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Title: Studies of Birds Killed in Nocturnal Migration
Author: Tordoff, Harrison Bruce, Mengel, Robert M.
Language: English
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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
Volume 10, No. 1, pp. 1-44, 6 figures in text, 2 tables
September 12, 1956
Studies of Birds
Killed in Nocturnal Migration
BY
HARRISON B. TORDOFF AND ROBERT M. MENGEL
UNIVERSITY OF KANSAS
LAWRENCE
1956
* * * * *
UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
Editors: E. Raymond Hall, Chairman, A. Byron Leonard,
Robert W. Wilson
Volume 10, No. 1, pp. 1-44, 6 figures in text, 2 tables
Published September 12, 1956
UNIVERSITY OF KANSAS
Lawrence, Kansas
PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1956
[Illustration: Logo]
26-3856
* * * * *
Studies of Birds Killed in Nocturnal Migration
BY HARRISON B. TORDOFF AND ROBERT M. MENGEL
Contents
PAGE
Introduction 4
Accidents to Migrating Birds in early October, 1954 6
General 6
Accidents at Topeka, Kansas 6
Description of WIBW-TV tower 7
Weather conditions 7
Acknowledgments 7
Notes on the Species Killed at Topeka 8
Randomness of the Sample 17
Number of Migrants 18
Differential Migration of Sex- and Age-classes 20
History of the subject 20
Differential migration of sex- and age-classes as
shown by the Topeka sample 23
Molt in Relation to Migration 29
General comment 29
Molt in the Topeka sample 30
Size Differences according to Sex and Age 31
Linear measurements 31
Weights 32
Computations of Longevity and Survival 38
Processing of Samples 38
Summary 39
Literature Cited 41
Introduction
This paper is primarily an analysis of a sample of migrant birds
killed in the autumn of 1954 by striking a television tower one mile
west of Topeka, Shawnee County, Kansas. Secondarily, some aspects of
migration involved in studies of this kind are discussed and
historical background is presented.
Considerable interest has been occasioned in recent years in the
eastern United States by large-scale accidents to night-migrating
birds. Most accidents have occurred in the autumn. The widespread
adoption by airports of an instrument called the ceilometer, which
measures the height of cloud ceilings by reflecting from them a
high-powered beam of light, has proved under certain conditions to be
catastrophic to night-flying birds. Among the recent reports of such
accidents are those of Spofford (1949) and Laskey (1951) for
Nashville, Tennessee, Howell and Tanner (1951) for Knoxville,
Tennessee, and Lovell (1952) for Louisville, Kentucky. Recently
Howell, Laskey, and Tanner (1954) reviewed ceilometer "tragedies"
without being able to determine the exact reason for their lethal
effectiveness. Less publicized so far have been mass collisions of
birds with another class of obstacles, tall radio and television
towers. These slender towers, usually 500 to 1000 feet tall, are
increasing rapidly in numbers and there is reason to suppose that they
will take a correspondingly larger toll of bird life.
Notice has long been given by ornithologists to mass destruction of
birds by more conventional solid obstructions to passage, and
newspapers occasionally mention birds killed at such well-known points
as the Washington Monument and the Empire State Building.
Seventy-five years ago, J. A. Allen (1880) published the results of
questionnaires circulated by William Brewster to lighthouse keepers.
Brewster himself (1886) described destruction of birds at a lighthouse
in the Bay of Fundy, paying keen attention to behavior of the birds
and the exact conditions under which nocturnal flight and accidents
occurred. The subject also received attention in several countries
across the Atlantic. Destruction of birds at Irish lighthouses was
carefully noted over a period of years and the results were published
periodically, culminating in R. M. Barrington's massive report (1900)
which remains in some ways the most thorough of its type.
While conservation-minded individuals have been concerned with the
tremendous mortality involved in these various events, the ill wind
blows some good in that, properly used, the data provided by such
accidents can shed light on many obscure aspects of bird migration.
Each accidental kill of birds affords a cross-section, approaching in
variable degree a random sample, of the migrants passing a given point
on a given date. The types of information provided by such kills are
numerous, for example: (1) information on the presence of various
species and the dates of their occurrence; (2) information on the
relative abundance of species; (3) quantitative data on the relative
sizes of males and females, and immatures and adults (of importance to
taxonomic ornithology); (4) information on the relative times of
migration of males, females, adults, and young; (5) information on
molts and plumages; (6) quantitative information on composition by
subspecies of migrants of the same species; (7) physiological data
(fat condition, _etc._) pertinent to the study of migration; and
probably others.
In spite of the great potential of this kind of material, the majority
of ornithologists with access to such data have contented themselves
with listing the species and sometimes the numbers of birds killed. A
few have gone further. James T. Tanner (unpublished) attempted to
compute the longevity of the Ovenbird (_Seiurus aurocapillus_) by
analysis of ceilometer-killed birds at Knoxville, Tennessee (see
below). Mention should be made of the reports of Rintoul and Baxter
(1914) supplemented by Ticehurst (1916) who used rather small numbers
of birds killed at Scottish lighthouses in studies of molt. However,
the only effort to utilize the results of accidental kills on a large
scale over a period of years appears to have been that, already
mentioned, of Barrington (1900) and his co-workers in Ireland. An idea
of the potentialities of the large recent kills in the United States
may be obtained when it is recalled that in the 18 years of
Barrington's work, which embodied some 1000 reports from lighthouse
keepers, Barrington obtained for study only about 2000 specimens, many
of these consisting of wings and feet only (Barrington's paper not
seen in original; see J. A. Allen, 1901:205). More recently Dobben and
Bruyns (1939) have analyzed the age and sex classes of some birds
killed at lighthouses in Holland.
As far as we have learned, there is no previous thorough analysis in
the literature of large, accidentally-killed samples of birds. On the
following pages we emphasize some of the uses which can be made of
such material. We think that intensive analyses of such events,
whenever they occur, should become a regular part of ornithological
investigation and that integration of numerous studies of such
incidents will provide an unprecedented mass of information on
migration.
Accidents to Migrating Birds in early October, 1954
GENERAL.--The few days around the end of the first week of October,
1954, were notable for a series of accidents which occurred to
migrating birds over much of eastern United States. So far as we know,
these were all associated with an extensive belt of bad weather (cold
fronts and stationary fronts) which covered much of the country during
that period, and the accidents involved ceilometers and solid
structures alike. Accidents known to us occurred as far south as
Macon, Georgia (David W. Johnston, letter: Nov. 1, 1954), as far north
as New York City, where many migrants were killed at the Empire State
Building (_New York Times_, Thursday, October 7, 1954, p. 1) and
elsewhere, and as far west as Smoky Hill Air Force Base at Salina,
Kansas (ceilometer, October 7, some birds received at the University
of Kansas). Some of the above, and incidents from a number of other
localities, were mentioned in varying detail in _Audubon Field Notes_
(vol. 9, no. 1, pp. 6, 10, 15, 17, 18, 32, February, 1955). Still
other accidents occurred at Columbia, Missouri (Richard P.
Grossenheider, verbal communication), and Topeka, Kansas (present
paper). Some probably have escaped our notice; summaries of some of
these will probably appear in ornithological journals for some time to
come. At Robins Air Force Base near Macon, Georgia, at least 50,000
birds were killed, of which about 2500, representing 54 species, were
picked up (Johnston, _loc. cit._).
ACCIDENTS AT TOPEKA, KANSAS.--At Topeka, Shawnee County, Kansas, all
birds were killed by collision with the newly-erected (1954)
television transmitting tower of station WIBW-TV. This tower is one
mile west of the city.
The first casualties (see Table 1 for all others) were a Sora
(_Porzana carolina_) and a Yellow-bellied Flycatcher (_Empidonax
flaviventris_) found on September 7. The major accidents, however,
occurred on the nights of September 24-25, September 30-October 1,
October 5-6, and October 6-7. Totals of birds picked up (probably over
95 per cent of birds killed) are given in Table 1, in which each date
given is that of the day after the kill, _i. e._, the date on which
the birds were collected.
All major kills occurred on cloudy and foggy nights associated with
frontal weather. Throughout the period a few birds struck the tower
even on fairly clear nights, and minor but appreciable "falls"
occurred on the nights of October 4-5, 7-8, and 22-23. A few birds
killed probably were overlooked for a time and found their way into
later samples. This is especially probable in the case of some birds
entered under date of October 23, as many of these were somewhat
desiccated. Weights clearly altered by desiccation or mutilation were
not recorded. Reports of these accidents have been published by Carson
(1954 a, b, and c).
According to Carson (1954c:27), the majority of birds killed on nights
of heavy flight fell "between three and four o'clock in the morning
when skies were overcast and a cool front moved in from the north. Due
to the cooperation of the watchmen it is thought that most of the
birds that were killed were recovered. Of course some injured birds in
hiding were not found and some were lost to predators."
DESCRIPTION OF WIBW-TV TOWER.--The tower is 950 feet tall and stands
on a hill approximately 1000 feet above sea level. The fact that the
tower is on a hill places the top of the tower at 1010 feet above the
elevation of the average local terrain. The tower is triangular in
cross-section, each face seven feet wide, and is constructed of
six-inch steel L-beams with three-inch cross-members every seven feet
and smaller diagonal cross-members. It has no taper and bears a
transmitting antenna on the top. The tower is supported by 12 guy
wires, 3 wires attaching at each of 4 levels. The cables extend south,
WNW, and NNE from the tower and are 1-1/2 inches in diameter. The
tower is lighted by a series of red lights, some flashing and others
steady. The transmitter was not in operation when the accidents took
place.
WEATHER CONDITIONS.--All major kills at Topeka occurred when migrating
birds encountered either a cold front or a stationary front lying over
eastern Kansas. Typically, this frontal weather included rain, fog,
and cloud ceilings down to as low as 800 to 1000 feet. Weather of this
type presumably forces the migrating birds to fly below the cloud
ceiling and thus brings them within the altitudinal range of the
television towers.
Acknowledgments
We gratefully acknowledge our debt to the Topeka Audubon Society for
making this study possible by carefully collecting birds killed at the
television tower. L. B. Carson deserves special mention for his
general supervision of the bird collecting by the members of the
Topeka Audubon Society. Members of the Society and others who picked
up birds under the television tower were: Mrs. Lloyd Biggs, Elaine
Carson, L. B. Carson, Jesse A. Eddy, Elizabeth Fisher, Mrs. Walter
Huxman, Florence McKinney, Mrs. Charles Martin, Mrs. Fred P. Martin,
T. W. Nelson, Fred Prebble, Grace Prebble, Orville Rice, Mrs. G.
Warren Scholl, E. W. Senne, and Beatrice Swenson.
We received equally important assistance from students and staff of
the University of Kansas in recording of data and preparation of
specimens. The following helped in these ways: Rollin H. Baker, R. W.
Dickerman, David L. Hardy, J. W. Hardy, Jane S. Mengel, Larry D.
Mosby, Richard Van Gelder, South G. Van Hoose, and Glen E. Woolfenden.
We are indebted to the Interlibrary Loan Service of the University of
Kansas Library for help in securing certain reference works. Robert
Sokal of the University of Kansas gave helpful advice concerning
statistical procedures.
Notes on the Species Killed at Topeka
A list of numbers and kinds of birds killed is given in Table 1.
Discussion of data afforded by certain species for which, large
samples were available will be found below. There are additionally
certain data afforded by the sample and certain comments to be made on
various species which can be handled most conveniently in an annotated
list. In this list we have included all weight data (still scarce for
many North American birds), comments on status in Kansas of various
species, results of comparisons to determine subspecies, and
miscellaneous observations. Weights of birds are given in grams and
were taken on a triple-beam balance. Fat condition is given in the
scale proposed by McCabe (1943:556). Weight data from birds migrating
at night should be especially useful because these migrants all have
relatively empty crops and stomachs, thus reducing variability. Not
all birds were suitable for weighing and measuring, for a variety of
reasons. This accounts for discrepancies in totals between Table 1 and
the annotated list.
All passerine species were aged by noting the degree of ossification
of the skull. In no case, of the more than a thousand passerines aged
by examination of the skull, did we find difficulty in determining
whether an individual was a bird of the year or an adult. We found no
specimens in which ossification of the skull was nearing completion.
In the several species in our sample with distinctive first-winter
plumages, we found complete agreement in age as shown by plumage and
by condition of the skull. We think this is further proof, if such is
needed, that this method of aging is thoroughly reliable in early
autumn for the passerine species included in our sample and for others
with similar breeding seasons.
TABLE 1.--BIRDS KILLED AT A TELEVISION TOWER AT TOPEKA, KANSAS, IN
1954
See annotated list for division into sex- and age-classes. Where
discrepancies exist between totals given here and totals given in the
annotated list, these result from the fact that some specimens could
not be sexed and aged.
A: Sept. 25
B: Oct. 1
C: Oct. 3
D: Oct. 4
E: Oct. 5
F: Oct. 6
G: Oct. 7
H: Oct. 8
I: Oct. 9
J: Oct. 10
K: Oct. 23
L: Totals
=======================+===+===+===+===+===+===+===+===+===+===+===+====
| A | B | C | D | E | F | G | H | I | J | K | L
-----------------------+---+---+---+---+---+---+---+---+---+---+---+----
Pied-billed Grebe | | 1| | | | 1| | | 1| | | 3
Green Heron | | | | | | 1| | | | | | 1
Blue-winged Teal | | 1| | | | 1| 6| | | | | 8
Virginia Rail | | 3| | | | 1| | | | | | 4
Sora | 1| 6| | | | 1| 1| 1| | | | 10
American Coot | | | | | | 3| | | | | | 3
Mourning Dove | | 8| | | 1| | | | | | 1| 10
Yellow-billed Cuckoo | | | | | | 1| | | | | | 1
Black-billed Cuckoo | | | | | | | 1| | | | | 1
Yellow-shafted Flicker | 3| | | | | | | | | | | 3
Yellow-bellied | | | | | | | | | | | |
Flycatcher | | | | | | | 1| | | | | 1
House Wren | 2| 3| | | | 1| 2| 1| | | 1| 10
Long-billed Marsh Wren | | 1| | | | 1| 1| | | | | 3
Short-billed Marsh Wren| 1| 2| | | | | 1| | | | | 4
Catbird | 1| 28| 1| | 1| 6| 6| | | | | 43
Brown Thrasher | | 1| | | | 1| | 1| | | | 3
Wood Thrush | | 3| | | | | | | | | | 3
Hermit Thrush | | | | | | | | | | 1| | 1
Olive-backed Thrush | | 14| | 1| | | 1| | | | | 16
Golden-crowned Kinglet | | | | | | | | | | 1| 5| 6
Ruby-crowned Kinglet | 2| 1| | | | | 8| 1| 1| | 1| 14
Yellow-throated Vireo | | 1| | | | | | | | | | 1
Blue-headed Vireo | 1| 19| | 1| 2| 5| 8| 3| 1| | | 40
Red-eyed Vireo | 18| 36| | | 2| 13| 2| 3| | | 1| 75
Philadelphia Vireo | 3| 9| | | | | | | | | | 12
Warbling Vireo | 8| 19| 1| | 4| 1| 1| | | | | 34
Black and White Warbler| 1| 1| | | | 3| | | | | | 5
Tennessee Warbler | | 1| | | 1| 2| 1| | | | | 5
Orange-crowned Warbler | 7| 14| | | 1| 4| 19| 5| 1| 1| | 52
Nashville Warbler | 7| 94| 4| | 3| 39| 27| 5| | 1| 1| 181
Parula Warbler | | | | | | | 1| | 1| | | 2
Yellow Warbler | 3| 3| | | | 1| 1| | | | | 8
Magnolia Warbler | | 1| | | | 2| | | | | | 3
Black-throated Blue | | | | | | | | | | | |
Warbler | | | | | | | 2| | | | 1| 3
Myrtle Warbler | | | | | | | | | 1| | | 1
Black-throated Green | | | | | | | | | | | |
Warbler | | | | | | | | | | 1| | 1
Chestnut-sided Warbler | | 1| | | | | | | 1| | | 2
Bay-breasted Warbler | 1| | | | | 2| | | | | | 3
Palm Warbler | 3| | | | | | | | | | 1| 4
Oven-bird | 4| 21| | | | 2| 3| 1| | | 1| 32
Northern Water-thrush | | 5| | | | | | | | 1| | 6
Mourning Warbler | 15| 64| | | 2| 11| 2| 1| | | | 95
Yellow-throat | 10|115| 2| | 4| 25| 18| 1| 1| | | 176
Yellow-breasted Chat | | 1| | | | | | | | | | 1
Wilson Warbler | 1| 2| | | | | | | | | | 3
Canada Warbler | | 2| | | | | | | | | | 2
American Redstart | 1| | | | | | | | | | | 1
Bobolink | | 4| | | | | | | | | | 4
Rose-breasted Grosbeak | | 2| | | | | | | | | | 2
Indigo Bunting | | 1| | | 2| 3| 1| | | | | 7
Dickcissel | | 31| | | 1| 3| 1| | | | | 36
Savannah Sparrow | 1| 6| | 1| | 1| 5| 1| | | 1| 16
Grasshopper Sparrow | | 7| | | 2| 3| 3| 1| 1| | 1| 18
Leconte Sparrow | | | | | | | | | | | 3| 3
Sharp-tailed Sparrow | | | | | | 1| 1| | | 1| | 3
Slate-colored Junco | | | | | | | | | | | 1| 1
Clay-colored Sparrow | | 11| 1| | | 2| | 1| | | | 15
Fox Sparrow | | | | | | | | | | | 1| 1
Lincoln Sparrow | 41| 7| | | 5| 22| 3| 1| | | 3| 82
Swamp Sparrow | | 1| | | | 1| 2| | | | | 4
Song Sparrow | | | | | | | | | | | 2| 2
Total--species | 22| 41| 6| 3| 13| 31| 29| 16| 10| 8| 15| 61
Total--individuals | 94|585| 16| 3| 26|146|147| 31| 10| 8| 24|1090
------------------------------------------------------------------------
The annotated list may be consulted for further data in connection
with the species listed in Table 1. As is indicated below, we regard
the figures of this sample as unreliable to an unknown degree in
comparing the relative abundance of one species with another.
Accumulation of such data from various localities, however, should
prove useful in another type of comparison. Samples of the same
species killed in the same way at about the same time at different
localities should be directly comparable. Eventually, this should
provide us with a means of determining relative abundance of a species
in different parts of its migratory route.
Approximately 200 of the most interesting specimens were preserved as
study skins and are in the University of Kansas Museum of Natural
History. An effort was made to preserve at least one of each species,
and we fell only a few short of this goal. All of the forms rare in
Kansas are represented by skins. We could see no reason to list the
preserved specimens in detail here. Species of which no study skins
were made, however, are so marked.
So far as we can tell, no truly western subspecies (from west of the
Great Plains) occurred in the Topeka sample. Probably most or all of
the birds came from areas more or less directly north of eastern
Kansas.
In critical areas where different subspecies of the same species occur
together in migration, data from samples of this kind should prove
enlightening. In future analyses, conducted in such areas, it might be
possible to preserve all specimens of some of the variable species, or
at least to measure all individuals of species in which size is the
most important variable character. Quantitative study could then be
made of the different geographic variants occurring, their proportions
in the migrant population determined, and their origins deduced. In
studying populations of Painted Buntings (_Passerina ciris_) wintering
in Mexico, Storer (1951) has provided an interesting demonstration of
methods which can be applied to such samples.
A few bats killed at the tower provided a surprise. They will be
discussed separately by Richard Van Gelder.
_Podilymbus p. podiceps._ Pied-billed Grebe.--Weights: male,
394.8 (all weights in grams); females, 332.5, 289.7; all
fat.
_Butorides v. virescens._ Green Heron.--Weight: 1 (unsexed),
168.6.
_Anas discors._ Blue-winged Teal.--Weights: 4 males, mean
421.2 (391.3-458.1); 3 females, 367.7, 371.6, 393.2; all
fat.
_Rallus limicola._ Virginia Rail.--Weights: 3 males, 73.7,
83.2, 90.5; 1 female, 67.3; moderately fat to fat.
_Porzana carolina._ Sora.--Weights: 4 males, mean 76.8
(68.7-89.9); 3 females, 62.6, 63.2, 63.5; moderately fat to
very fat.
_Fulica americana._ American Coot.--Weights: 2 females,
385.3, 530.0, both fat. None preserved.
_Zenaidura macroura marginella._ Mourning Dove.--Weights: 2
adult males, 121.8, 140.2; 3 immature males, 113.1, 126.1,
130.0; 3 adult females, 122.5, 126.9, 136.0; 2 immature
females, 129.4, 132.7; moderately fat to very fat. The
presence of Mourning Doves in the sample is interesting as
these birds are not generally regarded as night migrants.
Conceivably the specimens were local birds going to roost.
None preserved.
_Colaptes auratus luteus._ Yellow-shafted Flicker.--Weights:
2 males, 126.0, 139.4, little fat. Flickers have several
times been recorded as night migrants.
_Empidonax flaviventris._ Yellow-bellied
Flycatcher.--Weight: 1 immature male, 11.9, moderately fat.
This is a rare species in Kansas, the present being the
ninth preserved specimen for the State.
_Troglodytes aëdon parkmanii._ House Wren.--Weights: 4 adult
males, mean 10.5 (9.8-10.9), 2 immature males, 9.0, 11.3; 1
adult female, 9.9, 1 immature female, 7.0; no fat (im.
[**Female symbol]) to fat.
_Telmatodytes palustris dissaëptus._ Long-billed Marsh
Wren.--Weights: 1 adult male, 10.8; 1 adult female, 9.2;
both moderately fat. The specimens are moderately bright and
rufescent above, being typical of the populations of the
central plains.
_Cistothorus platensis stellaris._ Short-billed Marsh
Wren.--Weights: 1 immature male, 8.2; 1 adult female, 8.1;
immature female, 8.2; all fat.
_Dumetella carolinensis._ Catbird.--Weights: 6 adult males,
mean 37.5 (34.1-42.5), little fat to very fat; 14 immature
males, mean 37.57 ± .94 (standard error), S. D. (standard
deviation) 3.37, little fat to fat; 11 adult females, mean
39.09 ± .94, S. D. 2.97, little fat to fat; 12 immature
females, mean 38.42 ± .83, S. D. 2.74, moderately fat to
fat.
_Toxostoma r. rufum._ Brown Thrasher.--Weight: 1 immature
male, 60.2, little fat.
_Hylocichla mustelina._ Wood Thrush.--Weights: 1 adult male,
54.2, moderately fat; 2 adult females, 44.6, 45.7, little
fat and fat, respectively.
_Hylocichla ustulata swainsonii._ Olive-backed
Thrush.--Weights: 6 immature males, mean 31.0 (28.1-33.2),
little fat to fat; 6 adult females, mean 29.6 (27.1-35.0),
moderately fat to fat; 3 immature females, 27.1, 33.8, 35.8,
little fat to fat. The absence of adult males in our sample
of 15 birds is noteworthy but inexplicable with our few
data.
_Regulus s. satrapa._ Golden-crowned Kinglet.--Weights: 1
adult male, 6.7, moderately fat; 2 immature males, 6.5, 7.4,
moderately fat and fat; 2 adult females, 7.3, 7.4,
moderately fat and fat; 1 immature female, 7.2, moderately
fat.
_Regulus c. calendula._ Ruby-crowned Kinglet.--Weights: 3
adult males, 6.2, 7.6, 8.2, little fat to fat; 1 immature
male, 6.6, fat; 4 adult females, mean 6.1 (5.6-6.7),
moderately fat to fat; 3 immature females, 5.8, 6.6, 7.0,
moderately fat to fat.
_Vireo flavifrons._ Yellow-throated Vireo.--Weight: 1
immature male, 21.5, very fat.
_Vireo s. solitarius._ Blue-headed Vireo.--Weights: 9 adult
males, mean 17.7 (16.6-19.5), little fat to very fat; 17
immature males, mean 17.53 ± .46, S. D. 1.83, no fat (13.8)
to very fat (21.3); 7 adult females, mean 17.6 (15.0-21.6),
moderately fat to very fat; 6 immature females, mean 17.0
(14.5-18.9), moderately fat to fat. Surprisingly numerous in
the sample.
_Vireo olivaceus._ Red-eyed Vireo.--Weights: 1 adult male,
16.1, moderately fat; 38 immature males, mean 21.21 ± .43,
S. D. 2.60, little fat (1 specimen) to excessively fat,
mostly moderately fat or fat; 2 adult females, 18.1, 18.1,
both fat; 23 immature females, mean 19.28 ± .46, S. D. 2.16,
little (2 specimens) to very fat, mostly fat.
Wing length: 1 adult male, 79.1; 38 immature males, mean
78.05 ± .30, S. D. 1.80; 2 adult females, 76.3, 79.0, 23
immature females, mean 75.83 ± .42, S. D. 1.99.
As mentioned below, the presence of only 3 adults in the
sample of 64 Red-eyed Vireos is highly significant and their
occurrence only in the earlier samples is strong evidence of
early migration by the adults.
_Vireo philadelphicus._ Philadelphia Vireo.--Weights: 2
adult males, 12.1, 15.9, moderately fat and very fat; 2
immature males, 11.1, 13.2, fat and very fat; 2 adult
females, 13.1, 14.2, both fat; 5 immature females, mean 14.1
(12.0-15.2), moderately fat to very fat.
This species previously has been collected in Kansas only
twice. Both records are from Doniphan County in September,
1922. Field observers occasionally record the Philadelphia
Vireo in eastern Kansas. Long (1940:450) calls it a "very
rare migrant in the extreme east." Our sample of 12 birds
killed on two nights (and probably after the peak of
migration of this species) leads us to think that this vireo
is actually a regular, but overlooked, migrant in fair
numbers.
_Vireo g. gilvus._ Warbling Vireo.--Weights: 12 adult males,
mean 15.92 ± .43, S. D. 1.44, moderately fat to very fat; 8
immature males, mean 16.64 (14.2-17.8), fat to very fat; 5
adult females, mean 16.1 (13.7-18.0), fat to very fat; 5
immature females, mean 15.4 (14.1-17.8), little fat to fat.
Wing length: 12 adult males, mean 73.08 ± .49, S. D. 1.64; 8
immature males, mean 71.15 (69.9-72.8); 5 adult females,
mean 70.0 (69.2-71.0); 5 immature females, mean 68.4
(67.7-70.3).
Tail length: 12 adult males, mean 53.33 ± .53, S. D. 1.77; 8
immature males, mean 50.03 (47.1-51.3); 4 adult females,
mean 48.6 (47.7-49.8); 5 immature females, mean 49.2
(47.3-53.0).
There is no indication that western birds (_V. g.
swainsonii_) make up any part of this sample.
The sample of 34 Warbling Vireos is too small to show the
significance, if any, of the 2:1 ratio of males to females
in the sample. Adequate samples of this species, taken at
intervals, would add interesting information on time of
migration of the four sex- and age-classes.
_Mniotilta varia._ Black and White Warbler.--Weights: 1
adult male, 12.5, fat; 2 adult females, 10.0, 10.0, little
fat, fat.
_Vermivora peregrina._ Tennessee Warbler.--Weights: 1 adult
male, 10.9, very fat; 1 immature male, 12.9, very fat; 2
adult females, 9.1, 12.5, moderately fat and very fat. The
relative scarcity of Tennessee Warblers in the sample is
surprising. They are common in the area in spring.
_Vermivora c. celata._ Orange-crowned Warbler.--Weights: 9
adult males, mean 8.8 (7.7-10.9), little fat to fat; 13
immature males, mean 8.92 ± .15, S. D. .53, little fat to
fat; 5 adult females, mean 8.8 (8.3-10.3), little fat to
moderately fat; 17 immature females, mean 9.13 ± .08, S. D.
.72, little fat to fat. Of the 19 Orange-crowned Warblers
killed on October 7, 11 had little fat, 6 were moderately
fat, and only 2 were fat. No one-night sample of any other
warbler killed at Topeka had less fat than this group of
warblers. Furthermore, our sample (including 11 males) from
October 7 (all sex- and age-classes) averaged 8.81 grams;
the sample of 13 (including only 4 males) from October 1
averaged 9.1 grams. If one can assume, for any one species,
that individuals undertake nocturnal migration only when
they are physiologically ready, and this includes a certain
amount of fat as a fuel source (Wolfson, 1954), then this
further assumption seems justified: birds killed in
migration with little fat must have flown longer or farther
or both than birds killed with more fat. No further
speculation on this point is permissible with our data, but
the possibilities for study of future large kills,
especially where actual time of death of the birds is known,
are obvious.
_Vermivora r. ruficapilla._ Nashville Warbler.--More
Nashville Warblers were picked up at Topeka than any other
species and they are discussed in detail elsewhere in this
report. The four sex- and age-classes can be identified with
fair accuracy on plumage characteristics alone. Adult males
have a large amount of reddish-brown in the crown, not
completely veiled by the gray tips of the crown feathers.
Immature males have a smaller but distinct crown patch,
usually completely veiled. All males, compared with females,
are grayer on the sides of the head, have a more nearly
white eye-ring, and show clearer yellow on the throat. Adult
females differ from immature females in that they more often
have a trace of rufous in the crown and tend to be brighter
below than the immatures. Of 177 specimens, 20 were very
fat, 108 were fat, 46 were moderately fat, and 3 had little
fat.
_Parula americana._ Parula Warbler.--Weight: 1 adult female,
7.9, fat.
_Dendroica petechia aestiva._ Yellow Warbler.--Weights: 1
immature male, 10.2, fat; 3 adult females, 8.8, 9.5, 10.1,
moderately fat; 2 immature females, 9.0, 9.4, little fat and
fat.
_Dendroica magnolia._ Magnolia Warbler.--Weights: 1 adult
female, 9.0, moderately fat; 2 immature females, 7.9, 10.3,
moderately fat and fat.
_Dendroica c. caerulescens._ Black-throated Blue
Warbler.--Weights: 2 immature males, 13.8, 14.1, excessively
fat; 1 immature female, 11.4, fat. This species is rare in
Kansas. Although its breeding range is almost entirely east
and north of Kansas, records in files at the University of
Kansas show that more specimens have been taken in western
than in eastern Kansas.
_Dendroica c. coronata._ Myrtle Warbler.--Weight: 1 immature
female, 11.6, fat.
_Dendroica pensylvanica._ Chestnut-sided Warbler.--Weights:
2 immature females, 8.1, 10.0, little fat. Only one specimen
from Kansas had been preserved previously although the
species is a regular transient in small numbers throughout
the state.
_Dendroica castanea._ Bay-breasted Warbler.--Weights: 1
adult male, 19.2, excessively fat; 1 adult female, 11.7,
little fat; 1 immature female, 11.2, moderately fat. Only 5
specimens of this warbler have been taken previously in
Kansas, 4 in spring (Ruth, 1952:18-19) and 1 in fall.
_Dendroica p. palmarum._ Palm Warbler.--Weights: 2 immature
males, 9.9, 10.9, moderately fat; 2 unsexed immatures, 9.1,
9.4, moderately fat. This species has been taken in fall in
Kansas only once before (KU 26353, taken by Wetmore, at
Lawrence, on October 5, 1907), but probably occurs regularly
in both spring and fall migration.
_Seiurus a. aurocapillus._ Oven-bird.--Weights: 2 adult
males, 22.5, 23.8, fat and very fat; 14 immature males, mean
21.89 ± .66, S. D. 2.46, fat to very fat; 8 adult females,
mean 21.4 (18.3-25.7), moderately fat to fat; 6 immature
females, mean 18.2 (15.6-20.0), moderately fat to fat.
_Seiurus noveboracensis notabilis._ Northern
Water-thrush.--Weights: 3 immature males, 18.1, 18.6, 22.2,
moderately fat to fat; 1 immature female, 22.2, fat.
Referring these birds to _notabilis_ is a somewhat arbitrary
procedure. They display some intermediacy of characters and
probably stem from a population, intermediate between
_notabilis_ and _noveboracensis_, occupying much of central
North America (cf. McCabe and Miller, 1933).
_Oporornis philadelphia._ Mourning Warbler.--Weight data
presented elsewhere. The birds killed at Topeka provide the
latest fall dates for this species in Kansas. Fifteen were
killed on September 25, 64 on October 1, 2 on October 5, 11
on October 6, 2 on October 7, and 1 on October 8. We find no
other records later than September 15. Of 93 specimens
examined, 1 was excessively fat, 22 were very fat, 45 were
fat, 21 were moderately fat, and 4 had little fat. The
abundance of this secretive species in the sample was a
great surprise. It had previously been considered a rather
rare migrant in this area.
_Geothlypis trichas occidentalis [>brachidactyla?]._
Yellow-throat.--Weight data presented elsewhere. This
species was second in numbers only to the Nashville Warbler
in the total kill at Topeka. Of 167 birds examined, 29 were
very fat, 114 were fat, 23 were moderately fat, and 1 had
little fat.
The Yellow-throats are greatly in need of meaningful and
comprehensive revision, which to date has been restricted to
the western subspecies (Behle, 1950). Since the appearance
of the 1931 A. O. U. Check-List a great deal of scattered
taxonomic work on the species, as yet unsynthesized, has
made the picture of its geographic variation a blurry one so
far as the details are concerned. Made in the absence of
adequate comparative material, the above identification is
to be regarded as tentative. Also, it is, unfortunately,
based only on those 6 of our 176 specimens preserved as
skins. Five of these are adult males, the sixth being an
immature female. Compared with a series of Kentucky
specimens regarded as typical _brachidactyla_, these birds
are paler and brighter above (tending toward gray-green
rather than brownish olive), brighter and more extensively
yellow below, with broader, more nearly white superciliary
stripes above their black masks (in males). In size they are
close to _occidentalis_ (see Behle, 1950:202). Five males
have an average wing-length of 56.6 mm. (53-59); one female
measures 53. Six males from Kentucky: 55.1 (53-56); four
females, 51.1 (48-56). Our birds may be assumed to have
stemmed from a population to the north and west which, if
not _occidentalis_ (or _campicola_ Behle and Aldrich, of
which no comparative material is at hand), is intermediate
between _brachidactyla_ and more western birds. Judging from
Behle's map (1950:fig. 32), these birds may have come from
an area near the confluence of three subspecies
(_campicola_, _occidentalis_, _brachidactyla_). Long
(1940:452) reports three subspecies breeding in Kansas
(_brachidactyla_, northeast; _occidentalis_, west;
_trichas_, southeast). The occurrence in Kansas of _G. t.
trichas_ as currently understood is completely out of the
question.
_Icteria v. virens._ Yellow-breasted Chat.--Weight: 1
unsexed immature, 29.7, moderately fat.
_Wilsonia p. pusilla._ Wilson Warbler.--Weights: 2 adult
females, 7.5, 7.8, fat, moderately fat; 1 unsexed adult,
8.3, fat.
_Wilsonia canadensis._ Canada Warbler.--Weight: 1 immature
female, 10.0, little fat. We know of only five other
specimens from Kansas, although this warbler seems to be a
regular migrant in small numbers in the state.
_Setophaga r. ruticilla._ American Redstart.--Weight: 1
immature female, 9.1, moderately fat.
_Dolichonyx oryzivorus._ Bobolink.--Weights: 2 adult
females, 39.5, 42.9; 2 immature females, 38.8, 42.0; all
excessively fat. Specimens of the Bobolink previously have
been taken in fall in Kansas only on September 20 and 24,
1933, near Lawrence, by Long and Preble (Long, 1934).
_Pheucticus ludovicianus._ Rose-breasted Grosbeak.--Weights:
1 adult male, 50.4, fat; one immature male, 54.5, very fat.
_Passerina cyanea._ Indigo Bunting.--Weights: 1 adult male,
18.4, fat; 2 immature males, 17.2, 17.2, fat and very fat; 2
adult females, 14.3, 16.9, moderately fat and very fat; 1
immature female, 13.4, little fat. The sample was carefully
checked for Lazuli Buntings (_Passerina amoena_); none was
found.
_Spiza americana._ Dickcissel.--Weight data presented
elsewhere in this paper. Dickcissels were picked up at the
television tower on October 1 (31), 5 (1), 6 (3), and 7 (1).
These birds, together with an adult female taken 3 miles
east and 3 miles south of Lawrence, on October 11, 1953, by
Tordoff, are the only specimens of this species taken as
late as October in Kansas. The Dickcissel becomes
inconspicuous in late summer and many observers here and
elsewhere have thought the species disappeared much earlier
than it really does (see Ganier, 1949). Of 34 specimens, 20
were very fat and 14 were fat.
_Passerculus sandwichensis nevadensis._ Savannah
Sparrow.--Weights: 1 adult male, 19.4, fat; 2 immature
males, 18.3, 19.0, moderately fat; 5 adult females, mean
17.2 (14.8-19.5), little fat to fat; 4 immature females,
mean 18.0 (16.9-19.6), moderately fat to fat. Many of the
Savannah Sparrows migrating through Kansas have in the past
been referred to the subspecies _P. s. anthinus_ (=
_alaudinus_ of the 1931 A. O. U. Check-List) by various
workers (see Long, 1940:454). As Peters and Griscom
(1938:464-5) have shown, true _anthinus_, breeding in the
far northwest, ordinarily occurs in migration only in the
western part of the country, the breeding Savannah Sparrows
of a large part of the central continental region (east to
southern Wisconsin) being _P. s. nevadensis_ as now
understood. Migrants of this pale, clay-colored subspecies
should be abundant in Kansas, and all of the specimens in
the present sample are referable to it.
_Ammodramus savannarum perpallidus._ Grasshopper
Sparrow.--Weights: 3 adult males, 16.4, 17.6, 20.6,
moderately fat, fat, fat; 5 immature males, mean 18.1
(16.0-20.2), little fat to fat; 5 adult females, mean 17.9
(16.8-18.9), moderately fat to very fat; 5 immature females,
mean 18.1 (16.8-20.6), fat to very fat.
_Passerherbulus caudacutus._ Leconte Sparrow.--Weights: 1
immature male, 11.2, moderately fat; 1 immature female,
12.2, moderately fat.
_Ammospiza caudacuta nelsoni._ Sharp-tailed
Sparrow.--Weights: 2 adult males, 15.2, 17.1, moderately fat
and very fat; 1 adult female, 13.3, little fat. Five
specimens of this species have been taken previously in
Kansas, all in October in the eastern part of the state.
Additionally, several observers have reported birds seen but
not collected. The three birds from Topeka were picked up on
October 6, 7, and 10 and are the only specimens taken since
1907. Possibly our specimens from Topeka struck the tower on
the same night. Tordoff noticed, upon preparation, that the
specimens from October 7 and 10 showed progressive drying of
the extremities and spoilage as compared with the bird
picked up on October 6.
_Junco hyemalis cismontanus._ Slate-colored Junco.--Weight:
1 immature female, 16.4, little fat. Juncos of hybrid type,
whether _J. h. hyemalis_ × _J. oreganus_ subsp. or true _J.
h. cismontanus_, are fairly common in eastern Kansas.
_Spizella pallida._ Clay-colored Sparrow.--Weights: 2 adult
males, 11.6, 12.2, both fat; 1 immature male, 11.8, fat; 1
adult female, 12.5, fat; 7 immature females, mean 11.1
(9.7-12.5), little fat to fat.
_Passerella iliaca iliaca._ Fox Sparrow.--Weight: 1 adult
female, 29.4, little fat. A trifle grayer above than any of
several Kentucky specimens, this bird nevertheless seems
well within the range of variation of _iliaca_.
_Melospiza l. lincolnii._ Lincoln Sparrow.--Weights and
measurements are discussed elsewhere. Of 81 specimens, 15
were very fat, 47 were fat, 12 were moderately fat, and 7
had little fat. Interestingly, there is no evidence that the
large southern montane subspecies (_M. l. alticola_) has
contributed to the present sample. No bimodality is evident
in the curve of wing-length in our birds, the largest of
which barely approach, the small extreme recorded for
_alticola_ by Miller and McCabe (1935:156).
_Melospiza georgiana ericrypta._ Swamp Sparrow.--Weights: 3
immature females, 14.3, little fat, 16.7, 17.0, moderately
fat. Swamp Sparrows examined were all more or less brightly
colored and seem to belong to this northern subspecies.
_Melospiza melodia juddi._ Song Sparrow.--Weights: 1 adult
female, 19.4, little fat; 1 unsexed immature, 16.0, little
fat. A large proportion of the migrant and wintering Song
Sparrows in eastern Kansas probably originate from the range
of this subspecies in the northern plains. _Melospiza
melodia euphonia._--One immature female (not weighed) was
picked up below the tower on October 27, 1954, and thus does
not appear in Table 1. The specimen proved typical of this
generally more eastern subspecies upon comparison with a
large series from Kentucky. For what it may be worth we
refer the single specimen to this subspecies. Long
(1940:456) reported two eastern subspecies from Kansas
("_beata_," _melodia_). All Kansas specimens genuinely of
eastern origin probably originate from the range of
_euphonia_, as now understood.
Randomness of the Sample
The reliability of certain of the conclusions which might be drawn
from data of the kind presented herein depends largely on the
randomness of the sample. To what degree does this sample provide a
true cross-section of the nocturnal migrants present over the area on
a given night or succession of nights? As far as the relative
abundance of species in the sample is concerned, there is little doubt
that it is not at all random. The absence of such species as the
Gray-cheeked Thrush (_Hylocichla minima_), among the passerines, and
many of the shorebirds known to be migrating through the area at the
time is evidence for this statement. Quite possibly many seminocturnal
species did not strike the tower at all for the simple reason that
they could see it, and certain large-eyed diurnal species (such as
thrushes and shorebirds) may avoid collision to some extent, thus not
appearing in the sample in proportion to their actual numbers.
Finally, some or all of the species concerned probably migrate partly
by day. The sample may to some degree reflect the true relative
abundance of closely related species. For example, there is little
doubt that, as shown by the sample, Nashville Warblers are more
numerous locally at this season than Tennessee Warblers, a fact that
can readily be corroborated by ordinary field observation. Also, the
sample is useful in suggesting the actual abundance of species which
are furtive and/or difficult to identify under normal field
conditions, for example, the Mourning Warbler and Philadelphia Vireo.
It is obvious that the sample should reflect the true relative
abundance at one place and time of any two species with equal tendency
to migrate by night and equal tendency to strike the tower. Since the
facts in regard to both tendencies are at present unknown for most
species, we think that interspecific comparisons should be avoided or
approached with extreme caution.
In respect to the relative abundance of the various sex- and
age-classes within a given species, the sample is, we think, as close
to random as is possible to obtain. Certainly it is greatly superior
to samples obtained by field collecting, where possible differences in
habits, wariness, and experience of the birds, and subconscious (if
not conscious) selection by collectors can all bias the results.
Dwight (1900:128-9) believed that the greater wariness of adult birds
was almost entirely responsible for the seemingly disproportionate
number of immatures in autumn and gave some observational evidence in
favor of his views. The large percentage of adults in some of the
samples here treated tends to reinforce Dwight's position. To a
somewhat lesser extent, this advantage in randomness of accidental
kills over routine collecting may be supposed to apply also in
demonstrating the composition by subspecies of a single migrant
species.
So far as particulars already mentioned are concerned, the present
sample or other samples of tower-killed birds would seem to be in no
way superior (that is, more nearly random) to samples obtained in
connection with lighthouses and other lighted objects, and
ceilometers. In one important respect, however, it is probably
somewhat superior to these as the dimly red-lighted structure has not
been shown to have any important collecting or attracting influence.
Thus, in computations intended to estimate the over-all abundance of
migrants, the sample should be more reliable than samples involving
bright light with its possible attracting effect.
Number of Migrants
If it can be assumed that nocturnally migrating birds are
approximately uniformly spaced across the sky and that the red lights
did not attract birds which would otherwise have missed the tower, it
is possible to compute the volume of migration from the sample killed.
In regard to the first assumption, both Stone (1906:250-251) and
Lowery (1951:409-413) have presented evidence of fairly uniform
distribution of nocturnal migrants. We have no information on the
second assumption beyond the facts that birds do not strike the high
towers on clear nights or lower towers even on stormy nights.
On nights when large numbers of birds struck the 950 foot Topeka
tower, only a few struck a 500 foot radio tower, also lighted with red
lights, at Lawrence, 24 miles east, under similar weather conditions.
Most of the birds found at Topeka were fairly close to the base of the
tower, indicating that they struck the tower itself or that they were
flying high enough to strike guy wires only fairly close to the tower.
The scarcity of birds under the guy wires some distance from the tower
at Topeka and at the radio tower at Lawrence causes us to think that
most of the birds were flying more than 450 feet above the ground. On
this basis, we have computed numbers of migrants passing through a
plane one mile long and 500 feet high (2,640,000 square feet),
intersecting the assumed path of migration at right angles.
Vertically, the theoretical plane begins at 450 feet above ground and
has its top edge at 950 feet. The solid (discounting spaces between
girders, _etc._) cross-sectional area of the tower intersecting this
plane was computed by actual measurement to be 1685 square feet. On
the night of September 30-October 1, 585 birds were killed. By
computation (585/1685 = X/2,640,000), approximately 916,000 birds
passed through the mile-long plane that night. On each of the nights
of October 5-6 and October 6-7, approximately 230,000 birds passed
through this plane. By comparison, Lowery (1951:436) recorded maximum
station densities in one night in spring of 63,600 birds at Tampico,
Mexico, and 51,600 at Lawrence, Kansas, as determined by
moon-watching. Lowery's figures refer to numbers of birds crossing any
part of a circle one mile in diameter and are roughly comparable to
ours if, as we think, most of the birds at Topeka were flying at
altitudes between 450 and 950 feet above the ground.
It must be realized that these figures are only approximations. One
variable ignored is the frontal extent (or area, viewed from the
front, subject to damage by striking an obstruction) of the birds
themselves. Since practically all birds killed showed head or trunk
injuries, rather than a high proportion with only broken wings, we
chose to disregard frontal extent of the birds in our calculations. If
our figures are inaccurate by as much as 50 per cent in either
direction, which seems unlikely to us, they still give some idea of
the tremendous volume of nocturnal migration under some conditions.
It may be more meaningful to compute numbers of migrants by species.
This can be done readily by making appropriate substitutions from
Table 1 in the equation given above. For example, on the night of
September 30-October 1, approximately 147,000 Nashville Warblers
passed through the mile-long plane and on the same night, 100,000
Mourning Warblers and 14,000 Philadelphia Vireos. Neither of the last
two species would be judged to be abundant migrants in autumn in
eastern Kansas by ordinary field observations; the television tower
sample, however, indicates that these as well as other species must
often be overlooked when they do stop in Kansas.
Differential Migration of Sex- and Age-classes
HISTORY OF THE SUBJECT.--For a long time it has been known in a
general way that old and young birds and males and females of some
species do not always migrate at the same times, by the same routes,
or even to the same places. This is a subject about which much has
been written. Reading the summaries of some general texts, it is easy
to acquire the impression that the facts of the matter are well known.
On the contrary, they are poorly known and much remains to be learned
before differential migration is understood. This can best be
indicated by a brief survey of the literature.
The importance of the subject was emphasized by Meinertzhagen
(1930:52) in one of the later reviews of differential migration: "The
main problem concerns the Cause of Migration, the Stimulus which
compels Migration and the Origin of the Migratory Habit.... There is,
however, a minor problem which affords valuable evidence in helping us
to solve the major problem, bearing very directly on it, namely, the
order of sex and age on migration."
The mystery of how birds, especially the young, find their way in
migration has fascinated students since the earliest times. The quite
natural though purely anthropomorphic conclusion of early scholars was
that the old birds led the young on migration. This attractive idea
persisted long after ornithology began to grow into a science. The
classic theory was restated by Palmén (1876:267), in one of the first
thorough reviews of the subject of migration, as follows: "Directe
Beobachtungen in der Natur ergeben, dass die Schaaren von ziehenden
Vögeln allgemein ältere und stärkere Individuen als Anführer des Zuges
haben." Variously modified, this view continued to crop up for some
time and still found support in the 1890's (see Dixon, 1892:69). Gätke
(1895:101) correctly questioned the credibility of Palmén's "direct
observations."
With the gradual abandonment of the unsupportable classic theory,
diametrically opposed views were adopted by workers on opposite sides
of the Atlantic. The American stand was ably expressed by Brewster
(1886), who went to great pains to state his case and give evidence,
and who was later supported by Allen (1896:144-147; 1909:17). The
Americans held that adult birds nearly always preceded the young in
migration, and this was based on much evidence, whether or not
correctly interpreted. Dwight (1900:127) also gave evidence in favor
of this theory. Equally definite, if, as has later been shown,
somewhat vaguely documented, was the famous work of Gätke (1895:see
pp. 100-113), who after many years' observation of migrant birds in
Heligoland concluded the exact opposite, that young in general precede
adults (see critiques of Allen, 1896:144-147; Wiegold, 1926:5).
Gätke's dissenting opinion was for a time supported enthusiastically
by British workers (Gurney, 1923:579-580).
As so often happens, neither extreme has withstood the test of time,
and more recent summaries (Meinertzhagen, 1930:55-56; Thomson, 1926,
1936:488-489; Wiegold, 1926) have tended to compromise. Many
exceptions to Gätke's extreme conclusion have been detected.
Exceptions to the Brewster-Allen stand have also been discovered,
although work along these lines on the American side has lagged
somewhat. Rowan (1926) has given further evidence on the migration of
certain shorebirds, and some evidence has accrued in relation to
particular species and groups as a result of life-history and banding
studies (see Pitelka, 1946). Authors of major works on migration,
however, have either been preoccupied with other phases of migration
or avoided the issue. In an able study (one of several on related
subjects) of the composition by sex and age of migrant populations in
north Germany, Drost (1935:177) did not go into the question of order
on migration.
One is left with the impression that the whole subject is still
unsettled. While earlier workers sought to reduce the entire matter to
law, the latest disclaim the possibility of generalization. After
summarizing Brewster's and Gätke's opinions, Thomson (1926:79) wrote:
"It would seem, in any event, that no general rule can be laid down."
Meinertzhagen's summation (1930:56-57) still represents fairly well
the status of our knowledge: "Order of sex and age on autumn passage
is very difficult to arrive at, as evidence is conflicting. But, on
the whole, it seems that birds flock together, old and young,
preparatory to moving south, and do in many cases initiate migration
in company.... But once movement is initiated, among birds which do
not habitually fraternise in flocks, adults, and especially males,
will naturally outstrip the less virile females and still less virile
offspring.... The consequence is that any observer at an intermediate
station such as Heligoland is, in noting birds of the year as first
arrivals, has not had an opportunity of noting the flocks of adults
which have passed without alighting. On the other hand, there is very
definite evidence to show that among certain species, adults follow
their offspring on migration. The reason for different behaviour
among different types of birds remains obscure." We regard much of
this as still theory.
[Illustration: FIG. 1. Composition by age and sex as found in one or
more series of each of eight species of birds included in the Topeka
sample. Each separate series is represented by a single histogram, the
histograms for a species being grouped with the earliest series on the
left. Each histogram expresses the numbers of adults (left-hand
column) and immatures (right-hand column) in terms of percentage of
the whole series. Thus the two bars of each couplet add up to 100 per
cent. The hatched portion of each bar represents males, the clear
portion females.]
It would be difficult to imagine a better way of resolving the
problems and uncertainties just reviewed than by the detailed
analysis of large samples of migratory birds killed at random at
various points and times. An analysis of the sample of birds
accidentally killed at Topeka is presented here as an initial step in
this direction. Although the conclusions based on this sample are
tentative and may in time be altered, the data themselves are
definite. If this general type of analysis is repeated again and
again--applied to samples taken on many dates and in many
localities--a mass of hitherto unparalleled evidence for the study of
migration will emerge.
DIFFERENTIAL MIGRATION OF SEX- AND AGE-CLASSES AS SHOWN BY THE TOPEKA
SAMPLE.--Smaller samples have not been treated. Species affording
samples seemingly large enough to justify at least preliminary
analysis were: Catbird, Red-eyed Vireo, Mourning Warbler, Dickcissel,
Nashville Warbler, Orange-crowned Warbler, Yellow-throat, and Lincoln
Sparrow (Fig. 1). For all of these except the Catbird and Dickcissel,
at least two samples from a week or more apart were available for
comparison in an effort to detect trends in migration. Fig. 1 shows
the actual ratios of sex- and age-classes observed in samples of the
species listed above. Each of the last four species provided two
separate samples, of sufficient size to warrant an attempt at
measuring the statistical significance of the observed changes in
adult-immature ratios (Table 2).
TABLE 2.--STATISTICS OF THE RATIOS OF ADULTS TO IMMATURES IN FOUR
SPECIES
=======================================================================
Species | Dates |Total | Number | Differ- |P[3]
| of samples |number| and | ence |
| | |percentage |(in %)[2]|
| | |of adults[1]| |
--------------|--------------------|------|------------|---------|----
Nashville |Oct. 1 (93) | 156 | 45 (.484) | .071 |.36
Warbler |Oct. 5-7 (63) | | 26 (.413) | |
| | | | |
Orange-crowned|Sept. 25-Oct. 1 (19)| 44 | 3 (.158) | .282 |.05
Warbler |Oct. 5-9 (25) | | 11 (.440) | |
| | | | |
Yellow-throat |Oct. 1 (115) | 159 | 62 (.540) | .085 |.34
|Oct. 5-8 (44) | | 20 (.455) | |
| | | | |
Lincoln |Oct. 1-3 (44) | 71 | 27 (.614) | .318 |.01
Sparrow |Oct. 6-10 (27) | | 8 (.296) | |
----------------------------------------------------------------------
Footnotes:
[1] Percentage of immatures equals 1.000 minus percentage of
adults.
[2] Standard error of the difference between ratios was
computed by the formula
[Greek: s]^e = sqrt(P_e Q_e (1/N_1 + 1/N_2))
where P_e equals percentage of adults and Q_e equals
percentage of immatures in the entire sample.
[3] Probability of error; _i. e._, a P of .01 means there is
one chance in 100 that the difference observed does not
represent an actual difference in nature.
Upon the application of statistical methods it soon became evident
that, unless changes in ratio between two samples are marked, large
samples would be required in order to reach conclusions of high
statistical significance in a single study of the present type. In
this case (see Table 2), the Lincoln Sparrow and Orange-crowned
Warbler, though represented by only moderate-sized series, show marked
changes in age composition over the period studied, and the
statistical treatment indicates a high degree of probability that
these changes are real. Assurance that the lesser changes observed in
the Nashville Warbler and Yellow-throat are real, on the other hand,
is much less, even though the samples are larger. Few if any of the
samples here discussed are as large as might be desired. Therefore,
conclusions based upon them (see below) are to be regarded as
tentative. Many other, future, samples will perhaps also be
insufficient in size in themselves. There are, however, statistical
advantages to repetition which will serve to make the repeated
analysis even of small samples significant and valuable.
Certain of the samples not treated statistically show ratios that can
be seen by inspection to be probably significant. For example the
almost complete absence of adults from the three samples of Red-eyed
Vireos (Fig. 1E) cannot be disregarded in view of the size of the
whole sample of the species. The same applies to the high percentage
of adult females and the near absence of adult males in the sample of
the Dickcissel (Fig. 1F). The continuity in direction of changes
observed in the three samples of the Mourning Warbler (Fig. 1G) and
Red-eyed Vireo is likewise probably significant, even though some of
the samples compared are small. It seems to us that the application of
statistical methods to these species should await the accumulation of
more material. For anyone desiring to treat them statistically now,
the data are inherent in this paper.
We have not computed the standard errors of the ratios of sexes within
age groups (except experimentally in a few cases). This can easily be
done, however, and the significance of a given ratio determined, on
the assumption (perhaps sometimes dubiously justifiable) that the
sex-ratio in the species concerned is one:one. Obviously there is no
point in computation of the standard errors of adult-immature ratios
in single samples (such as that of the Dickcissel) until the actual
ratio prevailing in the species in nature at the season in question is
known for comparison with the observed ratio. Our formal statistical
treatment, therefore, has been limited to an examination of the
significance of the _changes_ between adult-immature ratios in samples
of the same species taken a number of days apart.
The samples suggest several patterns of differential migration of sex-
and age-classes. Indeed, the important consideration brought out--in
our opinion not hitherto sufficiently emphasized in literature--seems
to be that in generalizing about adults and immatures, one must be
careful to take sexes into account, and conversely, in generalizing
about males and females, one must consider also age. In other words,
there are really four classes to be considered. This poses additional
problems in analysis and introduces the need for still larger samples
in order to reach significant conclusions. To illustrate: an
adult-immature ratio of 40:20 (N = 60) may be satisfactorily
significant, while within the 40 adults a ratio of 25 males:15 females
may not be. Were the original sample 80:40 (N = 120) with male adults
50 and female adults 30, it is obvious that the significance of the
latter ratio would be greater. The same applies in reverse if the
greater emphasis is placed on sex and the lesser on age. Because of
the moderate size of the samples this problem has been felt in the
present study in respect to sex ratios within age groups, many of
which must at present be regarded as of tentative significance.
In short, what the earlier ornithologists regarded as a simple problem
is in reality a complex one. There are only two patterns in what may
be called the Brewster-Gätke argument: adults first or immatures first
(with of course the further possibility of both at the same time).
Both patterns occur, as is now known, at least to some extent. But
actual patterns, as suggested by our samples, are more complex when
all classes are considered. It will readily be seen that, if adult
males, immature males, adult females, and immature females be regarded
as units, each with certain migratory characteristics, the
combinations of these units in various orders of migratory precedence
are potentially numerous. In fact, of course, they do not behave
strictly as units (or perhaps very rarely so), but our data strongly
indicate that the tendency exists in many cases. This may be stated
another way. The present samples may be reduced to two basic patterns,
fitting the classic early American (adults first) and early European
(immatures first) theories. But, either such simple arrangement is
compounded in some, perhaps in truth in all, instances by differential
migration of the sexes _within_ each age class. This proposition can
also be stated backwards: the samples show differential times of
migration of the sexes, compounded by differential times of migration
of the age groups within each sex. The order in which these matters
are approached depends on what one is trying to find out. Influenced
by the literature, in which most emphasis has been placed on age, we
have approached the problem from that standpoint. The data and figures
here given, however, can be juggled if one wishes to place first
emphasis on the order of sexes in migration.
Bearing in mind what has just been said, particularly in respect to
sizes of samples necessary for significance, let us consider the
patterns of migration suggested by the Topeka sample. These are as
follows:
(1) _An early migration largely composed of adults, giving way later
on to a preponderance of immatures._ Regardless of variations among
them, samples showing this basic pattern are in line with the opinions
of Brewster (1886) and his followers. This pattern is here shown by
the Lincoln Sparrow, Yellow-throat, Nashville Warbler, Catbird (one
sample only), and Red-eyed Vireo (Fig. 1, A, B, C, D, E). The evidence
of these and all other samples would admittedly be more conclusive if
the samples were further apart in time or, better still, were there
more of them. There is evidence that differences in migration of the
sexes, within age classes, influence this pattern, sharply in some
instances. In the later samples of Lincoln Sparrow, Yellow-throat, and
Red-eyed Vireo (Fig. 1, A, B, E) there are relatively fewer males,
both adult and immature, than in the earlier samples and this may be
true also of the Catbird, judging from the single sample. The Red-eyed
Vireo (Fig. 1, E) is characterized by small number, or absence of,
males in each sample but the samples are not significantly different,
and can be regarded as one. Although the samples of the Dickcissel and
Mourning Warbler (Fig. 1, F, G) show a somewhat different over-all
pattern and are discussed further on, they also contain few adult
males. Since these samples are from a period that is near the end of
the migration of Red-eyed Vireos, Mourning Warblers, and Dickcissels,
it may be assumed tentatively that the adult males have already
migrated. Meinertzhagen (1930:56) postulated that in many species
there is an earlier or more rapid migration of adults, particularly
males, and the data for the above species in our sample tend to
support his assumption. But our data suggest in addition that in some
species _immature males_ migrate earlier, or more rapidly, than do
_immature females_, just as adult males precede adult females in some
instances. Within this general pattern (adults first) another
variation is shown by the Nashville Warbler (Fig. 1, C) in which the
later sample of adults is heavily weighted towards males, even though
an increasing over-all proportion of immatures is evidenced. In this
case, and contrary to Meinertzhagen's suggestion, it would seem that
adult females have preceded or outstripped adult males in migration.
(2) _An early preponderance of immatures, followed by a preponderance
of adults._ The several species of birds at Topeka that display this
pattern conform with the conclusions of Gätke and other early Old
World ornithologists that in most species immatures precede adults in
migration. In the present sample two variations of this pattern occur.
(a) In the Dickcissel (Fig. 1, F) and the Mourning Warbler (Fig. 1,
G), immatures decrease more markedly than adults (visible in samples
of Mourning Warbler; inferred in Dickcissel), leaving the adults in
the majority in the closing phase of migration. The distinctive and
interesting feature in each of these two species is the ascendancy in
numbers of adults _despite_ the almost complete disappearance of adult
males. The relative increase of adults is here caused by a retarded
migration of adult females, which linger conspicuously behind all
other classes. Something of this nature was suggested, in theory, by
Dixon (1892:70) who thought that adult females are delayed by
"maternal duties." It was hinted at also by Dwight (1900:127) who
thought that in some species females molted later than males as a
result of prolongation of parental responsibilities. As mentioned
already, there is need for caution in interpreting the present samples
because the Dickcissel is represented only by one sample and two of
the three samples of Mourning Warblers are small. In the case of the
Mourning Warbler, the samples may be regarded as one, nearly lacking
in adult males. The progressive increase of adult females, however,
may be significant; at least there are enough of these to make
division of the birds into three samples enlightening. There is, of
course, some chance that the majority of adult males have not yet
migrated, or are migrating by a different route. This seems unlikely
in both cases. October 1 is late in the migration of the Dickcissel
and it seems that large-scale migration would not occur much later,
and in the case of the Mourning Warbler adult males are rare in all
three samples, extending over a considerable period and reaching late
into the probable migration period of the species. It is interesting
to conjecture just when and where adult male Mourning Warblers do
migrate in autumn. Brewster (1886:16) wrote: "This species arrives at
Cambridge [Massachusetts] about September 12, and during the remainder
of the month is ... abundant.... The adults, however, are so very
uncommon that I have never known them [to] represent more than five
per cent of the total number of individuals. They do not seem to be
more numerous in the earlier flights than towards the close of the
month, and I am very sure that they cannot be found in this locality
before the young begin to appear." While the present samples show an
abundance of adult _females_ of this species (could Brewster have
failed to recognize these as adults?) the whereabouts of the adult
males remains a mystery.
(b) Another variation is displayed by the Orange-crowned Warbler (Fig.
1, H). Here also there is an increase of adults towards the end of
migration, but this increase is marked by a growing percentage not of
females but of males. Locally this species is a late migrant compared
with most others of the Parulidae. Thus the first sample, composed of
birds taken September 25-October 1, may be regarded as fairly early in
the fall migration. Immature birds compose 84.2 per cent of this
sample, there being no adult males at all. By October 5-9 the picture
has changed markedly, the sample being composed of 44 per cent adults
(82 per cent of which, in turn, are males) and 56 per cent immatures.
In view of this trend one can not help suspecting that a still later
sample would show a majority of adults, perhaps nearly all males. This
of course does not necessarily follow; the migration of immatures
could simply be more protracted, and could have commenced earlier,
than that of adults.
Little imagination is required to see how enlightening it might be
could we analyze thoroughly the patterns of all migrating species.
When the detailed facts are available, it seems likely that general
trends will emerge which may be of great significance to the study of
migration in general. A final point which must eventually be clarified
is determination of the extent of variability in the pattern of each
species from year to year and locality to locality.
Once patterns of precedence in migration of different classes are
established, search into the life-histories of the species concerned
may help to explain the peculiarities discovered. In the present case,
for instance, we find a possible clue to the reason for the high
proportion of adult females of the Dickcissel late in migration, as
shown by our sample. Gross (1921:14-15) presented evidence that adult
female Dickcissels molt considerably later than their mates, and we
have independent evidence that individuals of this species are at
times almost flightless when molting the remiges!
Molt in Relation to Migration
GENERAL COMMENT.--The exact relationship between molt and migration
seems not to have been definitely established. The subject has
received cursory attention in the literature and conflicting opinions
have been expressed. Dwight (1900:126-128) believed that molt is
completed or nearly completed before migration in nearly all passerine
species that occur in New York save for certain swallows and
flycatchers. Molt has since been found to precede migration of at
least one of the flycatchers (_Empidonax virescens_) considered by
Dwight to be an exception to this rule (Mengel, 1952). In Great
Britain the subject of molt in migration was considered in some detail
by Rintoul and Baxter (1914) and Ticehurst (1916), who arrived at
conclusions similar to Dwight's. These workers also found certain
swallows to be exceptions to the rule.
The above authors and others have shown that, at least among
passerines, some body molt is frequently found in migrating
individuals but that molt of tail feathers is much less often found
and molt of remiges almost nonexistent. Baxter and Rintoul noted only
four cases of molting wing feathers among hundreds of migrants. Among
the diverse non-passerine orders the picture seems to be more
complicated, as might be expected. We do not, however, comprehend the
reasoning which led Meinertzhagen (1930:56) to summarize: "... on the
whole it can be said that though birds seldom migrate when flight
feathers are in quill, moult in general does not influence migration."
This seems to us an obvious _non sequitur_. Meinertzhagen (_loc.
cit._) went on to say: "Males and females of one species are believed
to moult simultaneously [see, however, Dwight, 1900:127], and there is
no doubt that in some cases the two sexes migrate at slightly
different times, and occasionally prefer different winter quarters.
Birds of the year never moult their quills previous to their first
autumn migration [Consultation of Dwight, 1900, who gives many
examples of this, would have spared Meinertzhagen this error.], and
yet they frequently follow adults on passage and sometimes precede
them. There are no grounds for believing that adults have moulted
their quills before birds of the year are prepared to migrate [but
there are, in many cases; _cf._ Dwight, 1900:127], in the case where
adults precede the latter. Neither is there any evidence to show that
adults have not moulted their quills till after their offspring are
ready for passage, in the case where they follow their offspring. It
does not, therefore, appear that moult is an important factor."
Comments interpolated above show our feeling that this summary is
inadequate and misleading. To us it seems that the extreme rarity in
migration of birds with remiges in molt is strong evidence that molt
_does_ influence at least the time of migration. It is immaterial
whether this molt occurs before or after migration, although in the
majority of cases it seems to take place before. Much more needs to be
known of the migration pattern and molt of each species before
generalizations can safely be made.
Analysis of samples of migrants can show only the presence and nature
or the absence of molt in birds actually migrating. In the present
instance shortage of time and manpower for preserving some and
processing all of the sample resulted in incomplete data being kept on
molt. We include this section to emphasize uncertainties still
prevalent and to stimulate further work.
MOLT IN THE TOPEKA SAMPLE.--Our limited findings coincide with those
of Rintoul and Baxter (1914). Body molt was noted in a number of
individuals and species. When present, this molt almost invariably was
in its final stages. One immature male Rose-breasted Grosbeak (October
1) was in heavy body molt. It is perhaps worthy of mention here that
this grosbeak evidently migrates at times in extensive molt. An adult
male (RMM 1102) taken by Mengel near Henderson, Kentucky, on September
9, 1949, was molting plumage of body, wings, and tail, no feather of
the last being longer than one half inch. This remarkable specimen had
only five primaries on one side and four on the other fully
functional. The outermost on the left and two outermost on the right
were from the previous plumage, not yet dropped; the three innermost
of each wing were new and full-length.
In the present sample molt of remiges was noted in one specimen, an
adult female Indigo Bunting (October 1) with outer primaries sheathed
and with molt in progress in the body plumage. The one (immature)
Yellow-breasted Chat in the sample (October 1) had all of its tail
feathers nearly full-length but in quill, possibly as a result of
accident, and two feathers were being replaced also in the tail of an
immature Clay-colored Sparrow (October 6), which was also in body molt
and had some, juvenal feathers on the belly and flanks.
Body molt near completion was further noted as follows: immature male
Yellow-throated Vireo (October 1), adult male Blue-headed Vireo
(October 1), immature female Leconte Sparrow (October 23), several
Lincoln Sparrows (various dates).
Size Differences according to Sex and Age
LINEAR MEASUREMENTS.--Taxonomists long have recognized in many species
that males differ in size from females. Less attention, until
recently, has been paid to the relative sizes of adult and immature
birds. Many taxonomists, however, seem to have had an uneasy suspicion
that immature birds are "untrustworthy" in comparison with adults, and
immatures have often been excluded from samples when recognizable.
Since, however, there are still relatively few reliably aged specimens
in collections, for the most part only those immature birds
immediately recognizable as such by obvious plumage differences (which
are often present only in juvenal plumage) have been excluded from
series. The majority of birds in first winter plumage so closely
resemble adults that the two ages have been included in series for
measurement. In most passerines these younger birds still bear the
juvenal feathers in wing and tail and are, in size of these important
parts, quite as "untrustworthy" as birds still in juvenal body
plumage. Even if a complete postjuvenal molt occurs we still should
not assume that first winter feathers are as long as adult winter
feathers without first determining that this is so. Although aware of
this problem, systematists until recently seemingly have been more or
less content to disregard it, or forced to do so for practical
reasons. Miller (1941:179) had little choice but to hope that size
differences between adult and immature juncos were unimportant. Behle
(1942:217) wrote of Horned Larks, _Eremophila alpestris_: "... the
plumages of first-year birds and adults seem indistinguishable, though
I have never quite satisfied myself that there are no differences in
lengths of rectrices and remiges." He added, with logic confusing to
us: "Since it is a difficult problem to determine the ages of horned
larks that have passed the postjuvenal molt, this similarity of
plumages is fortunate for the systematist."
In recent years, some workers have analyzed size differences between
adults and immatures. Sibley (1950:115) showed that adult Red-eyed
Towhees (_Pipilo erythrophthalmus_) had notably longer wings and tails
than immatures, and the same was demonstrated in Red Crossbills
(_Loxia curvirostra_) by Tordoff (1952). In work with jays
(_Aphelocoma_), Pitelka (1951:199) found that: "... in comparisons of
dimensions of sex and age groups within a given sample, although
magnitude of difference varies from one character to another, most of
the averages are successively smaller for first-year males and adult
and first-year females." He listed exceptions and concluded:
"Segregation [of sex and age classes] has proved to be of extreme
significance in an interpretation of individual and geographic
variation."
Much along these lines can be learned by examination of large random
samples such as that afforded by the Topeka accident. Although only a
few species in this sample were measured, the results secured seem to
show further the need for segregation of age classes in taxonomic work
with some species.
Figure 2 shows the variation in the lengths of wing and tail in the
Nashville Warbler. It is evident from the figure that in both of these
characters the four sex- and age-classes differ significantly. An
accurate idea of geographic variation in this species could not be
obtained without separating these classes in comparisons. Age classes
in spring and summer, long after the skull is completely ossified, can
be segregated only if differences in plumage can be found. In the
Nashville Warbler, such differences occur in fall (see annotated list)
but these probably are obliterated by the partial prenuptial molt.
These facts emphasize the importance, for taxonomic studies, of aged
specimens collected in late summer or early fall on their breeding
ground and in fresh winter plumage. Figure 3 shows the variation in
length of wing in the Lincoln Sparrow in which age seems to be of much
less importance than in the Nashville Warbler. Males and females of
the Lincoln Sparrow differ significantly in wing-length, but adults
and immatures are of nearly the same size. It would seemingly not be
necessary to separate age classes in studies of geographic variation
in the Lincoln Sparrow. Size data for some other species are given in
the annotated list.
WEIGHTS.--Little seems to have been done to determine the correlation
of weights with sex- and age-classes. Weight may be the best single
index of over-all size and is especially useful to students of the
physiology of migration. Weight, however, is subject to the
considerable variable imposed by fat condition. In large and
comparable series, this variable is probably removed insofar as
comparability of means is concerned, but the high variability of
weight in most cases naturally results in more overlap (less
separability) between populations with means close together than is
found with most linear measurements.
[Illustration: FIG. 2. Statistics of variation in length of wing and
tail in the Nashville Warbler. The solid lines represent the observed
ranges in millimeters. The stippled boxes represent two standard
errors ([Greek: s]m) to each side of the means (vertical lines). The
open boxes represent one standard deviation ([Greek: s]) to each side
of the means.]
[Illustration: FIG. 3. Statistics of variation in length of wing in
the Lincoln Sparrow.]
Figures 4-6 show variation in weight in the samples of Nashville
Warbler, Mourning Warbler, Yellow-throat, Dickcissel, and Lincoln
Sparrow. Each figure is essentially self-explanatory. It will be seen
that in the Nashville Warbler and Lincoln Sparrow, weight is roughly
proportional to wing-length (shown in figs. 2 and 3), giving about
equally good separation of sex- and age-classes in the latter and
poorer separation in the former. Data for these and other species
indicate a generally greater weight of males than of females, but less
difference according to age. In some other species, for example the
Yellow-throat, immatures seem to be a little heavier on the average
than adults. It is unfortunate that wing-lengths are not at present
available for these.
[Illustration: FIG. 4. Statistics of variation in weight in the
Nashville Warbler and Mourning Warbler.]
[Illustration: FIG. 5. Statistics of variation in weight in the
Yellow-throat and Dickcissel.]
[Illustration: FIG. 6. Statistics of variation in weight in the
Lincoln Sparrow.]
These comments on weight suggest an additional factor which may play a
part in rate of migration and which some day may be profitably
studied. Suppose that in some species adults and immatures are nearly
the same in weight but that immatures have shorter wings. In such a
species the immatures are relatively shorter-winged for their weight
than adults. In aerodynamic terms, they would have a higher
"wing-loading." (Wing-loading is the result obtained by dividing area
of effective wing by total weight; it is here assumed that in a single
species wing area is directly proportional to wing length.) This being
the case, immatures with higher "wing-loading" would require more
energy (derived from fat) to fly the same distance as adults, or with
the same amount of fat they would fly a shorter distance. Thus they
might tend to be outstripped in migration by adults starting at the
same time. The reverse, of course, would also be theoretically true,
if adults possessed a higher wing-loading than immatures. Physical
factors such as these rather than the differential "virility"
postulated by Meinertzhagen (1930:56) might account for the arrival of
certain classes of some species on the wintering grounds in advance of
others. There are, of course, many other factors which must be taken
into account before the effect, if any, of the wing-loading factor can
be evaluated. Data for illuminating calculations will become
available, however, with the accumulation of abundant information on
weights, measurements, and migration patterns.
Computations of Longevity and Survival
Tanner (ms., and letter, April 21, 1955) recently devoted considerable
ingenuity to computing by actuarial methods the longevity of the
Oven-bird, using the adult-immature ratio in samples killed at a
ceilometer at Knoxville, Tennessee. Tanner's computations were based
on the simple assumption that
S (survival rate) = Number of adults in population (or sample) / Total
size of population (or sample).
Further application of such techniques may prove desirable and
rewarding. It would seem at present, however, to be a risky procedure,
as it has been abundantly shown (see above) that adults and immatures
often do not migrate at the same times and rates, and the ratios of
adults to immatures in samples of migrants are likely to be far from
representative of the true proportions in the populations concerned.
It should be added that Tanner is perfectly aware of this objection.
Processing of Samples
Thorough processing of large samples of birds killed accidentally is
time-consuming. We were fortunate in having considerable assistance;
even so, all desirable data could not be obtained from the 1090 birds
of the present sample. As aids to others conducting studies of this
kind we should mention a few points which may be of assistance.
Birds should be picked up as soon as possible after death, certainly
by the end of the day after the accident and preferably much sooner.
They should be weighed as soon as possible after collection (weights
decrease rapidly, even under refrigeration), and the weights (in
grams, to one tenth of a gram) written on tags attached to a leg of
each specimen. The sample should then be sorted by species or groups
of species of approximately equal size (to avoid crushing of smaller
birds by larger ones), placed in boxes, paper bags, or better,
air-tight containers clearly marked with date, locality, and other
necessary particulars, and relegated to a deep-freeze compartment. For
all but the smallest samples, such freezing units are indispensable to
complete study. Once frozen, the birds may be selected for study at
leisure, but time is still important, as, even when frozen, gonads may
eventually deteriorate, and birds eventually become desiccated which
is a disadvantage if skins are to be made.
In the cases of large kills, or limited manpower, or both, it may be
impossible to process all birds, however desirable this might be. If
possible, however, all should be collected, identified, the numbers
and species recorded, and rarities saved. Further, partial analysis,
or more properly, complete analysis of a partial sample, can be made.
Analyses which for any reason require randomness of sample pose a
special problem. We think that in very large kills the best way to
solve this problem is probably to make one or more transects across
the area where dead birds are found. These transects should cross both
the areas of greatest and least density (to allow for fast and slow
flying species). Their width may be adjusted to give the desired
number of birds, that is, the number that can be adequately processed.
Another alternative would be to decide to study certain abundant
species and pick up all of these. There are other possibilities, but
in any event the method of sampling should be thoroughly described
wherever all birds have not been processed.
Summary
The foregoing paper discusses accidents in which large numbers of
night-migrating birds are killed. A brief historical review of
ornithological interest in such occurrences is given, and the types of
data provided by the accidents are listed and discussed. In
particular, recent accidents occurring in early October, 1954, through
much of eastern United States are cited, and detailed analysis is
presented of a sample of 1090 birds killed one mile west of Topeka,
Shawnee County, Kansas, between September 25 and October 23, 1954.
At Topeka during the period mentioned, 1090 birds representing 61
species were collected and were processed at the University of Kansas.
For all specimens, weight, sex, age, and fat condition were recorded,
and certain species were measured as well. Some notes on molt were
taken. A total of 193 birds was preserved as study skins, and 49 as
skeletons. Comments on weight, size, sex, age, subspecific identity,
and status in Kansas are presented in an annotated list.
Randomness of this and other similar samples is discussed. A
theoretical computation is given for several nights of the numbers of
migrants passing through a plane one mile in width, from 450 to 950
feet above ground level, and oriented to face the arriving migrants.
The computed totals give some idea of the tremendous volume of
nocturnal migration under some conditions. Potentialities of further
study of such events are discussed and a comparison is made with lunar
observations.
Differential migration of sex- and age-groups as shown by the larger
samples taken at Topeka (Catbird, Red-eyed Vireo, Nashville Warbler,
Yellow-throat, Mourning Warbler, Dickcissel, Lincoln Sparrow) is
discussed and the history of theories on this subject reviewed. It is
shown that age and sex must both be taken into account in studies of
differential migration. Several patterns of migration are displayed by
the species analyzed, adults migrating earlier than immatures in some
instances, young earlier than adults in others, but all seemingly
being complicated to varying degrees by differential migration of
sexes within age groups. It is suggested that explanations of these
patterns may be sought in the life histories of the species involved.
Molt in relation to migration is discussed briefly, and it is held
that there is an important relationship between molt and time of
migration. Specimens noted to be in molt are listed.
Size differences, in wing length, tail length, and weight are
discussed in relation to sex and age, and variation in one or more of
these characters is analyzed as found in the Topeka samples of
Nashville Warbler, Mourning Warbler, Yellow-throat, Dickcissel, and
Lincoln Sparrow. It seems that in some instances significant size
differences prevail between adults and immatures and that these age
classes should be separated in taxonomic work with species where
differences in size are known to exist. When the facts are not known
they should be determined, and the large samples collected in
accidents to nocturnal migrants present excellent opportunities for
ascertaining the facts.
Other uses of material obtained in large migration accidents are
discussed, such as computations of longevity and the problems of
processing large, accidentally-killed samples. Care should be taken to
select samples as nearly random as possible when all birds cannot be
processed.
Repeated and thorough analysis of accidental kills should provide a
mass of valuable data bearing on many questions and problems which
have thus far been little studied.
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_Transmitted June 30, 1955._
Transcriber's Notes:
Words surrounded by _ are italicized.
Small capitals are presented as all capitals in this e-text.
Obvious printer's errors have been repaired, other inconsistent
spellings have been kept, for example inconsistent use of hyphen (e.g.
"age-classes" and "age classes") and diacritical mark (e.g. "Zool."
and "Zoöl.").
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