Robert Morrow Mengel

“ Also, the sample is useful in suggesting the actual abundance of species which are furtive and/or difficult to identify under normal field conditions, for example, the Mourning Warbler and Philadelphia Vireo. It is obvious that the sample should reflect the true relative abundance at one place and time of any two species with equal tendency to migrate by night and equal tendency to strike the tower. Since the facts in regard to both tendencies are at present unknown for most species, we think that interspecific comparisons should be avoided or approached with extreme caution. ”

“ 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. ”

“ While conservation-minded individuals have been concerned with the tremendous mortality involved in these various events, the ill wind blows some good in that, properly used, the data provided by such accidents can shed light on many obscure aspects of bird migration. Each accidental kill of birds affords a cross-section, approaching in variable degree a random sample, of the migrants passing a given point on a given date. ”

“ 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 [Pg 27] 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. ”

“ 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. ”

“ 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. ”

“ 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 data from birds migrating at night should be especially useful because these migrants all have relatively empty crops and stomachs, thus reducing variability. Not all birds were suitable for weighing and measuring, for a variety of reasons. This accounts for discrepancies in totals between Table 1 and the annotated list.
All passerine species were aged by noting the degree of ossification of the skull. In no case, of the more than a thousand passerines aged by examination of the skull, did we find difficulty in determining whether an individual was a bird of the year or an adult. ”

“ 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. ”

“ 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. ”

“ Nashville Warbler.—More Nashville Warblers were picked up at Topeka than any other species and they are discussed in detail elsewhere in this report. The four sex- and age-classes can be identified with fair accuracy on plumage characteristics alone. Adult males have a large amount of reddish-brown in the crown, not completely veiled by the gray tips of the crown feathers. Immature males have a smaller but distinct crown patch, usually completely veiled. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Furthermore, our sample (including 11 males) from October 7 (all sex- and age-classes) averaged 8.81 grams; the sample of 13 (including only 4 males) from October 1 averaged 9.1 grams. If one can assume, for any one species, that individuals undertake nocturnal migration only when they are physiologically ready, and this includes a certain amount of fat as a fuel source (Wolfson, 1954) , then this further assumption seems justified: birds killed in migration with little fat must have flown longer or farther or both than birds killed with more fat. ”

“ 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 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. ”

“ If our figures are inaccurate by as much as 50 per cent in either direction, which seems unlikely to us, they still give some idea of the tremendous volume of nocturnal migration under some conditions.
It may be more meaningful to compute numbers of migrants by species. This can be done readily by making appropriate substitutions from Table 1 in the equation given above. For example, on the night of September 30-October 1, approximately 147,000 Nashville Warblers passed through the mile-long plane and on the same night, 100,000 Mourning Warblers and 14,000 Philadelphia Vireos. ”

“ 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. ”

“ All males, compared with females, are grayer on the sides of the head, have a more nearly white eye-ring, and show clearer yellow on the throat. Adult females differ from immature females in that they more often have a trace of rufous in the crown and tend to be brighter below than the immatures. Of 177 specimens, 20 were very fat, 108 were fat, 46 were moderately fat, and 3 had little fat. ”

“ The scarcity of birds under the guy wires some distance from the tower at Topeka and at the radio tower at Lawrence causes us to think that most of the birds were flying more than 450 feet above the ground. On this basis, we have computed numbers of migrants passing through a plane one mile long and 500 feet high (2,640,000 square feet) , intersecting the assumed path of migration at right angles. Vertically, the theoretical plane begins at 450 feet above ground and has its top edge at 950 feet. ”

“ An idea of the potentialities of the large recent kills in the United States may be obtained when it is recalled that in the 18 years of Barrington's work, which embodied some 1000 reports from lighthouse keepers, Barrington obtained for study only about 2000 specimens, many of these consisting of wings and feet only (Barrington's paper not seen in original; see J. A. Allen, 1901:205) . More recently Dobben and Bruyns (1939) have analyzed the age and sex classes of some birds killed at lighthouses in Holland. ”

“ 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. ”

“ 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. ”

“ 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.... ”

“ 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. ”

“ In spite of the great potential of this kind of material, the majority of ornithologists with access to such data have contented themselves with listing the species and sometimes the numbers of birds killed. A few have gone further. James T. Tanner (unpublished) attempted to compute the longevity of the Ovenbird (Seiurus aurocapillus) by analysis of ceilometer-killed birds at Knoxville, Tennessee (see below) . ”

“ 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. ”

“ Migrants of this pale, clay-colored subspecies should be abundant in Kansas, and all of the specimens in the present sample are referable to it.
Ammodramus savannarum perpallidus. Grasshopper Sparrow.—Weights: 3 adult males, 16.4, 17.6, 20.6, moderately fat, fat, fat; 5 immature males, mean 18.1 (16.0-20.2) , little fat to fat; 5 adult females, mean 17.9 (16.8-18.9) , moderately fat to very fat; 5 immature females, mean 18.1 (16.8-20.6) , fat to very fat. ”