On nights when large numbers of birds struck the 950 foot Topeka tower, only a few struck a 500 foot radio tower, also lighted with red lights, at Lawrence, 24 miles east, under similar weather conditions. Most of the birds found at Topeka were fairly close to the base of the tower, indicating that they struck the tower itself or that they were flying high enough to strike guy wires only fairly close to the tower. 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. The solid (discounting spaces between girders, etc.) cross-sectional area of the tower intersecting this plane was computed by actual measurement to be 1685 square feet. On the night of September 30-October 1, 585 birds were killed. By computation (585/1685 = X/2,640,000), approximately 916,000 birds passed through the mile-long plane that night. On each of the nights of October 5-6 and October 6-7, approximately 230,000 birds passed through this plane. By comparison, Lowery (1951:436) recorded maximum station densities in one night in spring of 63,600 birds at Tampico, Mexico, and 51,600 at Lawrence, Kansas, as determined by moon-watching. Lowery's figures refer to numbers of birds crossing any part of a circle one mile in diameter and are roughly comparable to ours if, as we think, most of the birds at Topeka were flying at altitudes between 450 and 950 feet above the ground.
It must be realized that these figures are only approximations. One variable ignored is the frontal extent (or area, viewed from the front, subject to damage by striking an obstruction) of the birds themselves. Since practically all birds killed showed head or trunk injuries, rather than a high proportion with only broken wings, we chose to disregard frontal extent of the birds in our calculations. 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. Neither of the last two species would be judged to be abundant migrants in autumn in eastern Kansas by ordinary field observations; the television tower sample, however, indicates that these as well as other species must often be overlooked when they do stop in Kansas.
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.