Timing of nocturnal passerine migration in Arctic light conditions

Abstract

The nocturnal migration of many passerines starts after sunset and reaches peak intensity during the dark hours of the night. Birds destined for high Arctic breeding grounds encounter a special situation, as they will experience continuous daylight when reaching the high latitudes during the final part of spring migration. How does this affect the pattern of nocturnal migration? We consider three alternative hypotheses; that the period of nocturnal flight activity may become compressed, remain unchanged or become disorganized under Arctic light conditions. We tracked migrating birds by radar north of the Arctic Circle (at Abisko, 68°21′N, 18°49′E, in Swedish Lapland) and show that the pattern during the night, with a migration peak around midnight, persisted even in continuous daylight when the sun remained above the horizon throughout the 24 h of the day. The flight intensity peak under continuous daylight in spring (midnight sun) was very similar to the corresponding peak in autumn, when the migration took place under twilight conditions. The duration of the flight period under continuous daylight in spring lasted 8–10 h and did not seem to be compressed. We hypothesize that the flight period under midnight sun conditions may in fact be more protracted than during short nights, because of release from twilight cues that tend to synchronize initiation and termination of migratory flights. These cues will thus capture and confine the flight period. The results of this provisional study suggest interesting dynamics in timing of nocturnal migratory flights under seasonally and latitudinally changing day length conditions, which will need detailed documentation by future studies of migration intensity at high-latitude sites.