Predation and Piracy by Gulls at a Ternery in Maine

Abstract

THE current increase in the numbers of large gulls, exemplified by the population of Herring Gulls in New England which is doubling every 12 to 15 years (Kadlec and Drury, 1968), has engendered much concern and speculation about its consequences for other seabirds. The general conclusion is that terns, in particular, suffer heavily from predation and competition for nesting sites (for example, Gross, 1954; Drury, 1965; Howard, 1968). Arctic and Common Terns (Sterna paradisaea and S. hirundo) are probably unaffected in the nonbreeding season by changing gull populations. Thus the interactions between terns and gulls observable at breeding colonies can be a realistic indicator of the changing fortunes of the terns in the face of predation of eggs and young, competition for nesting sites and cleptoparasitism (seizing food gathered by another, which can be considered a special form of food competition). This paper considers predation of tern chicks by Herring and Great Black-backed Gulls (Larus argentatus and L. marinus) and stealing of fish by Laughing Gulls (Larus atricilla) at a colony of Arctic and Common Terns on the island of Petit Manan, Maine. These observations were made from 17 June to 13 July 1968 during th'e course of other studies on the island. It would, of course, be unwise to attribute every reduction in the numbers of breeding terns to the gulls, especially in view of the accumulation of persistent biocides in the oceans (Risebrough et al., 1967), a potential hazard not limited to the breeding season. Nevertheless, it will be shown that a substantial number of chicks are now lost to the large gulls and that fish-stealing is likely to be significant when the terns find fishing difficult. Petit Manan (44?22' N, 67?52' W) is a small, low, treeless island 4 km SE of the point of the same name in southwestern Washington County, Maine. It lies between two outer islands with large colonies of Arctic Terns (Matinicus Rock, 104 km SW and Machias Seal Island, New Brunswick, 64 km E by N), but is closer to the mainland than either of these. The island is approximately rectangular 300 X 250 m, predominantly grassy, with exposed rocks along the north and east shores and a large shingle bank along the southern and western edges. A small part at the southeastern edge of the island that rises to 6 m above mean sea level is the site of the tall lighthouse tower and the five associated buildings are nearby. Terns nested all over the island except where disturbed by the activities