Coloniality in Terns: The Role of Social Feeding

Abstract

Coloniality in ground-nesting seabirds is believed by most workers to have evolved primarily as an adaptation to avoid predation (Tinbergen 1953, Cullen 1960, Kruuk 1964, Lack 1968). Because locations free of mammalian predators are limited, many marine species breed in dense congregations. Birds in large breeding colonies may effectively reduce avian predation either because many individuals jointly mob an intruding predator (Kruuk 1964) or because nests in the center of the colony are relatively invulnerable to predators (selfish herd phenomenon of Hamilton 1971). The upper limit in colony size is presumably set by food supply (Ashmole 1963, Bourne 1963, Lack 1968, Nelson 1970). Recent evidence suggests that food-finding is another important function of coloniality (Ward 1965, Horn 1968, Fry 1972, Ward and Zahavi 1973, Krebs 1974, Emlen 1975). When food is unevenly distributed and unpredictable, individuals may learn about good feeding sites by observing the flight direction and success of individuals nearby (Turner 1964). In this view, colony size per se might be an important selective factor because large colonies provide more sources of about food location than small ones. Ward and Zahavi's (1973) informationcentre hypothesis predicts that, given equal habitat and predation factors, species which depend upon food that is unpredictable in space and time and/or distributed over large areas should nest in larger colonies than species feeding on a more uniformly-distributed food source close to the nest site. Also, gregariousness and group feeding should be more highly developed in species relying upon local enhancement (Thorpe 1956) to find dispersed food patches over large feeding areas. To test the hypothesis, I compared the six abundant beach-nesting tern species along the Atlantic coast of the United States: Arctic Tern (Sterna paradisaea), Common Tern (S. hirundo), Roseate Tern (S. dougallii), Least Tern (S. albifrons), Royal Tern (S. maxima), and Gull-billed Tern (Gelochelidon nilotica). Sandwich Terns (S. sandvicensis) were not included because of their low numbers and the lack of information about their feeding ecology. Five of the six species are primarily piscivorous (i.e., all except Gull-billed Terns), and all nest primarily on beaches or rocks of open islands. Breeding space seldom appears to be limited and most colonies are on sites