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Before presenting some of the present theories concerning the origin of bird migration, it seems well to consider briefly the ends that are served by this annual round trip between breeding grounds and winter quarters. It is apparent that the migratory habit enables a species to enjoy the summers of northern latitudes while avoiding the severity of the winters. In other words, migration makes it possible for some species to inhabit two different areas during the seasons when each presents favorable conditions. In the performance of its reproductive duties, every pair of birds requires a certain amount of territory, the extent of which varies greatly in different species. Generally, however, this territory must be large enough to provide adequate food, not only for the parent birds, but also for the lusty appetites that come into being with the hatching of the eggs. Thus, if all birds were to remain constantly either in tropical or in temperate regions, there would be intolerable overcrowding during the breeding season. By the spring withdrawal to regions uninhabitable earlier in the year, the migrants are assured of adequate space and ample food upon their arrival in the winter-freed North, and it may be assumed that the nonmigratory species that are resident in the South are also benefited by the departure of the migrants.