Page:The Migration of Birds - Thomas A Coward - 1912.pdf/35

Rh The earlier students of migration insisted that temperature was the sole cause of change of abode: that the northern lands became unsuitable through their falling temperature, and that the birds deserted them for warmer climes, returning when the lands they wintered in became too hot. As a variant of this notion, which cannot be lightly cast aside, the suggestion was mooted that it was not cold but the lack of food during the cold months which drove them south, and that in the Tropics, where at one time it was thought that all migratory birds wintered, food was scarce during the months of extreme heat. Dr Wallace went further and stated that the incentive to northern migration was the inability to find sufficient soft bodied insects suitable for the nestlings in the Tropics during summer (54). Yet there are birds which do find food enough for their young, and some of them are insect caters.

Seebohm, arguing with reason that the first home of the Charadriidae, was the Polar Basin (44), suggests that the desire for light originated the idea or the action, and though this was only applied by him to Arctic birds, others have striven to show that the longer hours of daylight would he an advantage to all birds, even though the difference of dark and light in the zone retired from and in that arrived at might be inconsiderable (41). Against this must be taken into consideration the fact that