Orientation of passerine trans-Sahara migrants: the directional shift (‘Zugknick’) reconsidered for free-flying birds
Highlights
► Flight directions of European passerines along the western flyway to Africa. ► Testing orientation mechanisms based on field observations. ► Gradual shift of migratory direction until the end of Sahara crossing. ► Change of migratory direction after the Sahara crossing. ► Results from laboratory orientation experiments reconsidered.
Section snippets
Capture Data from Mauritania
We used capture data gathered in 2003 to determine the timing of the migratory passage of pied flycatchers and garden warblers in southern Mauritania. Birds were captured at different sites in the vicinity of the radar sites (Fig. 1) with standard mist-nets between 13 August and 26 October (Salewski et al. 2009). We calculated standardized daily trapping numbers by accounting for trapping effort. Out of a total of 10 573 individuals (recaptures of the same season at the same site were
Results
Along the latitudinal range of the study sites, all theoretical models predict a gradual shift from southwest towards more southerly directions (Fig. 2). The shift caused by a constant magnetic north route is less than 10° from the constant geographical direction. Migrants orienting along a magnetoclinic route would make the strongest shift, ending up with a due south direction at the end of the Sahara crossing. The great circle route and the sun compass route predict directions in between the
Discussion
To investigate flight behaviour with respect to orientation, we would prefer to look at long tracks of individual birds, rather than at the general flight directions at different sites along the migratory route. However, our results are based on representative samples of small passerines migrating along the western Palaearctic–African flyway including the two very common species, garden warbler and pied flycatcher. Because flight directions were highly concentrated at almost all sites, there is
Acknowledgments
We are deeply indebted to colleagues, volunteers and students who spent hundreds of hours collecting radar data in various projects around Europe and Africa. Through all the years Thomas Steuri maintained and improved our bird radar system; he and Erich Bächler developed the specific software to analyse the data. We acknowledge the ringing team that contributed new data on the migratory phenology in Mauritania, particularly Marc Herremans, Fränzi and Pius Korner-Nievergelt, Heiko Schmaljohann
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