Fuelling for the Sahara crossing: variation in site use and the onset and rate of spring mass gain by 38 Palearctic migrants in the western Sahel

Abstract

To African–Palearctic migratory birds that spend the non-breeding season in sub-Saharan Africa, the Sahara desert is an ecological barrier that must be crossed using large energy reserves stored at stopover sites either side. Given the coincidence of spring migration with the height of the dry season in the west African Sahel, spring stopover conditions may be a limiting factor that regulates populations. By determining where, when and how reserves are accumulated prior to crossing the desert, we can identify possible population regulation mechanisms in Palearctic migrants and effectively target stopover sites for protection. We present data on the non-breeding season use of Djoudj National Park in the western Sahel by 38 Palearctic migrants based on constant effort mist-netting data. Twenty-four species showed evidence of using Djoudj either throughout the non-breeding period or during spring migration. The remaining 14 species were largely absent from Djoudj and presumably accumulated energy reserves for spring migration south of the Sahel. Species preparing for spring migration at Djoudj began increasing their energy reserves in January or February. Energy reserves were generally accumulated at slow rates during long stopovers, and in all species examined, predicted take-off fuel loads were sufficient to cross the Sahara desert under certain conditions. Three species, Common Redstart Phoenicurus phoenicurus, Common Whitethroat Sylvia communis and Common Blackcap Sylvia atricapilla, achieved faster rates and had concurrently lower stopover durations. We predict that vulnerability to fluctuating environmental conditions in the Sahel region is greater in species that initiate fuelling later in the spring and in species with short stopover durations.