Breeding Lesser Black-Backed Gulls Larus graellsii and Herring Gulls Larus argentatus:: Coexistence or competition?

While Lesser Black-backed Gull Larus graellsii numbers increase substantially at the German North Sea coast since the late 1980s, Herring Gull Larus argentatus numbers have been roughly stable since the mid-1980s. In order to investigate whether a different ecology might explain the current trends, we studied diet, colony attendance, reproductive output and aggressive behaviour in a mixed-colony on Amrum, southeastern North Sea, in 1994 and 1995. During incubation Lesser Black-backed Gulls fed mainly upon crustaceans and molluscs which were taken from the intertidal zone. During chick-rearing, they took mainly crustaceans and fish which were gathered mostly as trawler discards. The main food of Herring Gulls throughout the reproductive season were molluscs and crustaceans which were obtained from the intertidal zone. Numbers of Lesser Black-backed Gulls in the colony mainly varied with season and time of day, those of Herring Gulls with tide and season. Numbers of Herring Gulls commuting to the open sea roughly equalled those commuting to the Wadden Sea. Lesser Black-backed Gulls generally flew to the open sea but hardly towards the Wadden Sea. Nest attendance was significantly higher in Lesser Black-backed Gulls than in Herring Gulls during the chick-rearing period. Hatching success and fledging success tended to be higher in Lesser Black-backed Gulls. Lesser Black-backed Gulls won interspecific aggressive interactions significantly more often than Herring Gulls in the chick-rearing period. Our study indicates that Lesser Black-backed Gulls currently enjoy a few ecological advantages compared with Herring Gulls, particularly because they feed on food of apparently higher quality. We conclude that Lesser Black-backed Gulls have filled an empty niche rather than have outcompeted Herring Gulls during the past decades.