Diet composition of the Caspian Gull (Larus cachinanns) in inland Poland: effects of breeding area, breeding stage and sympatric breeding with the Herring Gull (Larus argentatus)

Abstract

The food composition in breeding colonies of large gulls (mainly Caspian Gull Larus cachinnans) was investigated at three sites representing recently-colonized inland habi-tats in Poland: a gravel pit, an unregulated river and a reservoir. No differences were found in the diet of sympatrically-breeding Caspian and Herring Gulls (L. argentatus) that consisted mainly of fish: 5782% of discarded food items and 7194% of prey items detected in pellets were fish. The fish diet consisted of 614 species, depending on local-ity. At the gravel pit and at the river where fish ponds were located near the breeding colo-nies, Carp (Cyprinus carpio) was the dominant fish species in the diet, whereas at the res-ervoir, Roach (Rutilus rutilus) predominated. The size of fish caught by these gulls was relatively similar at all sites, median length being 14.8 cm at the river, 15.6 cm at the gravel pit and 17.0 cm at the reservoir. The proportion of species but not the size of fish differed significantly between egg-laying/incubation and chick-rearing periods at the gravel pit. These results highlight the importance of fish food for large gulls during the breeding season in recently-colonized inland habitats. We suggest that easily accessible foraging areas, such as fish ponds, crucially affect the food composition of these gulls. Such food opportunism could favor rapid colonization of inland habitats.