The availability of anthropogenic food subsidies has promoted an increase in generalist opportunistic gull species, which currently breed and forage on predictable anthropogenic resources (e.g. landfills). Here we investigated whether marine resources are still important to urban-dwelling gulls. We studied 4 natural and 2 urban yellow-legged gull Larus michahellis colonies and compared (1) diet composition (through pellet analysis) and (2) isotopic niches of adults and chicks, (3) diet delivered to chicks of different ages, and (4) fatty acid (FA) composition of fledglings, in order to assess diet composition, diversity and quality, and the relevance of marine prey for natural and urban gull populations. Adult urban gulls consumed considerably lower proportions of marine prey when compared to gulls from natural colonies; however, they fed their younger chicks (<20 d old) mostly with fish, representing 61-80% of their chick food deliveries. Refuse items were mostly delivered to chicks older than 20 d. Overall, urban isotopic niches were not completely distinct from those of natural colonies, in some cases sharing ca. 50% of their niche space. Fledglings from the most urbanized colony presented overall higher FA concentrations and diversity, but they were lacking some omega-3 FAs relevant to their physiology. Our results highlight the importance of marine resources in the diet of urban gulls, particularly during early chick rearing, the relevance of food sources in the area around the breeding colonies and the fact that urban gulls benefit from year-round reliable anthropogenic food resources.

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Gulls are highly opportunistic seabirds, and the exploitation of fishery discards has led to many population increases worldwide. We investigated the importance of fish in the diet of yellow-legged Larus michahellis and Audouin’s gulls L. audouinii and assessed the influence of fishery discards on their feeding ecology. We collected pellets from 4 islands along the western Iberian coast during pre-breeding, breeding, and post-breeding seasons from 2014 to 2018. Stable isotopes (from adult blood, and chick and adult feathers) were used to investigate spatial, seasonal, and inter-annual differences in their feeding ecology. We used pellet, stable isotope (δ15N, δ13C, and δ34S), and biochemical (triglycerides, uric acid, total protein, and carotenoids in adult plasma) analyses to investigate their relationships with fish landings across the annual cycle. Results revealed that the fish species consumed by gulls matched those landed by local fisheries on all study islands, and there was a positive association of pelagic and demersal fish diets with fish landing quantities for 2 islands. δ34S values suggest different self-feeding and chick-provisioning strategies in relation to fisheries. δ15N values exhibited strong negative correlations with fish landings, and triglycerides were positively correlated with pelagic but not with demersal fish landing quantities, suggesting that gulls fed more on lower trophic level and higher energetic content pelagic fish than on demersal fish. Overall, our results based on several techniques suggest that gull feeding ecology was linked to fishery discards, which in view of the new EU landing obligation may have major implications for both gull populations across Europe.

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