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,

68210N, 18490E, 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.