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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