of
the connection, in what we may term British migration, than Mr Eagle
Clarke, but it must not for a moment be imagined that his conclusions
and the data from which he arrived at them are purely insular. The
British Islands are merely the field of observation, the centre of the
field, of the movements of Holarctic birds which travel regularly or
occasionally through Britain. Mr Clarke points out repeatedly that in
studying the phenomena it is the conditions at the point of departure
not at the point of arrival--generally the point of observation--which
are important.
The oft-repeated assertion that birds can foretell the nature of
approaching weather--that they are living barometers--is not supported
by any satisfactory evidence, but it is certain that on many occasions
the weather into which they have passed in moving from one zone to
another has not only retarded, checked, or exhausted them, but has
proved fatally disastrous. During the westward rushes in winter,
when exceptionally severe weather has cut off the food-supply of
ground-feeding birds, observers who have seen the birds moving in
front of the storm have maintained that they had felt its approach and
retreated in time. The truth seems to be that the birds start so soon
as the supply is cut off but in many cases speedily outstrip the storm.
When these exceptional winter migrations take place the birds in the
lowlands of Lancashire and Cheshire move westward towards Ireland, and
are observed at different points along the North Wales coast. They are
sometimes seen travelling in a snow-storm and sometimes in advance
of it. In eastern Cheshire I have seen parties of lapwings passing
over westward just in advance of snow, which when it reached the East
Cheshire fields, started the local lapwings after their relatives from
farther east.
During regular migration birds start in favourable weather but
frequently meet with unfavourable weather before their arrival at the
point aimed at; most of the bird "disasters" at the lighthouses and
lightships, and more occasionally inland, can be explained in this way.
In his digest of the observations at lighthouses and lightships Mr
Eagle Clarke shows that spells of genial weather are favourable and
that during these spells migration is even flowing and continuous
(15). Slightly unsettled conditions have little effect, but an
increase of the irregularities accelerates migration. Sooner or later
cyclonic disturbances interrupt regular movements, and, if these are
extraordinary, act as barriers, either holding the birds in one place or
forcing a hurried departure or "rush." Favourable weather immediately
following a check or "hold up" often causes a rush; a sudden fall in
temperature may force large numbers of birds on in autumn or retard them
in spring. Temperature, he declares, is the main controlling factor
in all extraordinary movements, other meteorological conditions being
suitable.
In the autumn migration to Britain, the chief movements take place when
a large and well-defined anticyclone has its centre somewhere over
Scandinavia, with gentle gradients in a south-westerly direction over
the North Sea. Coincident with this we usually find cyclonic conditions
prevailing to the west of the British area, with low-pressure centres
off the west or south-west of Ireland. The weather is clear and cold,
with light variable airs over Scandinavia, but in Britain the sky is
overcast, and the wind easterly and moderate to strong; not infrequently
these conditions mean fog on our eastern coasts. If the birds leave
Scandinavia under favourable conditions they may be met by the
approaching cyclonic system, which usually, though by no means always,
travels in a north-easterly direction across the Atlantic. Migration
is thus checked, but a return of favourable anticyclonic conditions
starts the birds again, often with a fresh impulse in the shape of
falling temperature. When the anticyclonic area is exceptionally
large, extending from the Scandinavian peninsula in a south-westerly
direction and embracing the whole of the British Islands, simultaneous
immigration and emigration may be witnessed.
Cyclonic spells are not always unfavourable to migration. In spring,
when they are of a mild type with soft rain and warm winds following
after a cold anticyclonic period, a northward movement is frequent.
Mr Eagle Clarke says that the importance of winds is overstated, but
as an incentive only. The direction of the wind has no influence as an
incentive but its force is an important factor; in a strong wind a bird
may be blown out of its course. Birds will not start in a high wind
but may pass into the influence of strong winds which may affect both
progress and direction. He adds that particular winds usually prevail
during the season of great autumn movements, which are not incentives
but are the result of pressure distribution which is favourable to
migration. These are usually north-east to south, but a westerly wind
would serve as well, but it indicates a pressure distribution which is
fatal to migration between north-west Europe and Britain--cyclonic areas
to the north-east and east of our area.
All this, no doubt, is perfectly true. It is founded on the analysis of
a huge number of carefully recorded observations, and upon a general
knowledge of migration which few can ever hope to equal. Mr Clarke
understands his subject. It appears, however, to me that he may put
rather too much weight upon the barometric influence, and too little
on one side of the wind question. Are we yet in a position to say that
birds do not make direct use of certain winds? It may be that the use
of the prevailing winds at migration time is far more unconsciously
intentional (if such an expression can be used) than is at first
apparent.
One or two points must be kept well to the front which are often ignored
by observers. Firstly, very much visible migration is abnormal; that
is to say, most of the incidents of passage which are noticeable,
especially observations at the lightships and lighthouses, are during
spells of weather which are described as unfavourable; it is the
"hold-ups," checks, and "rushes," which attract attention far more than
the even-flowing normal migration.
Mr J. Tomison, in his valuable notes on observations made at Skerryvore
(52), shows that in ordinary clear weather birds pass at a great
height, beyond the power of vision. He proves this by instances of
the diurnal passage of redwings, birds which are generally supposed
to migrate at night, and undoubtedly do so frequently. He heard the
well-known passage-note in the daytime, but with the naked eye could
see no redwings; he found them with the telescope and later discovered
others which were passing above the range of normal vision. Mr
Eagle Clarke, commenting upon the extraordinary numbers of rare and
exceptional visitors which are noticed on many islands--Fair Island,
the Flannens, the Isle of May, and Heligoland may be taken as a few
examples--says that it is their detached position and comparatively
small size which makes these islands so useful to the observer. The same
variety of birds and greater numbers reach larger islands and tracts of
land, but they are unobserved when they are thinly distributed and not
massed or confined in a small area. "With all our great army of trained
observers," he declares, "we in Britain see only an infinitesimal number
of the migrants which visit our shores ..." and "this is especially the
case on the mainland."
During an anticyclone there is a descending movement of air currents
from a centre of high pressure in all directions, and these currents
or winds are deflected "clockwise" in the northern hemisphere; and
when cyclonic conditions prevail the air currents are directed inwards
towards a low-pressure central area, rotating spirally at the surface
of the earth in the direction contrary to the hands of a watch. In the
southern hemisphere the directions are reversed. A cyclonic system is
usually carried forward by great drift winds like eddies upon a swift
stream, in the North Atlantic as a rule from south-west to north-east.
Do we really know the force and direction of the winds at a high
altitude during these movements? Are we not merely guessing at the
real aerial conditions by the movements near the earth at the time
of the departure of the birds? Is it fair, if I am right that the
meteorological observations are founded upon only those observed at
comparatively low altitudes, to lay down laws as to the particular
conditions which are favourable or unfavourable, or the particular winds
which are used or avoided? The direction of the wind may be the same
up to a great height, many thousand feet, or it may vary within 500
feet of the earth. Nearly fifty years ago, when Glaisher made his great
ascents, he sometimes met with three or four currents moving in opposite
directions. The more recent upper air investigations show that though
as a rule the wind at various elevations is in the main from one point
of the compass, its degrees vary considerably, and its force at the
various heights shows remarkable differences. Generally the force rises
to about 5000 feet, but there is no invariable rule. I tabulate a few
examples taken more or less at random from the Weather Reports for 1908.
The altitudes above the ground are measured in metres, roughly converted
into feet; the letters indicate the direction of the wind, the figures
its speed in miles per hour. The last one in the table, observations
made at Brighton on September 20th, is particularly useful. The
conditions on this date were anticyclonic, and favourable to migration.
At 400 feet above the sea the wind was blowing at 5 miles an hour; at
between 5000 and 6000 feet its force was 20 miles per hour. What then
would happen to a bird leaving Brighton for say the Spanish Peninsula?
If it flew at 20 miles an hour towards the French coast about Dieppe,
it would meet the wind blowing at 5 miles an hour, and take between
five and six hours to reach the coast, head to wind. If it rose to the
height of 3000 feet it would meet a wind blowing at the same speed as
it was flying, and it could make no headway. If, however, it flew in
a south-westerly direction the more it turned westerly the farther it
would drift down channel towards Normandy or Brittany, and be carried
out to sea! But this is exactly what would not have happened, for on
this date a feeble cyclonic system was approaching from the Atlantic and
extending its area of influence over southern England. In the Channel
the bird would meet westerly winds which would bring it safely to the
Brittany shores, or if it missed them, to the western shores of the Bay,
where the wind was actually from the north. I mention this merely to
show that apparently unfavourable winds may be really favourable.
--------+-----------+--------+---------+----------+----------+----------+
| | Ground |100 mtrs.|500 mtrs. |1000 mtrs.|1050 mtrs.|
Date. | Station. | Level. |(330 ft.)|(1660 ft.)|(3320 ft.)|(5000 ft.)|
--------+-----------+--------+---------+----------+----------+----------+
Jan. 2 |Petersfield| NE by E| ... | ENE | E by N | E ½ N |
| | | | 30 | 50 | 13 |
| | | | | | |
" 2 | Glossop | E by N | ... | E | E by S | ... |
| 1100 ft. | 8 | | 15 | 30 | |
| | | | | | |
" 3 |Pyrton Hill| ENE | ... | E by N | E | ... |
| 500 ft. | 14 | | 35 | 53 | |
| | | | | | |
" 4 | " | NE by E| ... | ESE | E by S | E by S |
| | 10 | | 25 | 25 | 30 |
| | | | | | |
" 11 |Petersfield| S by E | ... | S | SW by W | SW by W |
| | | | 10 | 3 | 5 |
| | | | | | |
April 9 | " | SE | ... | N by W | ... | N ½ W |
| | | | 7 | | 20 |
| | | | | | |
" 8 | Glossop | N | ... | N by W | NW by N | W |
| | | | 9 | 16 | 7 |
| | | | | | |
" 30 | " | S | S by E | S | W by N | ... |
| | 14 | 27 | 30 | 46 | |
| | | | | | |
May 16 | " | WSW | W by S | W by S | W | W by N |
| | 16 | 26 | 27 | 29 | 33 |
| | | | | | |
Sept. 5 | " | WSW | W by S | W by S | W by N | W by N |
| | 12 | 15 | 17 | 21 | 23 |
| | | | | | |
" 7 | " | S by W | SSW | SSW | SW | ... |
| | 9 | 16 | 20 | 33 | |
| | | | | | |
" 10 | " | NW by N| NW by N | NW by N | NW | NW |
| | 8 | 16 | 21 | 34 | 36 |
| | | | | | |
" 20 | Brighton | ESE | SSE | S | SSE | SSE |
| 380 ft. | 5 | 5 | 15 | 20 | 20 |
--------+-----------+--------+---------+----------+----------+----------+
--------+-------------+------------+--------------+-------------+
| 2000 mtrs. | 2500 mtrs.| 3000 mtrs. | 3500 mtrs. |
Date. | (6660 ft.). | (8320 ft.).| (10,000 ft.).|(11,660 ft.).|
--------+-------------+------------+--------------+-------------+
Jan. 2 | ENE | NE | NE by N | NE by N |
| 23 | 22 | 18 | 25 |
| | | | |
" 2 | | | | |
| | | | |
| | | | |
" 3 | | | | |
| | | | |
| | | | |
" 4 | ESE | SE by E | SE by E | |
| 35 | 20 | 15 | |
| | | | |
" 11 | N by W | NW | NNW | N1/2W |
| 9 | 8 | 7 | 11 |
| | | | |
April 9 | N | NW1/2N | NW by W | ... |
| 14 | 9 | 12 | |
| | | | |
" 8 | NE | N by E | W by N | NNE |
| 6 | 8 | 8 | 9 |
| | | | |
" 30 | | | | |
| | | | |
| | | | |
May 16 | WNW | | | |
| 36 | | | |
| | | | |
Sept. 5 | W by N | | | |
| 28 | | | |
| | | | |
" 7 | | | | |
| | | | |
| | | | |
" 10 | | | | |
| | | | |
| | | | |
" 20 | | | | |
| | | | |
--------+-------------+------------+--------------+-------------+
--------+-------------+-------------+-------------+-------------+
| 4000 mtrs. | 4500 mtrs. | 5000 mtrs. | 6000 mtrs. |
Date. |(13,320 ft.).|(15,000 ft.).|(16,700 ft.).|(20,000 ft.).|
--------+-------------+-------------+-------------+-------------+
Jan. 2 | NE by N | | | |
| 23 | | | |
| | | | |
" 2 | | | | |
| | | | |
| | | | |
" 3 | | | | |
| | | | |
| | | | |
" 4 | | | | |
| | | | |
| | | | |
" 11 | E1/2N | E by S | E by NE | ENE |
| 8 | 14 | 13 | 14 |
| | | | |
April 9 | NW by W | W by N | | |
| 18 | 20 | | |
| | | | |
" 8 | NW | SSW | NNW | SW |
| 1 | 3 | 5 | 7 |
| | | | |
" 30 | | | | |
| | | | |
| | | | |
May 16 | | | | |
| | | | |
| | | | |
Sept. 5 | | | | |
| | | | |
| | | | |
" 7 | | | | |
| | | | |
| | | | |
" 10 | | | | |
| | | | |
| | | | |
" 20 | | | | |
| | | | |
--------+-------------+-------------+-------------+-------------+
Under ordinary circumstances are we justified in saying that birds make
use of the winds blowing with a certain force at the point of departure,
or that they ignore them? Certainly we cannot judge by either the force
or direction of the wind at the point of arrival, as Mr Clarke points
out. The bird may have dropped into most adverse currents.
In Hungary, where migration has been very carefully studied, we find
evidence supporting Mr Clarke's theory, and yet giving it a slightly
different complexion. Low atmospheric pressure, depression (the warm
cyclonic conditions of spring) very often shows the greatest rate in
the arrival of the swallow. If there is a centre of depression west of
Hungary, and its path is directed north or north-east, swallows appear
in crowds. The fair side of the depression, with its warm southerly
winds, is therefore favourable. A list of twelve other birds, which also
appear in spring under these conditions in greatest numbers, is added.
The "bad" side, with cool northerly winds causes delays in the arrival
of these thirteen species. The depressions often have a sphere of
influence extending so far as North Africa, so that birds, on the fair
side, can cross the Mediterranean with southerly winds all the way (31).
I have endeavoured to show that often the force of wind is greater
at a high than a low altitude, and there is ample evidence to prove
that birds fly at a great height when conditions are favourable. Birds
usually leave Scandinavia when there are descending currents flowing
outwards from the centre of high pressure; is it wild speculation to
suggest that it is the southward flowing currents, which are also
deflected westwards, upon which the birds intend to travel? Thus the
bulk of the Scandinavian birds might not touch Britain at all, but
those which started upon light to moderate north-east to easterly winds
from the western shores of Norway would be helped to Britain. Mr Clarke
mentions that when he was at Fair Island, north-west to westerly winds
did not stop migration from the north, but is it certain that the birds
did travel in or against these westerly winds? May they not actually
have travelled on the "good side" of the cyclonic system, with these
very winds carrying them towards Fair Island? their actual visible
approach from the north does not prove that they had travelled all the
way in this line.
On September 22nd, he says--"The favourable meteorological conditions
of yesterday--fine weather and moderate south-east breezes,--has had a
marked effect, for to-day goldcrests are swarming everywhere." But what
does he mean? Favourable to him as an observer or to the goldcrests?
Surely the birds did not aim for Fair Island; were not these weak-winged
birds probably making for the south, when the south-east wind caught
them and drifted them to the west? Fair Island was a refuge, but hardly
the objective of their flight (17).
Compare this with Cordeaux's notes of another goldcrest immigration,
this time to the Lincolnshire coast (23). On October 13th the wind was
north to north-east in the afternoon, light but increasing in force, the
weather clear and bright--a few birds arrived. They had started under
favourable circumstances. Shortly after midnight on the morning of the
14th, the wind got full east, with quite half a gale and heavy beating
rain, continuous to the morning of the 16th; the nights were very dark.
"During this time the immigration was immense," and most of the birds
were goldcrests. Cordeaux's idea that these were not normal immigrants
but birds which were passing probably from north-east to south-west,
when the easterly gale caught them, is probably correct.
I have referred to birds starting at a high elevation. Service says
that in normal departure from the Solway, most birds mount to a high
altitude, but "a strong beam wind will bring the birds--even those of
strongest power--down to 200 to 500 feet of the surface, and it is
interesting to see whole flocks with heads turned almost completely to
wind, and yet travelling along at nearly their normal speed, at right
angles to their position" (46). Mr Tomison mentions rooks, daws and
hooded crows driven to Sule Skerry by south-east winds in March, leaving
two days later in a westerly gale. They, at any rate, did not object to
a strong wind which was in the right direction.
I have mentioned Mr F. J. Stubbs' paper on the "Use of Wind" (50),
and I believe that there is much more in it than is actually proved by
low-level observations. I doubt if birds always intentionally make use
of strong winds, currents which would carry them for great distances
at a considerable speed, but the preliminary ascent may be to search
for these currents. Cyclonic and anticyclonic winds, even when at
an altitude of some thousands of feet, would carry them easily, and
probably it is the wind-borne individuals, parties, or even hosts,
which drop for a refuge to the first island they see when carried far
from their migratory path. They are carried rather than drifted from
their pathway, borne in the moving current whether they wish it or
not. Provided that the cyclonic winds are fairly steady in direction
and force, sweeping round and inwards towards their centre, we may in
imagination trace the pathway of our so-called lost wanderers to far
distant islands; without many more upper-air observation stations, we
cannot actually prove the route.
But even putting aside the high altitude idea, and confining our
route-tracing to the known courses of air currents, we shall find
immense difficulty in mapping out the actual course of any bird on
any particular day. The study of some of the publications of the
Meteorological Committee, such, for instance, as the "Life History of
Surface Air Currents," by Shaw and Lempfert, published in 1906, shows
the great variation in the pathways, speeds, and formation of these
systems; a bird which accidentally entered a cyclone would unconsciously
alter its actual track and speed very many times before it passed beyond
the area of influence.
I am indebted to Mr Stubbs and Mr Herbert Taylor of King's College,
London, for some interesting mathematically worked-out routes of birds,
travelling at a given speed in a cyclone rotating at given speeds and
moving at a fixed rate; these show great variation both in direction and
speed according to the time and place of entering the system. The track
of the bird is, of course, influenced by its own rate of progress, by
the speed of the rotating currents, and by the rate at which the whole
system moves in any direction. Thus a migrant passing south and coming
within the influence of a cyclone which is moving north-east at a high
rate of speed, say 40 miles per hour, will, if it enters towards the
northern limits of the system, be at first retarded by the conflicting
forces of the easterly winds, the trend towards the north-east of
the rapidly travelling cyclone and its own southward flight. If it is
flying faster than the speed of the cyclone it will drift westward but
gradually approach the low pressure centre. After passing this its
course will at once change and its speed will be accelerated towards the
east.
[Illustration:
Map to show that a bird leaving Norway, near Aalsund, might be
carried round the British Islands in twenty-four hours. The arrows
indicate the actual directions and force of wind at the times marked
during a slow-travelling circular storm in autumn 1901. Speed of
bird about twenty-five miles per hour.]
Even violent storms move at varying rates, and it is conceivable that
a bird leaving Scandinavia on favourable anticyclonic winds might at
once come into the influence of a large, slowly-moving, circular storm,
with a low-pressure centre to the west of Ireland, and might, if the
air currents were strong, be carried westward at first, then south and
finally eastward, so that it would actually pass round the British
Islands. I have taken this exceptional case from the actual course of a
storm, which varied between forces 9 and II on the Beaufort Scale (say
an average of 50 miles per hour) but only travelled slowly eastward at
about 17 miles per hour. In some cases the storm centres are nearly
stationary for many hours.
It is easy to appreciate Herr Herman's statement that spring immigration
in Hungary is accelerated on the good side of a mild cyclone; the
direction of the bird, of the circulating air currents and of the whole
system may be coincident. Given a low-pressure centre west of the Bay of
Biscay, spring migration would be accelerated through Spain and France
towards Britain.
Mr Stubbs points out that the pathways of several birds, or parties of
birds, which started at different hours, would be divergent, for they
would come within the influence of winds blowing in various directions
according to the position of the system; this he argues is contrary to
the accepted idea of routes. This, however, entirely depends upon what
we mean by a route, as I endeavoured to show in an earlier chapter. The
journey from point to point is a route, although the bird may be drifted
many miles in one direction or another on the way; it is only when the
bird fails to reach its objective, a suitable breeding place or winter
station, that the route is a failure.
The frequent occurrence of rare birds, some of them almost or quite
unknown elsewhere in Britain, on out-of-the-way islands, has led to
strange theories. One is that there are regular fly-lines over Fair
Island, the Flannens, St Kilda and elsewhere, similar to the one which
is said to pass over Heligoland. Mr Eagle Clarke's long expected book
will contain the ideas of the man who is best able to theorise on this
point; I write, now, with the feeling that his knowledge may lead me
to alter my ideas. The suggestion I can offer at present is that there
are ornithologists directing their attention to these spots which,
through geographical position and isolation, are the likely refuges
for wind-borne migrants. Also that the accidental departure from the
directions aimed at by the birds is, where wind and barometric systems
are so variable, far more frequent than is usually suspected. Direct
routes are doubtless aimed at, but only accomplished under favourable
conditions for the whole journey; migration is less infallible than we
have been led to think. It is, too, an evolving habit, strengthened by
those which survive its perils, now as it was in its early days.
During a long overland journey, winds will probably have less influence,
though for rapid passages high flights certainly appear to be not
uncommon. There is, however, another aspect of the connection between
migration and weather which we have hardly touched, migration synchronal
to the change of season. Mr Cooke shows that in North America the push
forward in spring is not in most species so soon as the weather permits;
they do not actually move on the spring wave. Many warblers which nest
in the Great Slave Lake region in an average temperature of 47°, linger
in the Tropics, and reach New Orleans when the temperature is about
65°F. Then they hasten northwards, outstripping the advancing spring,
finding in Minnesota a temperature of about 55°, and 52° in Manitoba,
and gain another 5° on the season by the time they reach their home.
Thus they continually reach colder weather as they travel north.
The American robin, _Turdus migratorius_, moves more sedately; it takes
seventy-eight days for its 3000 mile trip, whilst spring takes some ten
days less to cover the distance. But the individual robins may advance
more quickly; it is the robin as a species which takes this time to
cover the area of distribution. The isotherm of 35°F., corresponding
to the beginning of spring migration, advances north at the rate of 3
miles per day from January 15th to February 15th; 10 miles a day is the
average for the next month, and 20 for the following month. But along
the eastern foothills of the Rockies, isotherms travel faster than in
corresponding latitudes farther east; spring rushes to this western
land. In mid-April to mid-June--the height of migration--the southern
portion of the Mackenzie Valley has about the same temperature as the
region of Lake Superior 700 miles farther south. This, coupled with the
diagonal course of the birds across the fast-moving region of spring,
exerts a powerful influence upon migration; the earliest robins reach
southern Iowa on March 1st, and travelling northward at about 13 miles
per day, find in central Minnesota a temperature similar to the one
they left. Those which breed near Lake Superior increase their speed
to a daily average of 25 miles, and arrive at latitude 52°, when the
temperature is still about 34°. The isotherm, however, has reached
central Athabasca, and the Mackenzie Valley and Alaska robins double and
quadruple their daily average on the north-west diagonal to keep pace
with the spring (19, 20, 21).
Instances worked out in America and elsewhere might be quoted to show
how some species forge ahead and others lag behind the vernal wave.
Each species needs separate tracing in its routes and times and habits,
but on the whole the movements have relation to the changes in seasonal
temperature. In autumn the journey varies according to the time of
starting. Early fall migrants, and indeed the majority of autumn
migrants all the world over, travel more slowly than in spring; they are
neither impelled by sex-impulses nor the need to escape from failing
food supplies. A little later the supply does slacken and with it the
temperature cools, and if the changes are sudden southward migration
is accelerated. Migration, however, is such an advantageous and
well-established habit that it usually begins before hurry is necessary,
and the birds loiter southward, feeding as they go.
Mr Cooke shows that in spring, weather seldom influences the start
from the winter home, but the _average_ weather conditions regulate
the _average_ rate of northward advance and the date of arrival at the
breeding home (22).
* * * * *
CHAPTER VIII
THE PERILS OF MIGRATION
The dangers to which migratory birds are subjected during their journeys
are but little less than those which would befall them if they remained
in unsuitable zones. During long oversea passages fatigue and hunger
weed out the weaklings, sudden storms and adverse winds strike them
where no land is near, and they are carried often far from the goal
they aimed at. Predatory birds accompany them, taking toll _en route_,
and predatory man waits for the tired wanderers with gun and net. Shore
birds may rest upon the waves; sandpipers have been seen feeding as
they walked upon the drifting weed of the Sargasso Sea, and steamers
and other vessels frequently provide a rest for weary birds; but what
happens to the many which find no haven? "Woe to the luckless warbler
whose feathers once become water-soaked!--a grave in the ocean or a
burial in the sand of the beach is the inevitable result," says Mr
Cooke. A storm on Lake Michigan during spring migration piled many
birds along the shore, and in the wider Gulf of Mexico many hundreds
of passage birds were seen to fall into the water when caught, but 30
miles from land, by a violent "norther." Other similar sudden disasters
have been recorded off our British coasts, even so far back as 1786,
when, as quoted by Southwell, a Newcastle collier passed through water
off the Suffolk shores black with vast numbers of drowned woodcocks.
During normal migration birds may be brought to a lower elevation by
strong contrary winds, or they may be bewildered by fogs and cloud and
dropped nearer the surface; it is then that the travellers meet with
disaster at our coastwise lights.
Mr Tomison records some of his experiences of migration at Skerryvore
(52). He never saw a bird at the windows when the moon was shining,
and on clear nights the passing crowds go on without a pause. But on
hazy nights, with an easterly wind and drizzle, or during fogs, if
large numbers of migrants are passing, hundreds may be seen flying in
all directions, "all seemingly of the opinion that the only way of
escape out of the confusion--is through the windows of the lantern."
On one September night, when he was standing on the balcony, he likens
the appearance of the birds to a heavy fall of snow. "Thousands were
flitting about; hundreds were striking against the dome and windows;
hundreds were sitting dazed and stupid on the trimming paths; and
scores falling to the rocks below, some instantaneously killed, others
seriously injured, falling helplessly into the sea." On the following
night when many fieldfares, redwings, thrushes and other birds were
passing, he says--"Sometimes we use the terms hundreds and thousands
without thinking what these figures mean but on this occasion when I say
thousands were killed I do not exaggerate in the slightest."
Mr W. Brewster's account of his experiences at the Point Lepreaux
lighthouse (8), shows that similar disasters occur in Canada and the
States, as indeed they do wherever there are passages of birds. On a
foggy evening in September 1885 "as soon as the sky became overcast
small birds began to come about the light--with the advent of the fog
they multiplied tenfold in the course of a few minutes" and many struck.
"About the top of the tower, a belt of light projected some thirty yards
into the mist by the powerful reflectors; and in this belt swarms of
birds, circling, floating, soaring, now advancing, next retreating,
but never quite able, as it seemed, to throw off the spell of the
fatal lantern.... Dozens were continually leaving the throng" of birds
which had flown to leeward, "and skimming towards the lantern. As they
approached they usually soared upward, and those which started on a
level with the platform usually passed above the roof.... Often for a
minute or more not a bird would strike. Then, as if seized by a panic,
they would come against the glass so rapidly ... that the sound of the
blows resembled the pattering of hail." During his stay no birds came to
the light except during dense cloud or fog, and they came in greatest
numbers when an hour or two before the fog the sky was clear.
The experiences of Eagle Clarke, Seebohm and others who have spent
migration seasons at lighthouses might be quoted, but these two give a
vivid description of what regularly takes place when weather conditions
are unfavourable. Steady white lights are the most fatal to migrants,
revolving lights, if white, are struck by some birds, but red lights
seldom attract the passers. Mr Eagle Clarke thinks that birds are
actually decoyed from their path and arrested in their course by the
action of the lights; he says that a change from white to red lights at
the Galloper Lightship stopped bird attraction.
On the mainland a new high building or tower, new telegraph wires or
other erections, until their presence is familiar, take toll of passage
birds.
Mr R. M. Barrington has for years collected information from the Irish
lighthouses and light-vessels; some of his results were added to the
work of the British Association Committee, and some he published himself
(5). He emphasises the fact that these phenomena depend largely upon
weather, and therefore are not trustworthy indications of the density
at any time or place of migration. Out of 115 song thrushes killed at
the lights and sent to him, 80 per cent struck during the fourth and
first quarters of the moon, and the same rule holds good for other
species. The intimate relation between the lunar phases and the number
of examples killed was shown by statistics from 1888 to 1894. Out of 673
specimens received only 116 were killed when the moon was more than half
full.
Apart from fog or cloud, birds may fail to hit the land aimed at, either
through accidental divergence from correct direction or wind drift. In
November 1884 Mr Barrington received information of large numbers of
rooks passing simultaneously at the Tearaght and Skelligs Lights--island
stations 20 miles apart and each 9 miles off the Kerry coast. The birds
arrived in continuous flocks from the westward--the open Atlantic--and
passed in an easterly and landward direction; they came in small parties
and in flocks numbering two or three hundred, on many days between the
2nd and 25th of the month. A few birds were noticed at the same time at
stations on the south and east Irish coasts, and all alike making for
the land. From similar observations made in other years he concludes
that these were portions of hosts which had overshot the mark, and
failing to find land had turned back. The weather charts, he adds,
show no sufficient reason for the birds to have been blown out of their
course by storms.
The weather charts, as I have pointed out, do not indicate the force
or direction of the wind at high altitudes; I suggest that these birds
were carried rather than blown out of their way by strong currents at a
higher altitude than recorded on the charts, and that having left the
air currents they descended to the elevation of about 700 or 800 feet at
which most of them were flying when they were observed making for the
land.
On the night of March 29th to 30th, 1911, the south-eastern extremity of
Ireland experienced a remarkable rush of migrants, and the local papers
were full of the avian disaster, for large numbers of birds struck
the lights as well as buildings and other objects in inland towns. Mr
Barrington collected information (4), and found that most of the birds
were starlings, though thrushes, blackbirds, and redwings were numerous.
He received specimens of woodcock, water-rail, snipe, dunlin, meadow
pipit, wheatear, goldcrest, starling, song thrush, redwing, blackbird,
black redstart, robin, skylark, and stonechat, whilst some thirteen or
fourteen other species were said to have been recognised, amongst them
oyster-catcher and wild duck. The area affected lay south-east of a line
drawn across country from Balbriggan to the Old Head of Kinsale, with
a coast line of some 200 miles; most of the birds noticed inland were
at towns on the rivers Suir, Barrow, and Nore. The flight was mostly
north-east, and at the lights offshore, towards the land. Mr Barrington
gives the following explanation. After crossing the Channel the coast
of Wexford was reached and the stream divided, some going north along
the east coast and others westward along the south coast, but changing
their direction when they reached the wide mouth of the Barrow. The
flocks which passed Lucifer Shoals, 10 miles offshore, proceeded north
without touching Wexford. Northerly and easterly winds had prevailed for
weeks prior to the 29th over France and the British Islands, and birds
would be held up in southern Europe; the milder coastwise temperature
of western France, he thinks, would cause them to take a more westerly
course than usual. On the morning of the 29th the wind changed to the
south at Valentia, Pembroke and the Scilly Islands, and there was an
average rise of 7° in temperature at French stations. This rise and the
southerly wind liberated the birds, but as the wind continued north-east
or east in England they "decided" to take a longer and more exhausting
course than usual, pass to Ireland and then turn north-east. The change
took place exactly on the last day of the last quarter of the moon--the
darkest night for travel. A bank of fog and drizzle met them off the
Irish coast, and baffled and weary they were attracted by the lights,
not only on the coast but in the inland towns they passed.
In the main I think Mr Barrington's explanation is correct, but even if
the birds were gathered farther west than usual, which I doubt, it was
the north-east wind which had drifted them, and the word "decided" is a
bold one to use when dealing with the behaviour of birds. Easterly winds
would drift them westward, and the striking Ireland was accidental; it
was the safety of the many, as well as the deathblow to the comparative
few. On the night of the 31st I received news of this visitation, and
later found that similar movements, without disaster, were noticed
on the north coast of Wales and in Cheshire. On the nights of the
30th and 31st birds in large numbers passed over Bangor and the Menai
Straits; amongst them were golden plover, and the next day these birds
with fieldfares and redwings were more abundant than before in the
mid-Cheshire fields. On the night of April 2nd, from dusk to midnight,
a large passage occurred over Mere in Cheshire, where curlew, golden
plover, oyster-catcher and wild duck were recognised by their calls,
and at the same time a passage was observed at Old Colwyn on the Welsh
coast. I do not even suggest that these were the same birds which passed
over south-eastern Ireland, but their presence within so short a time,
indicates the volume of the movement.
Welsh papers recorded an "Extraordinary feathered catastrophe" at
Pwllheli in Cardigan Bay which occurred on the night of March 17th,
1904, in which "thousands" of birds fell dead and dying upon the town
and shore. The journalistic description was lurid, but I am able to
give the explanation sent to me by a friend who was an eye-witness. The
night had been dark and foggy, and in the morning he found "scores of
dead starlings, redwings, thrushes and blackbirds lying on the beach at
high-water mark." During the night a steamer had been loading setts at
the quarry at the Gimlet Rock, a large outcrop outside the harbour, and
the artificial light used had been one of the powerful oil flares. The
fog-bewildered birds were led astray and had struck masts, rigging, and
rock in their confusion.
During a big fire in Philadelphia on March 27th, 1906, Mr W. Stone saw
large numbers of birds passing in its illumination, and many passed
too near and fell into the blaze; he picked up a few half-burnt song
sparrows and juncos.
Blizzards on continents, and to a less extent snow-storms in our
islands, account for the death of thousands of travellers. And even
in most favourable weather birds fall exhausted. During a stay on the
Yorkshire coast in autumn, when migration was even-flowing and unchecked
by adverse weather, I found several goldcrests which had reached land
only to die, and though most birds came in without showing signs of
fatigue, a few larks and starlings were so tired that they made little
effort to escape when approached.
Ornithological literature supplies many accounts of more or less similar
disasters to migrating birds, but these are enough to show that the
perils of migration are not exaggerated.
* * * * *
CHAPTER IX
EARLY IDEAS OF MIGRATION
The evolution of the study and knowledge of migration is an interesting
subject, dealt with more or less completely by several writers. In a
manual it is impossible to treat it fully. That the Greek poets--Homer
and Anacreon for instance, and the writers of Jeremiah and Job, knew
something about the regular movements of birds is evident, nor is it
surprising that in lands like Greece, Egypt and Palestine the passage of
birds should be noted and directly connected in the popular mind with
the seasonal changes.
In a measure similar observations and conclusions may be traced in
the history or traditions of most peoples, but in a northern detached
area, such as the British Islands, there is a marked tendency to
overlook passage and note only arrival and departure, mostly of summer
birds. Early observers noticed the swallow and cuckoo when they had
actually come, and missed them when they had gone, but they failed to
grasp whence they came or whither they went. Interchange of ideas with
inhabitants of other lands was limited, and few early travellers were
philosophers, at any rate so far as migration was concerned. In Germany,
however, the Emperor Frederic II. realised in the thirteenth century
many truths concerning migration (27), but in Britain uncertainty or
myth held sway until the end of the eighteenth century. Herr Herman,
reviewing the variation in thought, says--"But as in other fields, this
period is followed by a time of decadence, a natural consequence of
departing from immediate experience."
British, and many Continental observers too, saw when birds had come
and in autumn that they had gone. Early swallows and martins were
always met with near water, and were watched dropping to roost in the
reed beds, as they always do in autumn before departure. Next morning
none was visible. Certainly then they had vanished to hibernate in the
water. The discovery of masses of torpid swallows, dead or dying, by
no means an unknown thing when birds are overtaken by sudden falls in
temperature in autumn or by a severe setback in the spring, was to these
puzzled men confirmation of their theory of hibernation. Other details
of the many stories of swallow hibernation are due to exaggeration or
to misconception. In the second half of the eighteenth century a fierce
discussion waged for and against hibernation, and many, including
Geoffroy St Hilaire and Montagu, sat on the fence, admitting that it
might be possible with some species and probably was with swallows.
Later some Americans produced "evidence" in favour of avian hibernation,
and even Mr Charles Dixon, in his earlier book at any rate, did not
think it impossible (25). The only argument in favour of hibernation
is that it is a habit resorted to by other vertebrates to escape the
consequence of exposure to severe temperatures. The arguments against it
are that not a single instance of avian hibernation will stand the light
of reason and investigation, and that birds are provided with the means
of escaping from the cold zone and certainly use these means. There
are flightless birds, but they all live in climates in which they can
exist at all seasons. As Seebohm puts it--"The hibernation of birds is a
theory, the evidence in support of which has completely broken down. The
migration of birds is a fact, as completely authenticated as the fact of
their existence."
Dr Derham's "Physico-Theology" appeared in 1737 (24), and contained
some sound reasoning about migration, though he was a little puzzled
with the many hibernation stories. In 1780 an anonymous pamphlet--"A
Discourse on the Emigration of British Birds," flouted the theory of
winter sleep in no measured terms (33). This pamphlet was, at first,
attributed to George Edwards, and the 1811 edition has his name on the
title, but Mr A. C. Smith shows that the real writer was a comparatively
unknown man, John Legg. Legg must be looked upon as one of our first
real students of migration. It is Legg who refers to a pamphlet which
appeared in 1740 in which it was seriously argued that swallows migrated
annually to the moon.
All this time, from 1736 onwards, the family of Marsham in Norfolk, had
been quietly recording observations on the arrival of migrants, each
generation continuing the work. The accumulated results have been used,
and will be used again, in studying the science of "ornithophænology."
A myth, founded on mistaken observation as well as upon mere speculation
was, and to some extent still is, that the larger migrants assist the
passage of the weaker ones. How else, is still asked, can weak-winged
species cross the sea? It was an old legend when J. G. Gmelin heard it
from the Tartars in 1740; each crane they told him took a corncrake on
its back. There are men who know the corncrake well, who believe to-day
that the bird must skulk unseen through the winter, for they assert it
is quite incapable of lengthy flight. It is useless to argue with them;
the only answer is that it not only can, but regularly does perform a
long double journey; its range extending from northern Europe to South
Africa. In 1911 I handled a water-rail, a bird with short rounded
wings like those of the corncrake, which had struck the lantern of a
lighthouse with great violence. Its smashed head was nearly severed from
its body.
Herr Otto Herman's "Recensio critica automatica" (31) supplies much
information about the literature on bird migration, and the strange
divergence of opinion on nearly every point. It is carried up to the
beginning of the twentieth century, but much of the valuable work done
in America is altogether neglected.
A short bibliography is given at the end of the present volume,
including the more important works on the subject and a few of the
papers in periodical publications referred to in this manual.
* * * * *
CHAPTER X
SUGGESTIONS AND GUESSES
Several important migration phenomena have hardly been touched upon in
the previous pages; a few words about these may not be out of place.
There is no doubt that now and again American species are met with in
Europe, and European in America, though there is no evidence of direct
regular trans-Atlantic passage, except from Greenland. The appearance of
these birds has been explained in several ways, the general notion being
that it is impossible for a bird to fly unaided across the Atlantic, say
over 3000 statute miles, without rest. In considering the question we
are met with various points on which we still lack knowledge.
We know that strong-winged waders can accomplish 2500 miles, apparently
without a rest, and that if rest is necessary these birds can swim and
rise from the waves. We know, too, that there is regular passage between
Greenland and Europe. We do not know how long a bird can, without rest
and food, sustain flight; we do not know the speed it can travel when
aided by favourable winds, nor to what extent even passerine birds may
rest upon the water. My friend Mr J. A. Dockray, when punting in the
Dee estuary, has often seen birds alight to rest on his punt, and once
saw a tired thrush settle repeatedly on the water and finally safely
cross the estuary. There are several instances recorded of passerine
birds alighting upon and rising again from the water.
We do not know the extent of Greenland as a summer breeding home of
birds; the growing knowledge of this vast continent proves that its
summer avifauna is much larger than we thought, and that western and
eastern forms inhabit adjacent breeding areas; the possibility of birds
banding with the wrong set of travellers is greater than was suspected.
It is urged that the western shores of Scotland and Ireland should
receive these stragglers, but that the records of American birds
are fewer from these coasts than from the eastern shores and even
Heligoland. The best island route, however, would lead birds to join
the travellers from Scandinavia which pass by the safer eastern route
than the one round the western wind-swept shores of Ireland. Even this
reputed scarcity may be error, for how many reliable watchers are there
compared with the immense length of this wave-indented coastline? How
easy for a straggler to be overlooked! Mr S. F. Baird, in his paper on
the "Distribution and Migration of North American Birds," is emphatic
that the transfer of American birds to Europe is entirely due to the
agency of winds carrying them from their course (6). Mr A.L. Butler
met with snow-buntings in mid-Atlantic travelling east, and Mr J.
Trumbull supplies information about many passerine birds--especially
snow-buntings and wheatears--seen in September and October at various
points between Canada and the British coasts (53). Some joined ships
but others made no attempt to do so, even at 54° north 44° west.
Unfortunately there is the negative evidence of fraud, for when
unscrupulous dealers found that the public would give high prices for
rare birds, a trade in American skins began. It is not impossible that
even Gätke was victimised. Error or even accidental fraud may be taken
into account. Some years ago I heard that a hawk-owl had been killed
in Cheshire, at an inland port on the Ship Canal; I traced the bird,
the American species, but discovered that it had been captured on an
east-bound steamer in the Straits of Belle Isle, and had only died or
been killed when the vessel reached the coaling station at Partington,
where the taxidermist who received it thought it had been taken. A Cape
pigeon, which I saw in the flesh, reported as shot in Lancashire, I
found had been brought home in cold storage.
Birds may be carried on shipboard. When the "Mauretania" was between
400 and 500 miles out from New York, bound eastward on June 15th, 1911,
a curlew came on board and remained for three days, leaving when the
Irish shores were sighted on the 18th. My informant, an experienced
wildfowler, failed to catch the bird, but described it as like our
curlew. Probably it was the American _Numenius longirostris_, but
amongst the Irish curlews it would easily remain unrecognised.
When a seabird appears inland the usual explanation given is
"storm-blown," but increasing knowledge shows the frequent fallacy of
this idea. The Manx shearwater, for instance, is a regular migrant, and
the examination of the dates of the records of so-called "storm-blown"
birds found in inland localities, shows a remarkable regularity; the
majority are met with between the end of August and the end of the first
week in September. Not only do the birds move south in the early days of
September but many, usually at any rate, cross England; the weaklings
fall out and are found. Is it possible that some of these collapses of
passing birds are due to more than mere physical fatigue? Aviators have
discovered the existence of "wind pockets" or "holes in the air," where
the resistance of the air appears suddenly to fail; what is the effect
on a flying bird which suddenly enters one of these pockets?
The lesser black-backed gull also crosses England in large numbers; its
movements are more noticeable than those of the herring gull, common
gull, or even of the inland nesting and inland feeding black-headed gull.
Recent investigation has added the yellow-browed warbler, the
blue-throat, and many other "rare," or "casual" passerine birds to the
list of regular British birds of passage; evidently they have been
overlooked before. Even the crossbill, so long classed as a spasmodic
invader, is now seen to be a regular bird of passage to Britain, though
in varying numbers, and quite independently of the sub-specific form
which is always with us.
The wanderings to our islands of southern petrels and other oceanic
birds has occasioned much surprise. Take two examples of the genus
_Oestrelata_, one _O. brevipes_ taken at Borth in 1889, and _O.
neglecta_ in Cheshire in 1908, the known breeding range of both being in
the western Pacific; pelagic wanderings might lead a bird anywhere, but
it is conceivable that investigation may show that the breeding area is
wider than is supposed and that these species have stations even in the
South Atlantic.
Some writers affirm that birds only migrate on the wing, but the journey
by sea of many species is varied in method. Those very regular migrants,
the puffins and guillemots, which the light keepers assure us leave and
return to their stations almost at fixed dates, move by slow nautical
stages, swimming and feeding as they go. On May 2nd, 1911, I watched
a red-throated diver slowly travelling north; it actually travelled
farther beneath the surface than either by swimming or flying, so long
as I had it in view. The penguin's migrations cannot possibly be on
the wing. Dr Brooks rightly contended that the periodic assemblage of
wandering sea-birds at their "rookeries" is true migration, regular
as the almanack, although the feeding area is immense and the birds
do not reach home by any single path. Seebohm tells us of young bean
geese migrating in full moult, marching in an army to the interior of
the Tundra, and Mr W. H. Hudson, in "Birds and Man," relates a pathetic
story of a pair of upland geese in southern Buenos Ayres. His brother
saw them in August, the early spring of South America, leaving the
plains where they had wintered to breed in Magellanic islands. The main
flocks had departed, but these two birds, the female with a broken wing,
were steadily moving south, the male taking short flights and waiting
for her, as if to urge her on, and the female walking. "And in this
sad, anxious way they would journey on to the inevitable end, when a
pair or family of carrion eagles would spy them--and the first would be
left to continue the journey alone."
* * * * *
CHAPTER XI
SUMMARY
Migration owes its origin to the potentiality of flight, enabling
birds to advantage themselves by extended dispersals, which through
heredity become instinctive, regular and periodical. Geological changes,
especially the passing away of the glacial epoch, only influenced by
opening up new lands for summer colonisation, but climatic conditions
prevented these lands from becoming permanent abodes and fostered the
habit of periodical migration. Whatever the original home or centre
of distribution may have been, the dispersal from it was towards new
lands with a retreat towards the food-supply when these lands became
untenable. Fluctuating food-supply, love of home, sexual impulses,
desire for light, varying temperature, and other factors, all have more
or less influence, but the force exerted by any or all depends upon
the species operated upon and the locality in which it resides. The
present route followed or method of migration is little guide to the
history of past migration; during the evolution of present-day migration
alterations may have been occasioned by environment and changing
conditions. As Seebohm puts it, "The desire to migrate is a hereditary
impulse, to which the descendants of migratory birds are subject--a
force almost, if not quite, as irresistible as the hereditary impulse to
breed in the spring" (44).
The route is simply the course followed between the breeding area and
winter quarters; it is more or less restricted by the size of the area
in which food is to be found; it is usually the most direct way from one
food-base to the next, in a general direction from the seasonal bases.
Most birds move between north and south, but migrations are regularly
followed in other directions by some species.
Routes may follow coast-lines, these providing visible landmarks, and
also, for many species, plentiful food; islands, capes, estuaries and
inlets are landmarks, asylums, food-bases, and sites for congregation
and departure for cross-sea passages; at these places migration is
often specially noticeable. Overland routes may suggest "broad front"
migration, when there are no particular restricting influences and the
species have no special need for hurry. Migration at great elevations
and at high rates of speed is proved, but the highest and quickest
possible is as yet unascertained. It may also, under other conditions,
be performed at low elevations and very slowly. It is probable that
strong air-currents at a high elevation materially assist rapid and
lengthened migration. Force not direction of wind influences birds
moving at a low elevation.
Birds possess a certain power of orientation, a homing instinct,
which need not be called a sixth sense. Brain and eyes assist in the
development of this power; birds have an excellent memory. Young birds
lose their way more frequently than is generally supposed; variations
in routes are explained in many cases by these errors. Young may or may
not be guided by experienced adults; orientation is not infallible but
develops with age.
There is apparently no truth in the assertion that birds travel by
choice against a head wind or in a beam wind; a moderate wind behind,
on which they are carried, is most favourable. Leeward drift through
contrary winds explains many normal and abnormal routes, and the
occurrence of unexpected species in unexpected places. The distance
travelled not only varies according to species but in individuals of
the same species; the thesis that the most northerly breeder winters
farthest south does not always hold good.
Much may be learnt by the careful registration of arrivals and
departures of migratory birds, and by the marking of birds.
Ornithophænology, the science of migration study, as carried on at
present in many countries, would be materially assisted by some better
method of international registration and interchange of ideas.
In conclusion I would urge the value of the study, citing Herr Herman's
reasons put before the International Ornithological Congress in 1905.
The solution of the problem is in the interest of science, and therefore
of intellectual progress, teaching us the great part which migratory
birds play in the scheme of nature. The millions of birds which wander,
season after season, from one zone to another, represent an enormous
aggregate of labour, by flight and search for food, acting on "the
organic life of nature as does the regulator of a steam-engine, at
one time accelerating, at another retarding." Full insight into the
essence of the work done by birds will give us a correct notion of their
usefulness or injuriousness to man, and lead us to rational action for
their protection.
Whilst fully agreeing with Herr Herman I would go further. We live in
an age when aerial locomotion has become important, and will be more
and more important in the future. Every lesson we can learn from the
successes or failures of these most perfect aerial navigators must be of
use.
But putting aside economic and utilitarian considerations, there is to
some of us a greater stimulus to solve the problems of nature. With
the birds, and the insects and plants upon which they feed, we share
a common heritage, and the more we learn of the life of these, our
fellow workers, the nearer we approach solution of the great riddle of
the Universe, the mysterious law-abiding scheme of Nature. The book
of knowledge to which we may add some iota is marred with mystery,
superstition and error, but each proved fact cleanses its pages.
"Facts," says Laing, "are the spokes of the ladder by which we climb
from earth to heaven."
* * * * *
BIBLIOGRAPHY
1. ALLEN, J. A. Cooke's _Some New Facts about the Migration
of Birds_, _Auk_, xxi., 1904, 501.
2. ---- Gätke's _Heligoland_, _Auk_, xiii., 1896, 137.
3. ---- Walter's _Theories of Bird Migration_, _Auk_, xxv., 1908, 329.
4. BARRINGTON, R. M. "The great rush of Birds, etc." _Irish
Nat._, xx., 1911, 97.
5. ---- _The Migration of Birds as observed at Irish Lighthouses
and Lightships_, London, 1900.
6. BAIRD, S. F. "The Distribution and Migration of North
American Birds." _Amer. Jnl. Science and
Arts_, 2, 1866, xli.
7. BREHM, C. L. "Der Zug der Vögel," _Isis_, 1828, _Naumannia_,
1855.
8. BREWSTER, W. "Bird Migration." _Mem. Nuttall Orn. Club_
Cambridge, Mass. No. 1, 1886.
9. BROOKS, W. K. _The Foundations of Zoology_, New York, 1899.
10. _Bulletin of the British Ornithologists' Club. Reports on
Migration_, vols. xvii., xx., xxii., xxiv.,
xxvi., 1906-1910.
11. CARPENTER, F. W. "An Astronomical Determination of the
Heights of Birds," _Auk_, xxiii., 1906, 210.
12. CHAPMAN, ABEL. _Bird-Life of the Borders_, 2nd edit., London
1907.
13. ---- F. M. "Observations on the Nocturnal Migration of
Birds," _Auk_, 1888, 37.
14. CLARKE, A. H. "The Migration of Certain Shore Birds,"
_Auk_, xxii., 1905, 134.
15. CLARKE, W. E. "Bird Migration in Great Britain." _Report
of the British Association_, _London_, 1896.
16. ---- "Studies in Bird Migration," _Ibis_, 1902, 246, 1903, 112.
17. CLARKE, W. E. "The Birds of Fair Island; Native and
Migratory." _Ann. Scot. Nat. Hist._, 1906, 4.
18. COOKE, W. W. "Distribution and Migration of North
American Shorebirds." _U.S. Dept. Agric.
Biol. Survey_, Bull, 35, Washington, 1910.
19. ---- "Routes of Bird Migration," _Auk_, xxii., 1905, 1.
20. ---- "Some New Facts about the Migration of Birds." _U.S.
Dept. Agric. Year Book_, 1903, 371.
21. ---- "Our Greatest Travellers." _Nat. Geog. Mag._, 1911, 346.
22. ---- "The Migratory Movements of Birds in Relation to the
Weather." _U.S. Dept. Agric. Year Book_,
1910, 379.
23. CORDEAUX, J. "Migration in the Humber District," _Zool._,
1892, 418.
24. DERHAM, W. _Physico-Theology_, London. 1737. Lect. delivered
in 1711-12.
25. DIXON, C. _The Migration of Birds_, London, 1892.
26. ---- _The Migration of British Birds_, London, 1895.
27. FREDERICK II., (Emperor). _De Arte Venandi cum Avibus_,
Ed. Schneider, 1788, (Rhea. ii.. 1849).
28. GADOW, H. F. "Migration," _Encyclo. Brit._, 11th Edit.,
Cambridge, 1911.
29. GÄTKE, H. _Heligoland as an Ornithological Observatory_,
Trns. Rosenstock. London, 1895.
30. HERMAN, O. "A.M.O.K. Ornithophænologiæ anyaja,"
_Aquila_, 13, 1906, xx.
31. ---- _Recensio Critica automatica of the Doctrine of
Bird-Migration_, Budapest, 1905.
32. LAIDLAW, T. G. "Reports on the Movements and Occurrences
of Birds in Scotland during 1902 and
1903." _Ann. Scot. Nat. Hist._, 1903-4.
33. (LEGG, JOHN). _A Discourse on the Emigration of British
Birds_, London. 1795. (Salisbury, 1780, and
London 1811, the latter under name of
George Edwards.)
34. LINNÉ. C. _Dissertatio migratione Avium._ Upsaliae, 1757.
35. MIDDENDORF, A. T. VON. _Die Isepiptesen Russlands Grundlagen
zur Erforschung der Zugzeiten und
Zugrichtungen der Vögel Russlands_, St
Petersburg, 1853.
36. MENZBIER, M. "Die Zugstrassen der Vögel im Europäischen
Russland." _Bull de la Soc. Imp. d. Nat._,
Moscou, 1886, 291.
37. NEWTON, A. _A Dictionary of Birds_, London, 1893-1896.
38. ---- "Migration," _Encyclo. Brit._, 9th Edit., London.
39. PALMÉN, I. A. _Om foglarnes flyttingsvägar_, Helsingfors, 1874.
40. ---- _Über die Zugstrassen der Vögel_, Leipzig, 1876.
41. SCHÄFER, E. A. "On the Incidence of Daylight as a determining
factor in Bird Migration." _Nature_,
1907, 159.
42. SCLATER, W. L. "The Migration of Birds in South Africa."
_S. African Orn. Union_, 1906, II., 14.
43. SCOTT, W. E. D. "Some Observations on the Migration of
Birds." _Bull. Nuttall Ornith. Club_, vi. 97.
44. SEEBOHM, H. _Geographical Distribution of the Family
"Charadriidae,"_ London, 1888.
45. ---- _The Birds of Siberia_, London, 1901.
46. SERVICE, R. "Bird Migration in Solway." _Ann. Scot.
Nat. Hist._, 1903, 193.
47. STEBBINS, J. and FATH, E. A. "The use of Astronomical
Telescopes in determining the speeds of
Migratory birds." _Science_ (New York),
xxiv., 1906, 49.
48. STEJNEGER, L. "Do Birds Migrate along their Ancient Immigration
Routes." _Condor_, vii., 1905, 36.
49. STONE, W. "Bird Waves and their Graphic Representation,"
_Auk_, 1891, 194.
50. STUBBS, F. J. "The Use of Wind by Migrating Birds."
_Mem. and Proc. Manchester Lit. and Phil.
Soc._, vol. 53, 1909.
51. TAVERNER, P. A. "A Discussion of the Origin of Migration,"
_Auk_, xxi., 1904, 322.
52. TOMISON, J. "Bird Life as observed at Skerryvore Lighthouse."
_Ann. Scot. Nat, Hist._, 1907, 20.
53. TRUMBULL, J. "Notes on Land Birds observed in the North
Atlantic and the Gulf of St Lawrence."
1904. _Zoologist_, 1905, 293.
54. WALLACE, A. R., _Nature_, x., 1874, 459.
55. WHITLOCK, F. B. _The Migration of Birds_, London, 1897.
In addition numerous notes in the following periodicals
have been consulted:--_Annals of Scottish Natural History_,
_Auk_, _British Birds_, _Condor_, _Emu_, _Field_, _Ibis_, _Irish
Naturalist_, _Naturalist_, _Nature_, _Zoologist_.
INDEX
ALLEN, J. A., 29, 34, 49, 61, 64
American Golden Plover, 24, 61, 77
---- Robin, 102
Anacreon, 114
Anticyclones, 89
Arctic Tern, 3
BAIRD, S. F., 120
Barometric Influence, 88
BARRINGTON, R. M., 108, 109, 111
Beam Wind, 61, 96
Black and White Creeper, 68
Blackbird, 50, 109, 112
Blackheaded Gull, 75
Black Redstart, 109
Black-throated Blue Warbler, 69
Bluethroat, 123
Brambling, 7, 22
BRAY, R. A., 49
BREHM. C. L., 32
BREWSTER, W., 37, 106
BROOKS, W. K., 32, 124
BURROUGHS, J., 56
BUTLER, A. L., 121
Cape Pigeon, 121
CARPENTER, F. W., 48, 49
Chaffinch, 22
CHAPMAN, A., 45, 54
CHAPMAN, F. M., 48, 49
_Charadrius dominicus_, 77
_Charadrius fulvus_, 80
---- _plurialis_, 77
_Chrysomitris tristris_, 48
_Ciconia alba_, 71
CLARKE, A. H., 61
CLARKE, W. E., 6, 9, 38, 51, 59, 61, 64, 84, 85, 87, 88, 89, 94,
95, 100, 107
CLAYTON, H. H., 49
Common Tern, 75
COOKE, W. W., 6, 24, 25, 38, 41, 68, 69, 77, 79, 80, 101, 103, 104
CORDEAUX, J., 96
Corncrake, 117
COUES, E., 12
COX, P., 39
CRAWSHAY, R., 29
Crossbill, 123
Curlew, 8, 25, 111, 122
_Dendroica coerulescens_, 69
DERHAM, W., 116
Dipper, 7
Disasters at lights, 85, 105, 106
DIXON, C., 41, 43, 60, 116
DOCKRAY, J. A., 120
Dunlin, 8, 109
EDWARDS, G., 116
FATH, E. A., 55
Fieldfare, 7, 22
FREDERIC II., 115
GADOW, H. F., 20
GÄTKE, H., 33, 34, 47, 50, 51, 52, 62, 63, 121
GEOFFROY ST HILAIRE, 115
Geographical Distribution, 2, 10
_Geothlypis trichas_, 69
Glacial Epoch, 20
GLAISHER, 90
GMELIN, J. G., 117
Goldcrest, 95, 96, 109, 113
Goldeneye, 25
Golden Plover, 23, 24, 25, 26, 61, 77, 80, 111
Great Auk, 22
Great Crested Grebe, 44
Guillemot, 124
Hawk Owl, 121
HERMAN, O., 5, 6, 38, 40, 99, 115, 118, 129
Hibernation, 115, 116
_Hirundo rustica_, 65
Homer, 114
Hooded Crow, 51, 75, 97
HUDSON, W. H., 28, 124
Hudsonian Godwit, 28, 29
Isepipteses, 39
Jack Snipe, 7
Knot, 3, 28
Labrador Duck, 22
Land-bridges, 42, 43
Lapwing, 23, 74, 85
LEGG, J., 5, 117
Lesser Black-backed Gull, 123
_Limosa haesmastica_, 28
LUCANUS, 47
MACH-BRUER, 61
Manx Shearwater, 122
MAREK, M., 32
MARSHAM family, 117
MARTORELLI, G., 61
Maryland Yellowthroat, 69
Meadow Pipit, 109
MERRIAM, C. H., 6
MIDDENDORF, A. T. Von, 39, 58
_Mnistitta raria_, 68
MÖBIUS, K., 61
MONTAGU, G., 115
Moon-phases, 108
Moult, 67
NEWTON, A., 2, 11, 61
Nightingale, 43
Noddy Tern, 64
Non-breeding birds, 27, 28
_Numenius longirostris_, 122
_Oestrelata brevipes_, 123
---- _neglecta_, 123
Orientation, 56
Ornithophænology, 6, 117
Oystercatcher, 109, 111
Pacific Golden Plover, 26, 80
Pallas's Sand-grouse, 8
Palm Warbler, 69
PALMÉN, I. A., 39
Penguin, 124
Puffin, 124
QUINET, A., 41
_Quiscalus purpureus_, 48
RED GROUSE, 3, 7
Red-throated Diver, 124
Redwing, 22, 88, 109, 111
Ringing, 71-76
Robin, 7, 8, 109
Rook, 8, 97
Rushes, 85, 88
Sanderling, 28
SCLATER, P. L., 10
SCLATER, W. L., 66
SCOTT, W. E. D., 48
SEEBOHM, H., 19, 35, 36, 66, 107, 116, 124
SERVICE, R., 50, 96
Skylark, 109, 113
SMITH, A. C., 117
Snipe, 8, 109
Snow Bunting, 121
Song Thrush, 4, 7, 8, 73, 74, 109, 112
Sooty Tern, 64
Spoonbill, 50
Starling, 17, 109, 112, 113
STEBBINS, J., 49, 55
STONE, W., 112
Stonechat, 109
STUBBS, F. J., 52, 97, 98, 100
Submerged Coastlines, 42
Swallow, 36, 51, 65, 94
Swift, 18
TAVERNER, P. A., 2, 17, 30
TAYLOR, H., 98
Teal, 76
Thomson, A. L., 71
TOMISON, J., 88, 97, 105
Trans-Atlantic Migration, 119
TRISTRAM, Canon, 65
TRUMBULL, J., 121
Tufted Duck, 17
_Turdus migratorius_, 102
Turnstone, 28
Turtle Dove, 17, 44
Upland Goose, 124
VEREY, A. S., 48
WALLACE, A. R., 19, 31
WALTERS, H. E., 64
Water-rail, 109, 117
WATSON, J. B., 64
Wheatear, 67, 68, 109, 121
White Stork, 71
Wild Duck, 109, 111
Wind Pockets, 122
Wind Speed Tables, 92, 93
WINKENWERDE, H. A., 49
Woodcock, 75, 109
Yellow-browed Warbler, 123
Transcriber's Note: Although most printer's errors have been
retained, some have been silently corrected. Some spelling and
punctuation, capitalization, accents and formatting markup have
been normalized. Paragraphs which were split by illustrations, have
now been rejoined.
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