Introduction

Cassin’s Auklet is one of the most widely distributed of the Pacific alcids, and one of the best studied. A coastal, island-breeding species found from Alaska south to Baja California, this small, abundant auklet nests in shallow burrows, which the birds excavate with their sharp toe nails, and also in rock crevices or under trees or logs on the ground. During the nonbreeding season, this species spends most of its time at sea, sometimes lingering near nesting colonies but often traveling beyond the shallow waters of the continental shelf for its preferred food—small crustaceans, squid, and fish. Where such prey are abundant, these birds often gather in large flocks, diving after their prey propelled by their short, stubby wings. They gather their food and transport it back to the nest in a specialized throat pouch, a structure common to all true auklets.

The timing of Cassin’s Auklet breeding varies greatly with latitude. Birds nest in late fall through winter in Baja California, but in early to mid-summer in Alaska. This is the only alcid known to produce two broods in a single breeding season, at least in the southern part of the range where birds may be seen at nesting colonies every month of the year. Vulnerable to predators, especially large gulls, individuals tend to visit their nests at night.

Although the breeding biology is well known for certain populations of this species, based on studies at the Farallon Is., CA and Triangle Is., BC, populations nesting in Mexico, Washington, and Alaska have been little studied. In addition, throughout its range, little is known about the population dynamics, age structure, reproductive response to changes in oceanographic conditions, at-sea behavior and distribution, and migration of this abundant auklet.