Predator Behavior Favors Clumped Nesting in an Oceanic Seabird

Abstract

Most seabird and numerous landbird species nest colonially. Nesting near conspecifics can confer protection from predators, an effect that is frequently mentioned as a selective agent favoring colony membership (Wittenberger and Hunt 1985). For example, predatory gulls attempting to steal eggs and nestlings from Common Murres (Uria aalge) have better success when sufficient space surrounds a prey nest to permit maneuvering before attack. A murre nest with many close neighbors lacks this space; intruding gulls are deterred from approaching the intended victim by pecking from the neighbors (Birkhead 1977). Predator behavior should select for parents that surround themselves with close neighbors in this species and others with conspicuous nests and some defense capacity. As the proximity and number of neighbors decrease, so should protection from predators. Although it is widely accepted that increasingly dense grouping can confer protection from predators, and so improve survival and reproductive success (Krebs and Davies 1987), available data are equivocal. Among noncolonial species, predation rate typically increases with nest density (Wittenberger and Hunt 1985). For species that normally nest densely, several studies have demonstrated the expected negative relationship between per capita nest predation risk and nearest neighbor distance or colony size (Kruuk 1964, Andersson and Wiklund 1978, Siegel-Causey and Hunt 1981, Wilkinson and English-Loeb 1982, Gotmark and Andersson 1984). However, others have shown the opposite trend (Bergman 1939, Lemmetyinen 1971, Burger 1974, Pienkowski and Evans 1982), implicating higher predation risk as a cost of dense nesting. The role of predators in natural selection on avian nesting dispersion is, thus, far from clear. The most compelling comparative test of the hypothesis that predators preferentially take the most iso-