Stable isotopes reveal year-round sexual trophic segregation in four yellow-legged gull colonies

Abstract

Sexual segregation in trophic ecology is widespread among sexually dimorphic marine vertebrates. The degree of such segregation can change throughout the annual cycle, however, year-round data on seabird inter-sexual differences are scarce. Here, we investigated sexual trophic segregation in yellow-legged gulls Larus michahellis evaluating two hypothetical drivers of such segregation: the different nutritional requirements hypothesis and the sexual size dimorphism hypothesis. We used stable isotope analyses of blood and feathers to estimate year-round gull trophic ecology in four colonies along the western Iberian coast. δ15N and δ13C values were consistently higher in males than in females throughout the year and in all four study colonies, suggesting consistent sexual differences in trophic level prey as well as in foraging habitats. Significantly wider isotopic niches of females in the innermost primary and eight secondary feathers suggest that they are more generalists than males during most of the annual cycle. In accordance, isotopic mixing models estimated a higher proportion of marine invertebrates in female diet, while males consumed a higher proportion of fish, which could indicate that males foraged more in association with fisheries, having major implications in view of the newly implemented discard ban policy. Overall, our results revealed a clear pattern of sexual trophic segregation in yellow-legged gulls that was maintained throughout the year in four colonies along the western Iberian coast, arguing against the different nutritional requirements hypothesis and suggesting that sexual size dimorphism plays an important role in driving sexual segregation in trophic niches.