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.
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 of samples | Total number | Number and percentage of adults[1] | Difference (in %)[2] | P[3] |
| Nashville Warbler | Oct. 1 (93) Oct. 5-7 (63) | 156 | 45 (.484) 26 (.413) | .071 | .36 |
| Orange-crowned Warbler | Sept. 25-Oct. 1 (19) Oct. 5-9 (25) | 44 | 3 (.158) 11 (.440) | .282 | .05 |
| Yellow-throat | Oct. 1 (115) Oct. 5-8 (44) | 159 | 62 (.540) 20 (.455) | .085 | .34 |
| Lincoln Sparrow | Oct. 1-3 (44) Oct. 6-10 (27) | 71 | 27 (.614) 8 (.296) | .318 | .01 |
[1] Percentage of immatures equals 1.000 minus percentage of adults.
[2] Standard error of the difference between ratios was computed by the formula
| σe = | √ | Pe Qe ( | 1 | + | 1 | ) | , |
| N1 | N2 |
where Pe equals percentage of adults and Qe 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!