Many migrating species occupy totally different areas while some merely expand their range to the north in summer, with some individuals finding nesting space to the north and others remaining stationary. Eggs and nestlings cannot fly. Parents must be sure there is sufficient territory around the nest to support their voracious family. Predation is diminished by the fact that a predator species depending on a particular bird species for food cannot evolve since the bird species is not available as a food source full time. That's how the song goes and that's how the birds go.
Phonology is the study of biological rhythms.
Periodic activity such as flowering, reproduction, and migration all fall within this class of study. Much of migration activity in birds is controlled by an internal clock operating on a circannual rhythm. Each year, at a certain time, their biological clocks signal that it's time to fly northward to breed. Photoperiodicity may affect migration in some cases, when the birds are in the temperate zones where these ratios change. In the tropics, however, photoperiods don't have much effect.