Population and colony site dynamics in Black-headed Gulls Larus ridibundus breeding on the Finnish west coast

Abstract

Population and colony site parameters were examined in an inshore Black-headed Gull population over a period of 7 years. The population varied from 946 to 1445 pairs, but there was no significant difference in colony sizes between years. On average, the colonies contained 79-111 pairs each year. The gulls occupied a total of 24 breeding islands (26 colonies) in 7 years, which comprises 13% of the total islands in the study area . Each year 71-92% (mean 79%) of the colonies were situated in old sites, and 8- 29% (mean 21 %) in new ones. The most common reason for disappearance of colonies was overgrowth of the nesting islets, caused by manuring of the islands by the gulls.