Ptychoramphus aleuticus is a small alcid, weighing approximately 175 grams. They have a short dark bill with a yellow base, a white belly, blue feet, and white undertail coverts. Cassin’s auklets have dark gray underparts, with a gray chin, throat, flanks and breast. They have a broken white ring surrounding their eye.
Adult females have a shallower bill, but are otherwise similar to adult males in size and coloration. Juvenile P. aleuticus are slightly paler than adults and have white throats. The color of the eye can supposedly be used to age auklets. The subspecies P. a. australis is smaller in length and mass.
The simplest way to identify a Cassin’s auklet in the field is by their lack of head ornamentation (common with other alcids). If they are seen from afar, their rapid wingbeat and broad wings may be helpful in identification. If seen from a short distance, the broken white eye ring (like a white eyebrow) is useful for identification.
Can be Confused With
The mostly gray neck and breast make the Cassin’s auklet identifiable from other similarly sized murrelets. The rhinoceros auklet (Cerorhinca monocerata) is considerably larger, and has a yellowish bill.
The Cassin’s auklet breeds on offshore islands from Baja California to the Aleutians in Alaska. Subarctic species with a large marine range restricted to the eastern North Pacific and the Bering Sea. Southern extent of the marine distribution related to cool waters of the California Current and areas influenced by coastal upwelling, particularly off Baja California. Range extends north along the west coast of North America into the Bering Sea in summer and fall.
The two subspecies have different distributions: the Northern P. a. aleuticus is found from the Sanak Islands, Alaska to the Channel Islands in southern California. The northern populations engage in seasonal migrations. The southern P. a. australis breeds off the Baja Peninsula of California and Mexico. The southern populations do not appear to migrate seasonally.
Ecology and Behavior
Cassin’s auklets inhabit the continental shelf and slope. They are known to aggregate at the shelf-break (200 m isobath) to forage on euphausiids, particularly offshore of breeding colonies in central California. However, foraging distributions during the breeding season may be also influenced by the extent of frontal systems and upwelling plumes.
Cassin’s auklets come to land exclusively to breed. They are generally silent while at sea, but adults at breeding colonies call during the night.
Feeding and Prey
Cassin’s auklets pursue prey underwater, using their wings for propulsion. They forage nocturnally to avoid predation from large gulls at the breeding colonies.
Diet consists almost exclusively of Crustaceans (Euphausids and copepods), though larval fish and squid are occasionally taken (crustaceans > fish > squid).
Gaston, A.J., I.L. Jones and I. Lewington. 1998. The Auks: Alcidae. Bird Families of the World, Vol. 5. Oxford University Press, Oxford.
Hunt, G.L., Jr., H. Kato and S.M. McKinnell, eds. 2000. Predation by marine birds and mammals in the subarctic North Pacific Ocean. PICES Scientific Report No. 14. North Pacific Marine Science Organization, Sidney.
Vermeer, K., K.H. Briggs and D. Siegel-Causey, eds. 1993. The status, ecology, and conservation of marine birds of the North Pacific. Special Publications Canadian Wildlife Service, Ottawa.
Manuwal, D.A. and A.C. Thoresen, A.C. 1993. Cassin's Auklet (Ptychoramphus aleuticus). pp. 1-18 in A. Poole and F. Gill, eds. The Birds of North America, No. 50. The American Ornithologists’ Union, Washington, DC.