SPATIAL-CONCENTRATION EFFECTS AND THE IMPORTANCE OF LOCAL ENHANCEMENTIN THE EVOLUTION OF COLONIAL BREEDING IN SEABIRDS

The information center hypothesis (ICH) suggests that birds breed in c olonies because this behavior favors information exchange at the colon y about the location of food patches. However, the complex suite of be haviors the ICH requires implies that information center following is more likely to evolve after colonial breeding has become established t han to promote its initial development. A simpler hypothesis to explai n 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 inform ation about the location of patches than if foragers bred in a dispers ed fashion. To assess the effects of breeding dispersion on foraging s uccess, I simulated the foraging behavior of cliff-breeding seabirds ( nesting either solitarily or colonially) searching for patchily distri buted prey. Results show that colonial breeding is favored when food p atches 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 stra tegy can invade a population of solitarily breeding birds, and once a colonial breeding strategy becomes established, it generally is resist ant to invasion. These findings suggest that the spatial-concentration model is a plausible mechanism for the initial development of colonia lity.