Individual migratory patterns of two threatened seabirds revealed using stable isotope and geolocation analyses

Aim Intraspecific variability in the migratory movements of seabirds is being revealed far more complex than hitherto recognized, and our lack of understanding undermines their effective protection. Our aim is to test whether the isotopic values of a single feather of two threatened seabirds, the M editerranean ( P uffinus yelkouan ) and the B alearic ( P uffinus mauretanicus ) shearwaters allow the geographic assignment of birds to their non‐breeding areas. Location These two species are known to use three main non‐breeding areas: the NE A tlantic O cean, the W M editerranean and the A egean‐ B lack seas. Methods We clustered in three groups the δ 13 C and δ 15 N values of the first primary feather ( P 1), inferred to be grown during the non‐breeding period, of 34 M editerranean and 56 B alearic shearwaters accidentally caught by longliners. To link the isotopic values of P 1 to its corresponding non‐breeding area, we performed a discriminant function analysis ( DFA ) based on the three clusters and applied this function to feathers of known origin: P 1 from seven M editerranean shearwaters from H yères A rchipelago ( F rance) tracked with geolocators and body feathers from six chicks from B alearic shearwaters. To link the moulting patterns to the areas where the feathers were grown, we applied the DFA to a sequence of primary feathers of eight B alearic and five M editerranean shearwaters (caught by longliners). Results Isotopic and tracking data indicate that none of the M editerranean shearwaters migrated to the A tlantic. The cluster and discriminant function analyses revealed that 8% of B alearic and 54% of M editerranean shearwater moulted P 1 in the M editerranean S ea. Migratory movements were reflected in the changing isotopic values of the primary sequences. Main conclusions Stable isotope analyses ( SIA ) are a powerful approach to reveal the intraspecific variability in the migratory patterns of seabirds that use distinct isotopic areas over their annual cycle. The assignment of birds to their non‐breeding areas by means of SIA is a simple and effective tool that can help to evaluate the impact of human activities in remote areas not only at population but also at individual level, which is an essential knowledge for the management and conservation of threatened species.