Island breeding and continental feeding: How are diet patterns in adult yellow-legged gulls influenced by landfill accessibility and breeding stages?

We studied the diet of yellow-legged gulls, Larus (cachinnons) michahellis, at six colonies located at different distances from landfills during three breeding stages (territory establishment, pre-laying, and nesting periods) through the analysis of 1,192 pellets. Landfills were the foraging habitat most frequently used by gulls from all six colonies at all stages. Other terrestrial habitats were used with decreasing frequency as the breeding season advanced. Limited but consistent use was also made of the marine habitat. At each stage, landfill accessibility influenced the diet of the colonies, which showed both a varying degree of exploitation of landfills and a varying trophic niche width. We used principal components analysis (PCA) to assess diet variation patterns as the breeding season advanced as well as food accessibility parameters. The PCA revealed that gulls shift their diet toward a diet weakly diversified and increasingly centred on items from landfills and also showed that the diets of colonies were correlated with landfill accessibility and breeding stages. The shift in diet occurred earlier in the breeding season for colonies far from landfills compared to colonies near landfills. In light of these diet patterns, yellow-legged gulls appear to be strongly influenced by landfills; thus, the future closure of landfills is likely to have marked effects on yellow-legged gull population dynamics.