Loop migration, induced by seasonally different flyway use, in Northern European Barn Swallows

Seasonal variation in migratory routes seems to be a common trait among many Afro-Palearctic migrants, but there are only few examples of species or populations which shift between entirely different flyways in autumn and spring. To identify non-breeding regions and seasonal differences in migration strategies we tracked Barn Swallows (Hirundo rustica) from a Baltic breeding population by light-level geolocators. Using novel analytical tools, we provide the first full annual tracks for European swallows. The main non-breeding residency sites of the tracked individuals were located in Southern Africa and all birds followed a distinct counterclockwise loop migration pattern shifting from the central Afro-Palearctic flyway in autumn to the eastern Afro-Palearctic flyway in spring. Despite the elongated detour through the Arabian Peninsula and the Caucasus Mountains, spring migration was faster than autumn migration in all of the tracked males and one out of two females, implying favorable conditions en route that allow for more efficient and faster traveling.