Rhinoceros Auklet Cerorhinca monocerata


Justification of Red List Category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.

Population justification
The global population is estimated to number > c.1,300,000 individuals (del Hoyo et al. 1996), while national population estimates include: < c.1,000 wintering individuals in Korea; c.10,000-1 million breeding pairs c.1,000-10,000 wintering individuals in Japan and c.10,000-100,000 breeding pairs in Russia (Brazil 2009).

Trend justification
The population is suspected to be in decline owing to bycatch, predation by invasive species and climate change.

Distribution and population

The Rhinoceros Auklet is found in the North Pacific and breeds from California, USA, off the coasts of Canada and Alaska to the Aleutian Islands in North America; and on Hokkaido and Honshu, Japan, as well as on the northern tip of North Korea, Sakhalin (Russia) and at two places on the far eastern Siberian coast in Asia (del Hoyo et al. 1996).


Habitat: This marine species can be found both offshore and along sea coasts and islands. It breeds on maritime and inland grassy slopes, sometimes on predator-free islands, and rarely on steep island or mainland cliffs. It occurs in large aggregations at sea, often forming dense roosting flocks at night in sheltered bays. In winter it is normally pelagic in waters offshore from breeding areas and sometimes in near-shore coastal waters where food is highly concentrated due to oceanographic conditions. Diet: It feeds mostly on fish throughout the year supplemented in winter by small amounts of invertebrates such as squid and krill. Chicks are fed almost exclusively on fish, though invertebrates may also be given to late-hatching young. Breeding: It arrives at colonies in late March and early April, laying from the end of April to mid-June. It is monogamous with high site and mate fidelity. It is highly colonial in small to very large concentrations (sometimes over 100,000 individuals). Laying is often highly asynchronous within a colony. Birds lay in nest chambers at the end of a burrow which are excavated by both sexes (del Hoyo et al. 1996).


The ongoing decline has been attributed mainly to the ecosystem effects resulting from climate change, including the impact of long-term changes in sea surface temperatures and altered timing and intensity of upwelling (Gaston et al. 2009). Interaction with commercial fisheries poses another significant threat. In the 1980s, c. 5,000 birds were annually captured in the salmon gill-net fishery off Japan, with further mortality in gillnets off Washington and British Columbia (Gaston and Dechesne 1996). Predation by non-native species may be contributory to decline in certain parts of the species’ range; Arctic Fox Vulpes lagopus have caused population declines in Alaska, on islands where there were introduced for hunting (Gaston and Dechesne 1996), rats Rattus spp. have impacted populations in Russia (Gaston and Dechesne 1996) and raccoons Procyon lotor were introduced to the Queen Charlotte Islands in the early 1940s, with the support of the Provincial Game Commission, and likely caused the extirpation of the colony on Saunders Island and dramatically reduced the population on Helgesen Island (Gaston and Deschesne 1996). Individuals of this species are regularly found oiled and is evidently susceptible to oil spills, though the levels of mortality remain unclear.


Text account compilers
Butchart, S., Fjagesund, T., Martin, R., Calvert, R., Ekstrom, J.

Recommended citation
BirdLife International (2022) Species factsheet: Cerorhinca monocerata. Downloaded from on 25/06/2022. Recommended citation for factsheets for more than one species: BirdLife International (2022) IUCN Red List for birds. Downloaded from on 25/06/2022.