Effects of Territoriality on Breeding in a Population of Cassin's Auklet

Abstract

There are approximately 105,000 breeding Cassin's Auklets (Ptychoramphus aleuticus) on Southeast Farallon Island, California. Burrow density is high, from 1.09 to 0.02 burrows per square meter. Removal experiments conclusively showed a surplus population capable of reproducing. This floating population has an equal sex ratio, and 50% of the floaters were paired before or immediately after entering a recently vacated burrow. Fifty percent of the floating population is adult and 80% have no previous breeding experience. Most floaters do not maintain active permanent territories, but instead roost in areas where there are no territory—holders. Other floaters actively search for empty burrows. Fifty percent of the floaters fledged young, and at least 44% of 1970 floaters in the removal experiment returned to breed in 1971. Floaters enter the population at the rate of about 19% per year. It is clear that territorial behavior limits breeding density in this population of Cassin's Auklet.