There and Back Again - nocturnal migratory behaviour of birds during spring and autumn

Sammanfattning: This dissertation explores the migratory behaviour of nocturnally migrating passerines and common swifts (Apus apus). Birds passing Lund (southern Sweden) and Abisko (northernmost Scandinavia) where sampled using tracking radar. The aim has been to investigate potential diverging strategies between spring and autumn migration. We have found that both passerines and swifts compensate for wind drift, the swifts more so than passerines. Such compensation for displacement by winds where found both during spring and autumn migration. At Abisko the birds exhibited an exaggerated scatter of flight directions, particularly during spring migration. We have suggested that this behavior reflects the final approach orientation and navigation of the migrants. Additionally, both swifts and passerines flew at higher air speeds (speed relative the air) in spring than in autumn, indicating a selection for time minimization on migration to the breeding grounds. However, we saw no indications of birds favouring tailwind conditions for their migratory flights.