One of the most widely distributed of the Pacific alcids, and among auklets one of the best studied, Cassin's Auklet is an island-breeding species found from the Kurile Islands (Russia/Japan) through the Alaskan Aleutians and south to Baja California. The center of breeding is along the coast of British Columbia.
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, cacti or logs. During the nonbreeding season, this species spends most of its time at sea, with southern populations likely moving north and northern ones moving south to the central portion of its eastern Pacific range; it is most abundant in waters overlying the continental shelf and slope where its preferred food includes small crustaceans, squid, and larval/juvenile fish. Where such prey are abundant, these birds often gather in large flocks, diving after 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 with latitude and year. Birds nest in late fall through winter in Baja California, but in early to mid-summer in Alaska. Laying is late during El Niño and like oceanographically anomalous years, and early when upwelling occurs early. This is the only alcid known to produce two consecutive broods in a single breeding season, at least in the southern part of the range where birds may be seen at nesting colonies year round. Vulnerable to aerial predators, especially large gulls and falcons, individuals fly between sea and colonies at night.
Breeding biology is well known in this auklet, spanning the species' southern and central range, with major studies from the San Benito Is., Mexico; the Farallon Is., CA; and Triangle Is., BC, but little is known elsewhere (e.g. Manuwal 1974a, b, 1979; Vermeer 1987; Ainley et al.1990; Knechtel 1998; Hedd et al. 2002; Hipfner et al. 2004; Lee et al. 2007; Wolf 2007, Wolf et al. 2009, 2010). Key studies of auklets at sea include Abraham and Sydeman 2004, 2006; Adams et al. 2004a,b, 2010; Ainley et al. 1996, 2005, 2009; Ainley and Hyrenbach 2009; Boyd et al. 2008; Hyrenbach and Veit 2003; Lee et al. 2007. Conservation issues have been covered by Burger (1997), Everett and Anderson (1991), McChesney and Tershy (1998), and Wolf et al. 2006).