Overview
The Rhinoceros Auklet (Cerorhinca monocerata), despite its common name, is not an auklet at all, but rather the sole living member of the genus Cerorhinca and the closest living relative of the true puffins. Patrolling the cold, tempestuous open waters of the North Pacific Ocean, this formidable pelagic seabird is instantly recognizable by the prominent, vertical horn-like sheath that erupts from the base of its orange-yellow bill during the spring breeding season. Nesting colonially in deep underground burrows excavated into the forested slopes of remote, wave-lashed islands, the rhinoceros auklet is a master diver that bridges open-ocean marine food webs and coastal island ecosystems.
Biology and Physical Characteristics
The physical anatomy of Cerorhinca monocerata is built for pursuit diving in freezing subpolar waters. Its compact, heavily muscled body is propelled underwater by short, stiff, paddle-like wings that ‘fly’ through dense seawater with astonishing agility, reaching depths exceeding 180 feet (55 m). During breeding season, both males and females grow a conspicuous vertical ivory horn atop their heavy orange bill, along with twin plumes of white sensory feathers extending from the eye and mouth corners. Inside the upper mandible, specialized serrated denticles and a muscular tongue enable the bird to clamp up to 20 whole, wriggling sand lance and anchovies crosswise in its beak during foraging dives without dropping previously caught fish.
Behavior and Ecology
Rhinoceros auklets spend the non-breeding winter months entirely out at sea, drifting hundreds of miles from land across productive oceanic shelf breaks. In spring, hundreds of thousands assemble at traditional offshore island breeding colonies (such as Protection Island in Washington and Triangle Island in British Columbia). To protect their single chick from kleptoparasitic gulls and bald eagles, breeding adults forage far offshore during daylight hours, returning to the colony only under complete cover of night. Hundreds of birds crash-land simultaneously through the dark forest canopy, scurrying across the mossy floor into subterranean burrows up to ten feet deep to deliver fresh fish to their waiting chicks.
Conservation and Human Impact
The rhinoceros auklet is evaluated as Least Concern by the IUCN, with large, thriving breeding populations spanning North America and East Asia. However, localized colonies face acute threats from commercial gillnet fisheries, where diving auklets frequently drown as bycatch. Invasive mammalian predators—particularly introduced rats, raccoons, and feral cats on breeding islands—can devastate entire colonies by devouring eggs and chicks in burrows. Ongoing eradication of invasive rodents from offshore islands, alongside satellite monitoring of pelagic feeding zones, supports the stability of North Pacific populations.
Interesting Factoid
Every autumn after breeding season ends, the rhinoceros auklet sheds its signature ‘rhino horn’ completely, spending the winter at sea with a smooth bill before growing an entirely new horn the following spring.
Last updated: October 2026