Rhinoceros Auklets

The Rhinoceros Auklets, Cerorhinca monocerata, is a seabird considered, despite its name, a close relative of the puffins. It is the only living species of the genus Cerorhinca. Given its close relationship with the puffins, the common name Rhinoceros Puffin has been proposed for the species.

Its name is derived from a horn-like extension of the beak. This horn is only present in breeding adults, and like the elaborate sheath on the bill of puffins is shed every year.

It ranges widely across the North Pacific feeding on small fish and nesting in seabird colonies.

Description

The Rhinoceros Auklet (also known as the Rhino Auklet, Horn-billed Puffin, or Unicorn Puffin), is a medium-sized auk with a large, strong, orange/brown bill (with the ‘horn’ protruding from it).

The plumage is dark on top, paler below, and the breeding adults, both male and female possess white plumes above the eyes and behind the bill. Males are slightly larger than females (about 10% in mass).

The Rhinoceros Auklet’s plumage is darker on top and lighter underneath, which provides it with an effective camouflage when swimming in the ocean. This coloration allows the bird to blend in with the darker water from above, and the lighter sky from below. It makes it more difficult for predators. During the breeding season both the male and female Rhinoceros Auklets grow striking white plumes behind their bill and above their eyes. These feathers add to the already striking appearance of these birds and help them stand out during courtship displays. The combination of these plumes and the hornlike protrusion creates a look which is both elegant and unusual.

Males tend to be slightly bigger than females. They are typically 10% heavier. The size difference between males and females is common in many bird species. Males are typically larger or more robust, which helps them defend their territory and attract mates. This slight size difference also has an impact on the Rhinoceros Auklet’s mating dynamics, as larger males have an advantage when competing for nesting sites and securing mating opportunities. Both sexes have similar plumage patterns. This makes them look similar, and only slight differences exist in their size or intensity of seasonal plumes.

Rhinoceros Auklets are a fascinating species to birdwatchers, naturalists, and anyone who is lucky enough in the field to see them.

Distribution / Range

The Rhinoceros Auklet is a North Pacific auk that breeds from California (the Channel Islands) to the Aleutian Islands in Alaska in North America; and Hokkaidō and Honshū, Japan, as well as North Korea and Sakhalin Island in Asia.

It winters both in offshore and inshore waters, exhibiting some migration.

Breeding / Nesting

The Rhinoceros Auklet nests in burrows dug into the soil or in natural caves and cavities between 1 to 5 m deep. It prefers nesting sites on slight inclines to aid take-off (as it is a poor flier).

A single egg is incubated by both parents for 45 days, the semiprecocial chick is then fed each night with a bill full of fish (in the manner of puffins) for 50 days. The nocturnal (active at night) behaviour is believed to be a response to predation and kleptoparasitism by gulls.

Rhinoceros Auklets are nocturnal nesters. This means that they do the majority of their nesting, and other activities at night. This helps them avoid predators like gulls and ravens that may attack their eggs or chicks in the daylight. Rhinoceros Auklets nest in crevices and burrows, usually in caves or on rocky shorelines. This allows them to protect their eggs from predators. According to Cornell Lab of Ornithology, this species is well documented.

Nesting sites are usually small and hidden in rock or soil. They’re often located far away from human settlements. Auklets communicate using high-pitched calls and whistles during the breeding season. During this period, the birds are very territorial and each pair maintains a particular nest site. According to RSPB, this species is well documented.

Both parents incubate the egg laid by the female. The egg has a smooth, pale color with speckles. The male and female incubate the egg alternately, usually switching shifts at night. The parents will continue to feed the chick small fish, and other marine food, until the chick is ready to fly.

Rhinoceros Auklet hatchlings are precocial. This means that they are born with eyes wide open and can move about immediately after hatching. They still depend on their parents to provide food and protect them until they can fly and dive independently. After a few weeks, the chicks are more independent. They can eventually leave the nest to begin their life in the ocean.

Diet / Feeding

At sea Rhinoceros Auklets feed on fish, with some krill and squid taken also. They feed inshore during the breeding season in the midwater. To catch their prey, they dive to as deep as 57 meters (187 ft) for as long as 148 seconds.

Rhinoceros Auklets have a remarkable ability to dive under the surface of water and hunt for prey. The birds are good swimmers and use their wings to glide gracefully under the water in pursuit of their prey. Rhinoceros Auklets dive to depths up to 30 meters to capture small fish, krill and other marine organisms. The main food they eat is small fish such as anchovies and anchovies. They also eat crustaceans, squid, and herring.

Auklets hunt by diving quickly and deep, often holding multiple fish in their bill. This style of hunting allows them to bring large quantities of food to their nests. This is important, especially during breeding season, when they must feed their young. Birds have special adaptations to their bills which allow them to carry multiple fish, up to five depending on the size.

The Rhinoceros Auklet hunts alone, and spends most of its time on the ocean, outside of the breeding period. It is sometimes seen in large groups when it feeds, but prefers to stay to itself. The solitary nature of this seabird outside of breeding season, combined with its exceptional diving ability, makes it elusive.

Evolution and prehistoric species

The genus Cerorhinca evolved in the North Pacific, apparently in the mid-late Miocene. Although today only one species remains, it used to be much more diverse, both in number of species and in distribution; fossils have been found as far south as Baja California. Known prehistoric species are:

• Dubious Auklet, Cerorhinca dubia (Late Miocene of San Barbara County, USA)
• Cerorhinca minor (Late Miocene/Early Pliocene of Cedros Island, Mexico)

• Cerorhinca reai (San Diego Late Pliocene, SW USA)