Autumn Migration Strategy and Stop-Over Sites of the Globally Threatened Aquatic Warbler Acrocephalus paludicola on the Atlantic Flyway Migration Route

Abstract. Knowledge about migration strategy and availability of stop-over sites of globally threatened migratory birds is crucial for the implementation of efficient conservation plan. In this study, we investigated the refuelling strategy of the globally threatened Aquatic Warbler Acrocephalus paludicola at stop-over sites during the autumn migration in West Europe and Africa (40 and 72 sites, respectively, documented in published works about stop-over ecology and sites). Based on fuel reserves of captured birds we calculated potential maximal range of non-refuelling flight from stop-over sites located on the Atlantic flyway migration route. We tried to verify the hypothesis that Aquatic Warblers are able to cross the Sahara during one non-stop flight. We found that mean range of flights was affected significantly by the stopover geographic location but not by age of the birds and stop-over habitat. Individuals staging in France are able to cover smaller distances (mean 612.7 km) compared to individuals staging in Portugal and Spain (mean 857.5 km), i.e. closer to two important geographical barriers (the Gibraltar Strait and the Sahara desert). However, maximal ranges of the heaviest individuals were longer (2370 km in France and 1998 km in Portugal). Our study suggests that Aquatic Warblers are generally not able to cross the Sahara during one non-stop flight. Only some individuals with high fat loads starting from stop-overs in Portugal may be able to cross the Sahara without refuelling. However, due to diel air temperature pattern in the desert they need to stop at least once to rest and wait for nighttime drop in temperature to continue migration. Within the Sahara there are many small patches of suitable habitats which can facilitate resting and maybe even refuelling. Observed and predicted reduction of available area of stop-over habitats may have serious negative consequences for population dynamics of many land-bird species, including the globally threatened Aquatic Warbler.