Breeding success of Common Gulls Larus canus in West Scotland, I. Observation at a single colony

A colony of c. 100 pairs of Common Gulls Larus canus breeding on a 12 ha island 250 m off the coast of west Scotland was studied in the years 1988-95. Laying and hatching dates varied significantly between years. Length of incubation varied little, the annual mean always lying between 24.2 and 25.3 days. In 1988-90, mean clutch size was 2.4, 2.8 and 2.5; productivity (young fledged/pair laying) was 0.48, 0,74 and 0,27; and 34%, 47% and 23% of pairs fledged young. Of these successful pairs, the percentages fledging one, two and three young were 61%, 34% and 5% in 1988; 50%, 42% and 8% in 1989; and 83%, 17% and 0% in 1990. In 1988, the 18% of the colony that laid before 11 May fledged 1.09 young/pair; the 48% that laid in the next week fledged 0.48 young/pair; the 20% that laid in the subsequent week fledged 0.28 young/pair; and the 14% that laid thereafter fledged no young. In 1988-90, no or almost no young fledged from clutches completed after 20 May. In 1988, nests within 1 m of dense vegetation (usually Juncus) hatched and fledged more successfully than those further from dense vegetation. Breeding success was reduced by Peregrines Falco peregrinus Herring Gulls Larus argentatus and American Mink Mustela vison In 1988, Peregrines took 20% of large unfledged young, and nests within 55 m of a Herring Gull nest hatched and fledged less successfully than those further from Herring Gull nests. Earlier breeders tended to nest further from Herring Gull nests, and closer to vegetation, than did later breeders. Predation by American Mink, unrecorded on this island in 1980-89, was first recorded late in the 1990 breeding season. In 1991 and 1992, mink were established on the island and for the first time on record no Common Gulls fledged there. The study site was abandoned by Common Gulls in 1993-97 (although some bred elsewhere on the island).