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.