Observations on black-backed gull predation at the Cape Kidnappers gannetries: 1959–1963

Four colonies of black-backed gulls (of which onIy the three main ones are considered extensively) exist in the close proximity of Cape Kidnappers. While gulls move extensively within the area, no, large increase in numbers has been noted between 1959 and 1963. Distinct gull movements, or “patrolling” have been observed near all gannetries at Cape Kidnappers. Mainly adult birds are concerned in this activity. Edible matter, in particular regurgitations or eggs, may be taken by gulls from the Cape Kidnappers colonies, as elsewhere, if the opportunity occurs during “patrolling ” movements in the air or “parading” on land. The placing of Plateau Colony B on a grid has shown empty nests on the periphery. It was found that the gannets nesting in the centre of the colony had the highest percentage nesting success and returned earlier for the breeding season. Banding has established that young pairs of gannets arrive later at the gannetries, nest on the outer margins of the colony, and have smaller nesting mounds. This marginal area also coincides with the main areas of movement by tourists. Figures from 1945–62 indicate a steady increase in the gannet population of the Plateau gannetries. Because of publicity an increasing number of the public are visiting the gannetries each year. With gull populations at their present levels, there is no evidence that black-backed gull predation is having a depressing effect on gannet numbers.