Re-distribution of Black-headed Gull Larus ridibundus colonies in NE Vänern during 1985—1995

Abstract

Since 1985, some forty islands and groups of islets in NE Vanern are censused yearly for breeding larids, waders, cormorants, etc. This paper reports on the population changes, colony occupancy and re-distribution of Black-headed Gulls Larus ridibundus in the census area during an 11-year period. The number of Black-headed Gulls is relatively modest, with a peak of 790 individuals in 1995. There has, however, been an interesting change in their distribution. Islands close to land have been abandoned and the gulls have settled on previously unoccupied islands and islets in the outer archipelago. The causes of this re-distribution are not known, but habitat changes (near-shore islands having become overgrown with tall vegetation) and predation by mink are suggested as two important causes.