Formation and growth of new seabird colonies: The significance of habitat quality

Despite the central role that colonial breeding plays in the ecology of marine birds, few theoretical or empirical studies have addressed the process by which new seabird colonies form and grow. Documented colonization events are rare, but suggest an intriguing paradox - prospective breeders are reluctant to pioneer new colonies even when they may suffer substantial costs by recruiting into large established colonies. Once formed, however, new colonies become highly attractive to prospective breeders and grow rapidly. We evaluated the contributions of habitat quality and individual quality to processes of colony formation and growth in Black-legged Kittiwakes (Rissa tridactyla) using productivity and population data from 14 new colonies that formed in Chiniak Bay, Kodiak Island, Alaska, in the late 1980s. Two lines of reasoning suggest that habitat quality in established colonies was more important than individual quality in promoting formation of new colonies: new colonies exhibited greater productivity than old colonies, and new colonies formed at a time when compelling evidence existed of low habitat quality in established colonies. In addition, population modeling revealed that: i) immigration fueled rapid growth of new colonies, ii) some established breeders may have relocated from old to new colonies, and iii) Chiniak Bay did not constitute a closed metapopulation. We propose that, although inverse density dependence in small seabird colonies can explain both the reluctance of individuals to pioneer new habitat and the rapid growth of newly formed colonies, density-independent factors such as predation may also contribute to differences in habitat quality between old and new colonies.