Zugunruhe is the German term for migratory restlessness, the restless activity often seen in birds before seasonal departure.
The behaviour was identified in controlled studies in which captive birds became unusually active at the time of year when their wild counterparts would migrate. Birds may flutter, hop, and orient toward the direction of their normal route, especially during the night.
Zugunruhe helped scientists show that migration is not merely a reaction to immediate weather. It is influenced by an internal annual timing system, or circannual rhythm, that interacts with daylight, food availability, temperature, and other environmental signals. The timing and direction of the behaviour can differ between populations.
The term is especially associated with experiments using migratory songbirds, including species studied in orientation cages. It should not be confused with torpor, which is a state of reduced activity and metabolism, or with hibernation. Zugunruhe is an active, seasonal preparation for movement and has become an important concept in migration biology.