The food of breeding Herring Gulls Larus argentatus at the lower river Elbe: does fish availability limit inland colonisation?

In a Herring Gull Larus argentatus colony on an island in the lower river Elbe, northern Germany, diet during the breeding season was investigated. Fish was found in 79% of all pellets, followed by crustaceans (39%, presumably Mitten Crabs Eriocheir sinensis only) and human refuse (10%). Overall, 77% of all pellets only contained fish and/or crab, suggesting that feeding was predominantly at the river bank near the colony. Of (potential) food remains other than pellets, 46% were fish, 19% gull eggs, 17% crustaceans, 10% small birds and 8% human refuse. The strong tidal rhythm of the adults’ attendance at the colony coincides with fishery activities at the river banks and supports their dependence on unwanted bycatches from fisheries. From this and other studies, there is strong evidence to suggest that the inland breeding distribution of Herring Gulls is limited by the availability of fish during the breeding season rather than by the availability of human refuse or by the lack of breeding habitats.