Numbers and Habitat Selection of Cassin's Auklet Breeding on Triangle Island, British Columbia

Triangle Island, with approximately 360,000 breeding pairs, constitutes the largest Cassin's Auklet (Ptycoramphus aleuticus) colony in the world and contains 40% of the total breeding population of that species. Nesting habitat preferences were determined by stepwise multiple regression analyses. Cassin's Auklets preferred nesting in sparse shrubbery and short herbaceous vegetation on slopes and in the interior of the island. Their nesting densities ranged from 0.1 to 1.1 pairs per m2with an overall density of 0.37 pairs per m2. Highest densities occurred on the open summit, around the edge of the plateau, on the southeast plateau, and on the southwest slope. They nested least densely on the central plateau of the island, where dense, tall salmonberry was the dominant vegetation, as well as on slopes where other burrowing alcids, Rhinoceros Auklets and Tufted Puffins, were present in large numbers. The latter two alcids were restricted to certain slopes and cliff tops while Cassin's Auklets nested on all slopes and on relatively flat areas of the island. Differences in nesting distribution between the three alcids may partly be explained by the lower wing-loading of Cassin's Auklets.