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.
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 (σm) to each side of the means (vertical lines). The open boxes represent one standard deviation (σ) to each side of the means.
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.
Fig. 4. Statistics of variation in weight in the Nashville Warbler and Mourning Warbler.
Fig. 5. Statistics of variation in weight in the Yellow-throat and Dickcissel.
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.