Interactions between Herring and Lesser Black-Backed Gulls Feeding on Refuse

--The interactions among Lesser Black-backed Gulls and Herring Gulls during the breeding season while feeding on a refuse dump on Walney Island, England were analyzed. Herring Gulls were found to be more aggressive than Lesser Black-backs, and they were equally aggressive toward Lesser Black-backs and Herring Gulls. It is suggested that Lesser Black-backs are more timid than Herring Gulls and avoid being pecked at by them. Most of the Herring Gulls (77%) found their own food, 95% of the Lesser Black-backs stole theirs. In contrast to Herring Gulls, Lesser Black-backs rarely dug for food and those that fed independently (5%) found less food than Herring Gulls. Lesser Black-backs were very efficient in stealing food, mostly from Herring Gulls. It is suggested that the number of Lesser Black-backs feeding on the dump is dependent on the number of Herring Gulls. Department of Zoology, Animal Behaviour Research Group, Oxford, England. Present address: Department of Biology, Simon Fraser University, Burnaby, B.C. V5A 1S6, Canada. Accepted 5 April 1976. THE Herring Gull (Larus argentatus) and the Lesser Black-backed Gull (L. fuscus) are closely related species (Mayr 1963) that on occasion produce fertile hybrids (Tinbergen 1953). The species are very similar in body dimensions (Barth 1975). These gulls occupy different food niches (Brown 1967, Shaffer 1971) but considerable overlap occurs on certain feeding substrates (Hunt and Hunt 1973). Given this close similarity one might expect either extreme competition and aggression, or the avoidance thereof because each species uses the common substrate in a different way. This paper examines these possibilities in a situation where both species feed simultaneously on a refuse dump. The study was made on Walney Island, Cumbria, England, in 1973-74. Both species of gulls nest on the south end of the island, about 3 km from the dump. Shaffer (1971) showed that roughly three times more Herring Gulls than Lesser Black-backs frequented this dump, and he suggested that the Herring Gulls "simply drive the Lesser Black-backs away." This implies severe competition and aggression, which the detailed observations set out in this paper do not support.