Birds that migrate at night enter a state of sleepless mania and gorge on foods by day, behaviors mediated by their biological clocks. Migration requires dramatic seasonal changes in behavior and physiology, and these changes must be timed appropriately for successful migration. In late summer after nestlings fledge, birds begin to molt, replacing their ratty old feathers with sleek new ones. They also begin to gorge themselves. Migration likely brings to mind the familiar sight of geese flying overhead in their iconic V formation, honking stridently as they fly toward their faraway goal. But the migration of many birds is a rarely observed phenomenon. Most passerine birds, a group that includes songbirds and groups taxonomically related to them, migrate at night. Although nocturnal migration is common among passerine species, most of these birds are strictly diurnal during non-migratory periods.
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