LONG-TERM EFFECTS OF NORTH-AMERICAN MINK MUSTELA-VISON ON SEABIRDS INWESTERN SCOTLAND

In the years 1989-95, feral North American Mink caused widespread whol e-colony breeding failures of Black-headed Culls, Common Gulls and Com mon Terns at colonies on small islands in a study area along 1000 km o f mainland coast in west Scotland. After one or more years of such fai lure, most of the affected breeding sites held no birds or greatly red uced numbers. In some cases movement to new or existing colonies suns detected; birds sometimes found mink-free islands and bred successfull y By 1996, mink had had two distinct long-term effects on breeding num bers and distribution of these species. (1) In 1987 the study area con tained 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 Culls and Common Gulls decreased by 52% a nd 30%. There was no evidence that birds had left the area and, subjec t to certain assumptions, these decreases were largely accounted for b y 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 co nsidered as a possible cause when seabird colonies near the mainland d ecline and disappear.