At-sea distribution and behaviour of a surface-feeding seabird, the lesser black-backed gull Larus fuscus, and its association with different prey

The lesser black-backed gull Larus fuscus is a typical surface-feeding seabird with a widespread, patchy distribution in the SE North Sea. During the 1990s, breeding numbers along the German North Sea coast increased exponentially. Analyses of a multiple-year data set revealed changes in the at-sea distribution that seemed to reflect an increase in the numbers of breeding birds. Detailed recordings of feeding and resting behaviour at sea revealed area utilisation by the lesser black-backed gull. Observations at sea and dietary analysis revealed that swimming crabs Liocarcinus spp. are a major dietary item of this gull species. This natural food source was primarily captured close to the shore; at longer distances from the coast the gulls mainly fed on other natural prey items or discarded fishes from trawlers and avoided competition with other breeding gull species. During July and August 1990 to 1995 L. fuscus abundance was significantly correlated with fishing effort, while in May and June there was no such correlation. Time of day was the main environmental variable influencing the gulls' flight activity and indicated an association between activity and availability of their major natural prey, swimming crabs. Distinct patterns and changes in the at-sea distribution and foraging behaviour of L. fuscus in the SE North Sea reflect anthropogenic and natural influences in their habitats.