Dead birds for this study were collected in beaches by Itsas Enara Ornitologi Elkartea and anonymous volunteers, as well as kindly provided by the wildlife recovery centers of Cantabria, Bizkaia and Gipuzkoa, the beach cleaning services dependent on the Bizkaia and Gipuzkoa Provincial Councils and the city councils of Getxo, Zarautz and Donostia, and the Wildlife Care Center "Centre de Sauvegarde du Marais aux Oiseaux" (Ol éron Island). Our gratitude to Urdaibai Bird Center and Plentziako Itsas Estazioa (UPV/EHU) for their collaboration. We are very grateful to G.