Flight‐range estimates for small trans‐Sahara migrants

Arguments in support of the non‐stop and the intermittent strategies for crossing the Sahara have been based on data on the fat reserves of birds before the crossing and of birds grounded in the desert. In this paper, flight‐range estimates were calculated and the necessary assumptions about air speed, energy input during flight, and energy equivalent of body reserves were evaluated. As examples, Willow Warblers Phylloscopus trochilus and Garden Warblers Sylvia borin were investigated during autumn migration from two study sites north of the Sahara and two study sites in the desert. In still air, the flight‐range for both species at all study sites was too short to reach the Sahel zone without refuelling. It is concluded that birds depend on tailwinds for a successful crossing, independent of a non‐stop or an intermittent migratory strategy, and that weather conditions in autumn allow them to rely on tailwinds.