Trophic ecology of a resident Yellow-legged Gull (Larus michahellis) population in the Bay of Biscay

•We analysed diet of a resident Yellow-legged Gull population from Iberia.•We used δ13C, δ15N and δ34S stable isotopes from feathers of chicks and adults.•Prey consumption differed slightly between colonies and among years.•Diet did not differ between age classes and only slightly between seasons.•All this suggests a relatively homogeneous diet with only some slight variations.

Stable isotopes analyses (SIAs) are an efficient tool to obtain a general insight into the diet of generalist consumers, such as the Yellow-legged Gull (Larus michahellis). Here we analysed δ13C, δ15N and δ34S values in feathers of chicks and adults, and used Bayesian triple-isotope mixing models to reconstruct the diet of a Yellow-legged Gull population breeding in the southeastern Bay of Biscay. Questions to test were (1) whether adults and chicks rely on different feeding resources during breeding period; (2) whether there is a seasonal foraging effect involving a higher proportion of refuse food in winter compared to summer, and (3) the magnitude of the annual variation in diet. Prey consumption differed between colonies, among years, and also varied slightly between seasons, and this was mainly due to a differential use of prey of marine origin. However, diet did not differ between age classes. These results suggest a relatively monotonous diet with only slight variations from year to year, seasonally and at a local geographic scale.