Flight initiation of nocturnal passerine migrants in relation to celestial orientation conditions at twilight

The time of departure of nocturnal passerine migrants was studied by radiotelemetry and tracking radar in South Sweden in spring and autumn. Our objective was to analyse the time of flight initiation in relation to celestial orientation cues during the twilight period at dusk. The telemetry and radar results were compared with data on nocturnal flight departures reported in the literature. There was a considerable variation in time of flight initiation throughout the twilight period. A large fraction of the birds (mostly thrushes) departed mainly during the civil and nautical twilight periods, with a sun elevation between 0 degrees and -12 degrees while several migrants did not leave until much later in the night. This variation in time of departures speaks against the hypothesis that departure time is connected with a critical skylight situation with respect to the visual accessibility of celestial orientation cues. Characteristic times of departure, as reflected by the associated elevations of the sun below the horizon, seem to differ between different passerine species, latitudes and times of migratory season.