Stable isotope analysis of feathers reveals migration and molting strategies in Mediterranean shearwaters ( Puffinus spp.) across their breeding range
Cristina De Tena, Sven Kapelj, Maite Louzao Arsuaga, Jose Manuel Arcos Pros, David Garcia, David Grémillet, Jerome Fort, Clara Perón, Martin Austad, Nicholas Barbara, Danae Portolou, Nicola Baccetti, Marco Zenatello, Rhiannon Austin, Tim Guilford, Karen Bourgeois, Jacob González-Solís, Teresa Militão, George Karris, Stavros Xirouchakis, Raül RamosAbstract
Understanding the decline of migratory animal populations requires identifying their year-round distribution and overlaps with threats. In birds, stable isotope analysis of feathers grown outside the breeding period reveals molting areas and migratory patterns with minimal disturbance, especially valuable for cryptic and threatened species such as the endemic Mediterranean Puffinus mauretanicus (Balearic Shearwater) and P. yelkouan (Yelkouan Shearwater). Using tracking data of 64 individuals across 7 populations and stable isotopic ratios (δ13C, δ15N) from a single wing feather, we developed and validated a discriminant model assigning shearwaters to three post-breeding molting areas: the northeast Atlantic (n = 20), Mediterranean Sea (n = 27), and Black Sea (n = 17). The model achieved high cross-validated accuracy (98.70%; 95% CI: 95.50–100%; κ = 0.98) and correctly classified tracked individuals in 91% of cases. Unlike previous isotope-based studies, our study develops a validated three-basin isotopic discriminant calibrated using geolocator-tracked individuals across multiple colonies, while incorporating wing-feather series and applicable to bycaught and untracked birds. Isotopic differences among molting areas were consistent and exceeded temporal variability, enabling robust spatial discrimination. Analyses of primary molt series showed that >80% of individuals completed molt within a single basin, strengthening the biological basis for isotopic assignment. Tracking data revealed a longitudinal gradient in migratory behavior: most P. mauretanicus from Ibiza migrated towards the northeast Atlantic, while an increasing proportion of P. yelkouan breeding farther east moved toward the Black Sea. Application of the model to untracked and bycaught birds demonstrated its utility for inaccessible colonies, mortality assessments, and retrospective analyses. Our metapopulation study confirms the use of discrete post-breeding molting areas in the northeast Atlantic, Mediterranean, and Black Sea, and provides a minimally invasive tool for assessing migratory connectivity across populations and identify overlap with regions of intense fishing activity, colony-level risk assessments and targeted bycatch mitigation measures, including priority threat areas.