Rhinoceros Auklet (Cerorhinca monocerata)
Despite its name, the Rhinoceros Auklet is actually more closely related to puffins than true auklets, and some ornithologists have even proposed renaming it.
Each spring it develops a distinctive horn on its bill that is shed after the breeding season, glows under ultraviolet light, and, unlike other puffins, it returns to its nesting burrow only after dark, often carrying a row of fish across its bill.
Species Profile
Scientific Classification
1. Naming & Taxonomy
Peter Simon Pallas described the species in 1811. It is the only living member of the genus Cerorhinca.
The Name Is Wrong
This is the central fact about the bird, and it is not a minor quibble.
Despite being called an auklet, Cerorhinca monocerata is a close relative of the puffins. Molecular work by Friesen and colleagues in 1996 placed it in the tribe Fraterculini, alongside the three Fratercula puffins, and not with the true auklets at all.
The mismatch has been recognised formally. A proposal to the American Ornithologists' Union's classification committee in 2008 suggested renaming the species the rhinoceros puffin. Other names already in circulation include horn-billed puffin and unicorn puffin, along with the informal rhino auklet.
The name has not changed, but any source describing this bird as a true auklet is out of date.
A Genus That Once Spanned Two Oceans
Cerorhinca evolved in the North Pacific, apparently in the mid to late Miocene. It was once far more diverse than the single surviving species suggests, with fossils found as far south as Baja California.
- Cerorhinca dubia L. H. Miller, 1925 — Late Miocene, Santa Barbara County, USA
- Cerorhinca minor Howard, 1971 — Late Miocene or Early Pliocene, Cedros Island, Mexico
- Cerorhinca reai Chandler, 1990 — Late Pliocene, San Diego
- Cerorhinca sp. — Early Pliocene, southeastern United States
That last entry matters. Smith, Olson and Clarke reported the first Atlantic record of the clade in 2007, from the Pliocene of North Carolina. A genus assumed to be purely Pacific turns out to have had a more complicated history than anyone expected.
2. Identification & Appearance
Cerorhinca monocerata · Identification & Appearance
Least ConcernPlumage
Dark above, paler below, and altogether plainer than a puffin. Breeding adults of both sexes carry two white plumes on each side of the head, one running back above the eye and one back from the base of the bill.
The bill itself is large, strong and orange-brown.
The Horn
The feature the bird is named for is a vertical extension of the rhamphotheca, the horny sheath of the bill, projecting upward from the upper mandible.
It appears only in breeding adults. It grows in early spring and is shed in late summer, exactly as puffins shed the bright outer plates of their bills, which is one more piece of evidence for where this species actually belongs.
Its function is unknown. It is the same size in males and females, which rules out the simplest explanations.
The Horn Glows
Research published in 2019 found that the bill is fluorescent under ultraviolet light, and that the effect is strongest in the horn itself.
This is likely to matter. Birds can see into the ultraviolet, and several species are known to use UV cues, so a fluorescent ornament on a bird that chooses a lifetime mate is a reasonable candidate for a signal. There is no difference in fluorescence between the sexes, but the amount varies noticeably between individuals, which is exactly what you would expect from something being assessed by a potential partner.
3. Range & Habitat
Cerorhinca monocerata · Distribution at a glance
Least Concern-
Open Ocean
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Offshore Islands
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Grassy Coastal Slopes
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Nearshore Marine Waters
Breeding requires soil that can be dug, so colonies sit on islands with workable ground rather than bare rock. The birds prefer slight inclines, which help with take-off, because this is not a strong flier.
Migration is partial and regionally variable. Between October and April large numbers pass through pelagic Californian waters, most of them from colonies north of California. Birds that already live off the California coast simply stay put.
Within the family, the Rhinoceros Auklet generally occupies waters south of the Fratercula puffins.
4. Diet & Diving
Mainly fish, with some krill and squid. During the breeding season it feeds inshore, in midwater rather than on the bottom.
Dive performance is considerable. Recorded to 57 m (187 ft), and submerged for as long as 148 seconds.
Diet composition depends heavily on where a colony sits, and the contrast can be sharp over short distances. At one island in the Salish Sea off Washington, the birds feed mainly on sand lance. At another island off the same coast, the diet is anchovy and smelt.
5. Breeding & Nesting
Cerorhinca monocerata · Breeding & Nesting
Nocturnal NesterIt Comes Home in the Dark
This is what separates the Rhinoceros Auklet from the puffins it is related to.
Adults visit the colony at night. Cornell notes two things that follow from it: this is the largest auk to attend its breeding colony after dark, and it is the only nocturnal auk that carries fish externally. Every other night-visiting auk brings food back in a gular pouch, if it brings food at all.
The reason is thought to be gulls. Arriving in daylight with a visible beakful of fish invites both predation and theft, and darkness removes the problem. The trade-off is that the bird has to find its own burrow entrance, among thousands of others, in the dark.
The chick is semi-precocial and is fed each night with a bill full of fish, in the manner of a puffin, for 35 to 45 days.
Pairs Migrate Apart, Then Sync Up
A 2018 study produced one of the more surprising findings about this species. Pair-mates are monogamous and migrate across broadly similar areas, but they do so separately, not together.
Then, once they return to the colony in the pre-laying period, they synchronise their foraging activity. Whatever holds the pair together, it is not proximity through the winter.
6. Conservation
The IUCN lists the Rhinoceros Auklet as Least Concern.
Introduced rodents are the clearest historical threat. Populations at some former colonies crashed after rats and rabbits reached the islands, which is a familiar pattern for any seabird that nests in a burrow at ground level.
The recovery story is equally clear. Active removal of rats and rabbits from affected islands has led to successful recolonisation of former nesting sites. As with the Pigeon Guillemot and introduced foxes, the intervention that works is the removal of the mammal.
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Common Questions About the Rhinoceros Auklet
Sources & References
- Birds of the World: Rhinoceros Auklet
- IUCN Red List assessment for Cerorhinca monocerata
- Fluorescent ornamentation in the Rhinoceros Auklet
- Pair-mates migrate independently but synchronize foraging
Last reviewed August 2026.
