Assessing migration and moulting strategy in closely related taxa based on stable isotope analysis: a population study of Balearic and Yelkouan shearwaters across their breeding range

Animal migrations are unique phenomena involving mass movements of individuals, which pose significant challenges to develop conservation strategies. Migratory seabirds, particularly, face many anthropogenic threats across their distributions, and populations are declining worldwide. We provided a thorough isotopic method to characterise individual migratory patterns and identify main moulting areas of two closely-related Puffinus shearwaters breeding in the Mediterranean that produces minimal intrusion on birds. Using tracking data of 64 birds and the stable isotopic values (δ13C and δ15N) of one wing feather, we built a robust discriminant for rapid geographic assignment of post-breeding grounds, among Atlantic, Mediterranean and Black Sea basins. Afterwards, we applied the method to several wing feathers of non-tracked individuals and populations and assessed their most plausible post-breeding grounds. First, most tracked Balearic shearwaters from Ibiza and 7% from Minorca were moult-ascribed to the Atlantic; the remaining Balearic and all Yelkouan shearwaters displayed what resembled a longitudinal chain-migration, with an easternmost gradual proportion of individuals migrating towards the Black Sea. Second, the isotopic discriminant achieved high accuracy of correct classification (91.0%) with only six non-assigned individuals. Third, >80% of birds moulted all their primary feathers in a single location. Finally, we provided examples assigning moulting areas of non-tracked individuals (e.g., bycaught birds) and entire populations based on a single sampling. This isotopic approach and the examples we provided offer new insights into the study of seabird migration with minimal disturbance, requiring a single capture. The methodology offers valuable tools for studying endangered seabirds and supports conservation efforts by identifying key moulting areas and migration strategies.