Conservation through connectivity: can isotopic gradients in Africa reveal winter quarters of a migratory bird?

Conservation of migratory wildlife requires knowledge of migratory connectivity between breeding and non-breeding locations. Stable isotopes in combination with geographical isotopic patterns (isoscapes) can provide inferences about migratory connectivity. This study examines whether such an approach can be used to infer wintering areas in sub-Saharan Africa, where we lack such knowledge for many species, but where this method has not been used widely. We measured δ ²H, δ ¹³C and δ ¹⁵N in winter-grown feathers of a breeding Swiss and Spanish population of European hoopoe Upupa epops—a typical Palaearctic-Afrotropical migrant. δ ²H values predicted that ~70 % of the hoopoes spent the non-breeding season in the western portion of their potential winter range. This was corroborated by a shallow east–west gradient in feather-δ ²H values of museum specimens from known African origin across the potential winter range and by the recovery of Swiss hoopoes marked with geolocators. Hoopoes categorized as from eastern versus western regions of the wintering range were further delineated spatially using feather δ ¹³C and δ ¹⁵N. δ ¹⁵N showed no trend, whereas adults were more enriched in ¹³C in the western portion of the range, with eastern adults being in addition more depleted in ¹³C than eastern juveniles. This suggests that eastern juveniles may have occupied more xeric habitats than sympatric adults. We demonstrated that stable isotopes, especially δ ²H, could only very roughly delineate the winter distribution of a trans-Saharan Palaearctic migrant restricted primarily to the Sahelian and savanna belt south of the Sahara. Further refinements of precipitation isoscapes for Africa as well the development of isoscapes for δ ¹³C and δ ¹⁵N may improve assignment of this and other migrants.