Two major autumn migration strategies of Willow Warblers Phylloscopus trochilus in the eastern Mediterranean Basin

Willow Warblers Phylloscopus trochilus use two major migration strategies in the eastern Mediterranean Basin to overcome the sea barrier. The first is to cross the Mediterranean Sea after intensive fat refuelling and land on the shores of Egypt or Libya. The second strategy traverses the sea barrier using shortcuts or bypassing it through Israel. We show that Willow Warblers bypassing the Mediterranean Sea through Israel arrive with low fat reserves and low average body mass. Their body mass increased in suitable stopover sites. There was no significant gradient of body mass along a north-south axis in Israel. Our results indicate that Willow Warblers passing through Israel have a different endogenous plan of fat accumulation than the birds that cross the sea. The first migration strategy may allow Willow Warblers a staged crossing of the Sahara Desert without foraging, while the second implies an intermittent crossing with foraging. We suggest that Willow Warblers may cross the east side of the Sahara Desert in short flights with foraging during stopovers.