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."