Orientation of passerine trans-Sahara migrants: the directional shift (‘Zugknick’) reconsidered for free-flying birds

As most long-distance migrants leave their breeding ranges in Western Europe towards the southwest, they must change their migratory directions somewhere to reach their nonbreeding areas in Africa. Laboratory experiments have been the basis to suggest an endogenously controlled, abrupt directional shift in the south of the Iberian Peninsula, the so-called ‘Zugknick’. We compared own radar data gathered in Europe and the Western Sahara with this established theory. Radar tracks were filtered according to the wing beat pattern and migratory passage of garden warblers, Sylvia borin, and pied flycatchers, Ficedula hypoleuca, the species used in the former experiments. The flight directions were highly concentrated and the mean direction in the Western Sahara was only slightly more south than the southwesterly directions in central and southwestern Europe. At observation sites close to the southern edge of the Sahara, mean directions were south to east-southeast. We therefore conclude that an endogenously predetermined directional shift occurs towards the end of the Sahara crossing according to physiological or environmental triggers leading the birds towards their wintering ranges in the southeast.