INVESTIGATIONS INTO THE CAUSES OF THE DECLINE OF THE BLACK-HEADED GULL (Larus ridibundus) COLONY AT RAVENGLASS, CUMBRIA

Abstract

In the late 1970's, declines were noticed in the breeding populations of Black-headed Gulls and various tern species on the Ravenglass Local Nature Reserve, Cumbria. By 1983, terns were extinct as breeding species and Black-headed Gull numbers had fallen from 10,000 breeding pairs in 1976 to only 2,290.The annual cycle of Black-headed Gulls was examined and a number of hypotheses constructed as to possible causes of the declines. These hypotheses were tested using historical information and field data collected at Ravenglass and a number of other Cumbrian Black-headed Gull colonies.Historical data from Ravenglass suggest that Black-headed Gull breeding success was poor in a number of years. The reasons for this poor breeding success are not known but disturbance, predation and food shortage caused by agricultural change, vegetation change or weather factors appear to have been important. In 1984 the 1,514 pairs of gulls that settled at Ravenglass produced no young, probably as a result of predation by foxes on eggs and small chicks. No gulls settled to breed at Ravenglass in 1985 and none have done so since.Studies at other Cumbrian colonies since 1985 revealed different causes of reduced breeding success at different sites in different years. This may have been important in reducing the number of potential recruits to Ravenglass from other colonies. Other ground-nesting birds breeding at Ravenglass (except She1duck which breed in rabbit burrows) suffered fox predation in 1984-87.Gulls from both inland and coastal gulleries fed predominantly inland. Levels of heavy metal and radionuclide contaminants in gulls are below those recorded to be harmful.It is concluded that the decline was probably caused by a combination of a number of factors (see above). A number of changes in the colonial behaviour of Black-headed Gulls at Ravenglass were noted in the later stages of the decline suggesting ways In which the gulls responded to the reduction in colony size.