Spatial-concentration effects and the importance of local enhancement in the evolution of colonial breeding in seabirds.

The information center hypothesis (ICH) suggests that birds breed in colonies because this behavior favors information exchange at the colony about the location of food patches. However, the complex suite of behaviors the ICH requires implies that information center following is more likely to evolve after colonial breeding has become established than to promote its initial development. A simpler hypothesis to explain the evolution of colonial breeding is that coloniality concentrates foragers in space, which leads to more rapid discovery of food patches and, by means of local enhancement, more efficient transfer of information about the location of patches than if foragers bred in a dispersed fashion. To assess the effects of breeding dispersion on foraging success, I simulated the foraging behavior of cliff-breeding seabirds (nesting either solitarily or colonially) searching for patchily distributed prey. Results show that colonial breeding is favored when food patches are sufficiently large or short-lived that competition for food is ameliorated. Conversely, dispersed nesting is favored when patches are small or long-lived. Individuals playing a colonial breeding strategy can invade a population of solitarily breeding birds, and once a colonial breeding strategy becomes established, it generally is resistant to invasion. These findings suggest that the spatial-concentration model is a plausible mechanism for the initial development of coloniality.