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.