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The bi-annual crossing of the Sahara desert is a considerable energetic challenge faced by approximately a quarter of Europe's total bird population, as they migrate to and from sub-Saharan non-breeding grounds. Where, when and how migratory birds prepare for the crossing remains to be defined in many species, especially in sub-Saharan Africa. Here, we describe the migration and fuelling strategies employed by the Grasshopper Warbler Locustella naevia during the northern autumn and spring to cross the western Sahara using ringing datasets from Portugal and Senegal. Body masses in recaptured birds combined with flight ranges suggest that <10% of birds in Portugal could have reached sub-Saharan Africa without refuelling. Estimated rates of mass change [up to 3.4% of lean body mass (LBM)/day] and stopover durations (mean 8 days) also suggest that the average bird in Portugal required further stopovers, and point to the strategic importance of northwest Africa during autumn migration. In sub-Saharan Africa, Grasshopper Warblers began to leave Senegal as early as mid-January in order to spend up to 2 months at unknown sites in North Africa. Fuelling for the northward journey across the Sahara was characterised by a slow fuelling rate (1% LBM/day) and long duration relative to Portugal (19 days). The constraining factor on fuelling rates in Senegal is hypothesised to be low resource availability associated with the Sahelian dry season. These resources appear to vary annually with Sahelian rainfall, resulting in variable fuel loads and fuelling rates and potentially leading to fuelling shortfalls in years of low rainfall. [ABSTRACT FROM AUTHOR]