Group Adherence in Endangered California Least Terns (Sternula antillarum browni)

Colonial nesting in seabirds is advantageous for protection from predators—spotting a predator, mobbing, and predator swamping. Familiarity with nesting areas gives knowledge of protected sites and may promote site fidelity. Familiarity with nearest neighbors helps nesting success by lessening intraspecific aggression and increasing social facilitation, and may promote group adherence. Group adherence has been proposed as more important than site tenacity for some species where nesting areas are frequently disturbed. Ground-nesting terns often nest at disturbed sites, and their colonies are accessible to predators. Serendipitously, I was able to test the concept of group adherence in individually color-marked California Least Terns Sternula antillarum browni during early egg-laying when some nests in a colony in southern California were depredated, and the adults deserted. A week later, I found the majority of those birds nesting at the edge of a small Least Tern colony 28 km distant, where they laid a second clutch and remained at the site the rest of the breeding season. The following breeding season, no color-marked terns nested again at the small colony where they had moved after disturbance.