The Regulation of Seabird Colony Size: A Hinterland Model

In 1963, Ashmole proposed that pelagic seabird populations are limited by density-dependent competition for food around their breeding colonies. This hypothesis has received renewed attention because of recent evidence suggesting that seabirds may consume large quantities of marine food and deplete local food supplies (Wiens 1984; Birt et al. 1987) and that a variety of behavioral, reproductive, and population parameters are linked to food availability (Cairns 1988). Although food supply is widely viewed as having an important role in regulating seabird colony size (Birkhead and Furness 1985), the mechanism for such regulation is unclear. Furness and Birkhead (1984) found that colony sizes of four British seabird species varied inversely with populations of other colonies within their potential foraging range. They explained this result by postulating that birds from neighboring colonies compete for food in common foraging rounds, such that birds from a large colony deplete shared resources and limit the size of neighboring colonies. This argument is incomplete because it does not explain why the large colony is large in the first place. Furness and Birkhead offered the additional suggestion that some colonies might have restricted availability of nest sites, which would limit their populations and allow neighboring colonies to become large because of lack of competition in common feeding areas. However, this effect could operate only when a site-limited colony and a site-unlimited colony are adjacent; when adjacent colonies are both site-limited orboth site-unlimited, no prediction can be made. The supposition that a substantial number of colonies are site-limited also has a bearing on the expected correlation between colony size and the populations of neighboring colonies, since random variation in colony size resulting from site availability should relax the strong inverse correlations reported by Furness and Birkhead. This paper develops an alternative model of population regulation of seabird colonies, which is based on the notion that colony size is related to the amount of foraging habitat used by the colony (Diamond 1978). The model proposes that seabirds from neighboring colonies typically occupy nonoverlapping feeding zones and that colony population is a function of the size of these zones. The model assumes that birds recruiting as first breeders choose the colony in which feeding conditions are most favorable. If colonies are not limited in nestsite availability and if nest-site quality is uniform among colonies, differential