Breeding success of Common Gulls Larus canus in West Scotland, II. Comparisons between colonies

In a study of Common Gull Larus canus breeding success in west Scotland from 1991-1997, the number of colonies of five or more pairs monitored each year varied between 13 in 1991 and 27 in 1995. Observed mean clutch size varied greatly between colonies, from zero at colonies severely affected by predation by American Mink Mustela vison, to 3.0 at a colony where no predation was detected. Excluding mink-affected colonies, mean clutch for all colonies combined varied between 2.47 in 1993 (n = 663 clutches at 12 colonies) and 2.69 in 1995 (n = 843 at 14 colonies). Productivity (young fledged/pair nesting) varied significantly between colonies. In 98 observations of colonies during 1994-1997, there were 41 records of colonies producing no young, and mink predation of eggs or chicks occurred at most of these (between 25 and 38, 61 %- 93%). The highest productivity of a colony was 1.6 at a site where predation was not detected. Productivity for all colonies combined varied significantly between years, from 0.1 in 1993 to 0.4-0.6 in 1996 and 1997 when mink were removed from some of the colonies. For the years 1994-1997 combined, colonies apparently unaffected by predation fledged a mean of 0.7-0.9 young/pair (n = 23); colonies affected by raptors, usually or always Peregrine Falcons Falco peregrinus, fledged 0.4 (n = 11); colonies close to colonies of the larger gulls fledged 0.4 (n = 7); one colony affected by otter Lutra lutra predation fledged 0.76; and colonies affected by mink fledged 0.04-0.06 (n = 36). Predation by native species was rarely associated with whole-colony breeding failure (2 out of 19 colony observations), whereas predation by mink was commonly associated with whole-colony breeding failure (25 out of 36 colony observations). Between 1989 and 1997, at 32 colonies in 15 sealochs and sounds, total numbers of breeding pairs of Common Gulls decreased by 41% from 1248 to 734. Breeding Common Gulls disappeared from six of these 15 areas and decreased at six; at two sealochs where they increased, mink had been removed each spring to protect breeding seabirds; and, at one sealoch, breeding Common Gulls persisted only by nesting on a factory roof. These declines and disappearances are ascribed to the widespread annual breeding failures of Common Gulls caused by mink predation: too few young are reared each year to replace adults that die from all causes.