Rhinoceros Auklet
Cerorhinca monocerata
角嘴海雀
Introduction
The rhinoceros auklet (Cerorhinca monocerata) is a medium-sized seabird of the North Pacific, closely related to puffins and the only extant species in the genus Cerorhinca. During the breeding season, adults develop a prominent horn-like projection on the bill. This horn exhibits fluorescence under ultraviolet light, a trait unique among North American seabirds. Breeding birds also display white plumes above the eyes and behind the bill, with plumage brighter than in non-breeding season. Alternative common names include rhino auklet, horn-billed puffin, and unicorn puffin. The species breeds from the Channel Islands in California to Alaska and the Aleutian Islands, with major colonies in British Columbia. Wintering occurs offshore from Alaska to Baja California, and Japanese populations winter in the Sea of Japan. The species feeds by pursuit-diving on small fish and invertebrates. Conservation status varies by region.
Description
This medium-sized auk presents a robust profile dominated by its impressive bill. The large, strong bill displays an orange-brown coloration and bears the distinctive horn-like projection that gives the species its name—present only in breeding adults and shed annually like the decorative sheath on puffin bills. The overall plumage follows the typical auk pattern: dark slate-brown upperparts contrast with paler underparts. Breeding adults of both sexes acquire ornamental white plumes positioned above the eyes and extending behind the bill, creating a distinguished facial appearance. Sexual dimorphism is subtle but present, with males averaging approximately 10% larger in mass than females.
Identification
The rhinoceros auklet is most readily identified by its substantial orange-brown bill, which bears the characteristic horn projection during breeding season. Combined with the white facial plumes of breeding adults, this creates a unique profile among Pacific auks. The large bill size and horn distinguish it from smaller auklet species, while its darker upperparts separate it from some other auks. In non-breeding plumage, the horn is absent and facial plumes reduced, making careful attention to bill shape and size essential for identification.
Distribution & Habitat
This North Pacific seabird breeds across a broad range spanning both sides of the ocean. In North America, colonies extend from the Channel Islands of California through the Aleutian Islands of Alaska. Asian populations breed on Hokkaidō and Honshū in Japan, the Korean Peninsula, and Sakhalin Island. The species winters in both offshore and inshore waters, with migration occurring primarily between October and April. Large numbers concentrate in pelagic California waters during winter, comprising birds from northern breeding colonies, while resident birds remain off the California coast year-round.
Behavior & Ecology
Rhinoceros auklets are piscivorous, feeding primarily on small fish with krill and squid taken opportunistically. They hunt in midwater zones during the breeding season, diving to depths of 57 meters for up to 148 seconds. Diet composition varies by colony location—studies show birds near the Salish Sea favor sand lance, while other Washington island populations target anchovy and smelt. Breeding occurs in early summer, with pairs nesting in burrows or natural cavities extending 1-5 meters underground, preferring slight inclines to assist their relatively weak takeoff. Both parents share 30-35 day incubation of a single egg, then feed the chick nocturnally for 35-45 days—a strategy believed to reduce predation and kleptoparasitism by gulls. The species is monogamous, with pairs synchronizing foraging during the pre-laying period despite migrating separately outside breeding season.
Conservation
The rhinoceros auklet is considered a species of conservation concern across parts of its range. Like many colonial seabirds, it faces threats from habitat degradation, predation by introduced mammals at breeding colonies, and human disturbance. Oil spills and bycatch in gillnet fisheries pose additional risks during the non-breeding period when birds concentrate in offshore waters. Climate change effects on prey availability and ocean conditions may impact populations in coming decades. Conservation efforts focus on colony protection, predator control, and monitoring programs to track population trends across the species' extensive range.
Source: Wikipedia · CC BY-SA 3.0
Taxonomy
- Order
- Charadriiformes
- Family
- Alcidae
- Genus
- Cerorhinca
- eBird Code
- rhiauk
Distribution
breeds North Pacific islands from Sakhalin and Kuril Islands, Hokkaido and northern Honshu (northern Japan), Aleutian Islands, southern Alaska to southern California; winters coastal waters to southern Japan and Korean Peninsula and northern Baja California
Data Sources
Species description from Wikipedia, licensed under CC BY-SA 3.0.
Taxonomy data from AviList 2025.