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Résumé :
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During autumn migration stopovers on the offshore island Helgoland (North Sea), Northern Wheatears Oenanthe oenanthe varied behaviour with respect to foraging conditions. In both types of habitat studied (wrack beds along the beach, grassland), periods of improved foraging conditions (observed as increased rates of successful pecks) occurred due to the arrival of new algae at the beach (with very profitable kelp fly larvae instead of less profitable imagines as food supply) and the cutting of a patch of grassland (with presumably better accessibility of ground-dwelling invertebrates), respectively. Compared to periods with lower intake rates, a higher percentage of Northern Wheatears stayed in the beach habitat (instead of departing and continuing migration), stopovers were longer there and coincided with a higher rate of aggressive encounters and territoriality in both habitat types. For birds individually marked on the beach, in a multifactorial analysis only the rate of successful pecks and the evening wind conditions (tail wind component) explained the decision about settling or departing. These results show that migrating passerines are flexible in their stopover behaviour after landing, with skipping unfavourable habitats if reserves allow to continue migration. The high variability of shoreline habitats would have implications for the organisation of migration (sudden refuelling in case of favourable food supply) if passerines concentrated on such habitats.
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