Long-term effects of North American Mink Mustela vison on seabirds in western Scotland

Abstract

In the years 1989-95, feral North American Mink caused widespread whole-colony breeding failures of Black-headed Gulls, Common Gulls and Common Terns at colonies on small islands in a study area along 1000 km of mainland coast in west Scotland. After one or more years of such failure, most of the affected breeding sites held no birds or greatly reduced numbers. In some cases movement to new or existing colonies was detected; birds sometimes found mink-free islands and bred successfully. By 1996, mink had had two distinct long-term effects on breeding numbers and distribution of these species. (1) In 1987 the study area contained 1839 breeding pairs of Common Terns (one-eighth of the British Isles total). By 1996 they had decreased by 36% to 1179 pairs. Between 1989 and 1996, Black-headed Gulls and Common Gulls decreased by 52% and 30%. There was no evidence that birds had left the area and, subject to certain assumptions, these decreases were largely accounted for by known predation of eggs and chicks by mink. (2) Discrete areas such as archipelagoes, sea lochs, firths and sounds lost all or nearly all breeding seabirds as a result of such movements from affected islands. Mink predation is widespread but difficult to detect and should be considered as a possible cause when seabird colonies near the mainland decline and disappear.