Spring passerine migrants stopping over in the Sahara are not fall-outs

The strategy of migrants crossing the Sahara desert has been the subject of debate, but recent evidence from radar studies has confirmed that most passerines use an intermittent migration strategy. The latter has also been suggested from previous studies in oases during autumn migration. It was found that migrants with relatively high fuel loads rest in the desert during daytime and continue migration during the following night, whereas lean migrants stopover in oases for several days to refuel. However, data from the Sahara are scarce for spring migration. We captured passerine migrants near B?r Amrane (22°47′N, 8°43′W) in the plain desert of Mauritania for 3 weeks during spring migration in 2004. We estimated flight ranges of 85 passerines stopping over in the desert to test whether they carried sufficient fuel loads to accomplish migration across the Sahara successfully. High fat loads of the majority of birds indicated that they were neither “fall-outs” nor too weak to accomplish migration successfully. The flight range estimates, based on mean flight speeds derived from radar measurements (59 km/h), revealed that 85% of all birds were able to reach the northern fringe of the desert with an intermittent migration strategy. Furthermore, birds stopping over in an oasis (Ouadane, 370 km to the southwest of B?r Amrane) did not carry consistently lower fuel loads compared to the migrants captured in the desert.