Numbers, foraging and refuelling of passerine migrants at a stopover site in the western Sahara: diverse strategies to cross a desert

Abstract

Twice a year, songbirds breeding in the Western Palaearctic cross the largest desert of the world, the Sahara, to reach their African winter quarters. Recently, a radar study quantified this migration and demonstrated that almost all passerines cross the western Sahara with an intermittent strategy, i.e. they fly during the night and rest during the day. Before crossing the desert, most passerines accumulate fat stores because they will not find appropriate resting sites for feeding in the Sahara. However, it has also been reported that birds use the vegetation around oases for refuelling. Since birds resting at oases had smaller fat deposits than birds resting in the open desert, it was hypothesised that mainly lean birds or fall-outs use the oases for feeding. In this study, we investigated which species or individuals use oases in the western Sahara during spring migration and how they use them. We demonstrate that a minority of species adapted to dry vegetation may cross the Sahara with low energy stores and intermittent refuelling in vegetation patches. These birds avoid the costs of transporting large energy stores, in contrast to most other passerine migrants which fuel up before crossing the Sahara and adopt an intermittent strategy without refuelling. The birds which rely on refuelling at oases probably often have a slow refuelling rate and may even run the risk of not finding appropriate habitats. The available studies reveal that birds use a wide variety of strategies to cross the Sahara. The particular strategy adopted depends on the species, and is modulated according to weather conditions aloft at the time, existing energy stores, the availability of stopover sites, and the suitability (food availability, competitors) of stopover sites.