Bird migration studies have been given added impetus recently thanks to the miniaturisation of tracking devices. However, tracking methodologies have remained impractical for the smallest pelagic species and so important gaps in knowledge still exist. In the case of the European Storm-petrel Hydrobates pelagicus, while Atlantic populations are thought to overwinter along the south-western African coast, the winter quarters of Mediterranean birds remain more enigmatic. We performed stable isotope analysis (SIA) of C and N on P1, S8 and P10 feathers from 33 adult birds captured in three Atlantic colonies and 156 adult birds in seven western Mediterranean colonies to infer their wintering areas. In addition, we collated all observational field data, both from peer-reviewed publications and the wider literature, to complement our inferences from SIA. Within the Atlantic, isotopic profiles of feathers moulted at the breeding grounds (P1) differed between birds captured at northern Atlantic and Canary Islands colonies, but were similar for feathers moulted in winter quarters (S8 and P10), indicating low migratory connectivity. Isotopic values of feathers from western Mediterranean birds differed from those of Atlantic birds and showed Mediterranean values for all feathers, indicating that the former overwinter in Mediterranean waters. Variance in the isotopic values was greater in winter than in breeding season feathers, suggesting that birds disperse over larger areas in winter. Isotopic values of feathers moulted during the non-breeding period could match a post-breeding movement towards the southern and eastern Mediterranean. This inference matches the distribution of the few winter reports, which are mainly concentrated in the south-central Mediterranean, mostly in the Tunisian Platform. Our results suggest that this region is the principal wintering area of Mediterranean Storm-petrels.
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