Habitat segregation by breeding origin in the declining populations of European Robins wintering in southern Iberia

Mediterranean woodlands and associated shrub formations of southern Iberia are key habitats for conservation of migratory birds. In some bird species, migratory and sedentary conspecifics meet in these areas during winter, but our understanding of how each population group is distributed over available habitats and the factors that determine their spatial organization are still unclear. This seriously limits our ability to assess their vulnerability to ongoing environmental changes affecting wintering habitats in this region. We used hydrogen isotopic signatures of feathers (δDf) to shed light on the habitat distribution of seasonally sympatric European Robin Erithacus rubecula populations wintering in Campo de Gibraltar that are currently facing a drastic decline. In contrast to previous studies that used morphological methods to distinguish the migratory behaviour of wintering Robins in this area, our isotopic approach revealed that sedentary Robins were not outcompeted upon the arrival of migrants and remained in the woodlands where they reproduce, which agrees with results obtained in other regions. Interestingly, we also found that migratory Robins with a closer breeding origin (higher δDf values) had a higher probability of occupying woodlands than did migrants coming from further away. Overall, our results suggest that the role of breeding origin in shaping the distribution of Robins during winter in Campo de Gibraltar is more relevant than the effects of sex, age or body size, which might facilitate the evolution of local adaptations for the exploitation of each habitat type