The Rhinoceros Auklet, a medium-sized auk, is closely related to the puffins (Fratercula). Although dowdier in plumage, it is distinguished from puffins, and all other auks, by a prominent rhinoceros-like "horn" (actually a vertical extension of the mandibular plates) projecting above the upper mandible in breeding plumage. The precise function of this prominent ornament, which is the same size in males and females, is unknown.
This species occurs throughout temperate waters of the North Pacific, generally to the south of the Fratercula puffins. Most of the North American population breeds on a small number of islands in British Columbia and adjacent parts of Washington and southeast Alaska. Unlike puffins, the Rhinoceros Auklet is mainly a nocturnal visitor to its colonies. It is the largest auk to visit breeding colonies at night and the only nocturnal auk that carries fish externally; all of the other nocturnal auks bring food in a gular pouch, if they do so at all. The Rhinoceros Auklet resembles puffins in many aspects of its biology. Along with the fact that colony visits in some areas may be crepuscular or diurnal, these similarities suggest that nocturnal colony visitation may have evolved relatively recently, perhaps in response to kleptoparasitism or predation by gulls and raptors. During the breeding season, Rhinoceros Auklets feed mainly on schooling fishes and tend to forage closer to shore than the Fratercula puffins.