Scientific name:Cerorhinca monocerata
Alternate name: Horned-billed Puffin
Japanese: Utou
Russian: Shistik Odnorogii (Unicorn Auk)
Spanish: Alcuela Rinoceronte
French: Alques á bec cornu

The Rhinoceros Auklet gets its name from a horny protrusion on the upper mandible, but because of its overall dimensions, flight and morphology, it is really a puffin.⁵⁸³ It is intermediate in size among the other puffins being smaller and lighter than the Tufted, the largest puffin. This horned-billed bird is much too large to be classed with the auklets which are considerably smaller than the Atlantic Puffin. The Rhinoceros Aukletıs behavior however, falls between the puffins and the auklets. It breeds in burrows on grassy or shrub-covered slopes of islands and like a puffin, carries several fish in its bill at a time.

The Rhinoceros Auklet breeds in large numbers on the open coasts and the calmer waters of the inside passages among the coastal islands. It burrows more actively than most alcids, extending burrows up to 8 meters in length. It communicates by sounds as well as displays, but the voice is undoubtedly more important than visual signals for communication since it is primarily a nocturnal species.

Most Rhinoceros Auklets are active at their nests only at night, despite the fact that in the southern part of the range, diurnal activity is seen. At a small colony in Sea Lion Caves on the Oregon coast some have been seen carrying fish to their nests in the caves during daylight.⁵²⁵ In this situation, however, the young are still tucked away in the dark recesses of the cave. No one really knows why these birds resort to nocturnal activity. Gulls do not seem to prey on them to any great extent, and other puffins as well as guillemots survive in the presence of gull kleptoparasitism without resorting to nocturnal feeding of young. In July, 1977, on several different days, a Rhinoceros Auklet associated with Tufted Puffins on Williamson Rocks in Rosario Strait, Washington. On one occasion the bird landed on the rocks among several Tufted Puffins and began exploring rock cavities. It entered a burrow and a few minutes later came out to rest in the sun. In the same month, Rhinoceros Auklets were flushed during the day from other small islands at the south end of Lopez Island in the San Juans. On Teuri Island in northern Japan, they are frequently seen at their nesting places in daylight, although there, they are primarily crepuscular to nocturnal as they are elsewhere.⁶⁰¹ It appears, then, that these birds are not strictly pelagic during daylight.

The Rhinoceros Auklet has been extending its range southward from a present center of distribution in southern Alaska to reoccupy islands from which the species has been absent for more than a century. Two pairs of birds were seen during the early breeding season in 1960 on the Farallon Islands, and now several pairs are known to breed there. Nesting records are established for the northern California coast and several sites are now authenticated for the Oregon coast.^{463, 524} In Washington a population of 16,162 pairs nest on Destruction Island,351 and more than 17,000 pairs breed on Protection and Smith Islands.⁶⁶² Recent colonies have been discovered also on Cleland Island at the south end of Vancouver Island where 50 pairs nest. To the north on Triangle Island, an estimate of 30,000 birds now breed.^{240, 635} The largest colony in British Columbia is on Pine Island, northeast of Vancouver Island, with about 80,000 birds.⁶³⁵ At its center of distribution for North America, on the eastern side of Forrester Island, there is a large colony of 108,000 birds.⁵⁷⁰ To the north, smaller colonies are scattered from Chowiet Island, 386 pairs, East Amatuli Island, 500 pairs; the Chiswell Islands to the Shantarskie Islands,and the north maritime territory, Sakhalin.^{30, 339} Teuri Island, Japan harbors the largest known colony with recent estimates of up to 344,000 birds.⁶⁴² Earlier reports indicated twice this number on Teuri. Although more exploration may prove differently, the species is not known to breed on the Aleutians to the west of a small colony of about twenty pairs on Near Island of the Shumagins. Total numbers are probably not over 1.2 million birds.

On Protection Island, where the population has apparently greatly expanded in recent years, Rhinoceros Auklets first appears on the slopes early in March.^{504, 662} Cleaning, repairing, and digging their burrows continues nightly through April. Burrows are extended in easy stages by 18-20 centimeters each night. Like other puffins, the Rhinoceros Auklet digs with its bill and feet, kicking the debris out behind to spray soil for a meter or more. On Protection Island, some burrows are less than a meter in length, others are up to more than 4 meters and branched, ending in one or more larger nesting chambers. Available vegetation is used to line the nest, and before the single white egg is deposited, the birds leave the island for a period varying from six to thirty-five days.⁶⁶² On Protection Island, fresh eggs may be found as early as 21 April and as late as 30 May.⁵⁰⁴ On Destruction Island, 80 percent of the eggs in 1974 and 1975 were laid within a two-week period beginning the first week of May.³⁵¹ To the north of Chowiet Island, Alaska, the eggs are laid between 20 May and 17 June.¹¹⁷

The incubating adult is averse to being exposed to daylight when removed from the burrow, and immediately seeks a hiding place rather than taking to flight. Both sexes share irregularly in the duties of the incubation period which lasts thirty-nine to fifty-two days, and averages forty-five days. The wide range in incubation time is possibly because the egg is frequently left to cool temporarily during the first few days after laying. By the fourth day after hatching, the chick is able to maintain its body temperature and is left alone in its burrow by day.⁶⁶² The chick is usually fed at dusk or after dark.

At the Japan colony in 1981, most Rhinoceros Auklets were feeding young on 4 June. At that time tens of thousands of the birds gathered on the sea about 1 km from shore as early as two hours before sunset. An hour later they began flying toward land in large numbers. They filled the sky, flying at cliff-top height, streaming in and out, continuing into the night. At dawn thousands left the cliffs synchronously rocketing down with their wings swept backward, creating a sound of a roaring wind. Others left with more normal flapping flight, tilting from side to side as they descended to sea level. A few were seen to go from, and to come to, the island during daylight hours; especially on foggy days.⁶⁰¹

To sit at night near their burrows to watch the adults approach with bills full of fish is a highlight of one's ornithological experience. They may carry as many as nine sand lances, and up to thirteen fish in one load have been recorded.⁵⁰⁴ The larger numbers consist of smaller fish but loads may vary in weight from 19 to 31 grams or between 5 and 6 percent of the adult body weight. Three to seven fish is a more usual number.³⁵¹ An adult bird approaching its nest sometimes circles past the area two or more times before dropping in close to its burrow entrance. Such circling is common to most species of alcids.

During the feeding weeks on Protection Island, most of the nocturnal activity occurs between 2200 and midnight, and 0300 to 0400, but the birds begin congregating in small groups on the water near the island about one hour before sunset. About forty-five minutes after sundown, they proceed to their nests where activity continues until just before sunrise. Variation in this arrival pattern is reported on Destruction Island, Washington, and Teuri Island, Japan, where the adults arrive before or at sunset after the eggs hatch.^{351, 601}

Depending upon the locality and the availability of food, the nestling may be fed herring, night smelt, anchovy, surf smelt, rock fish, and sand lance.^{627, 628} It receives two loads of fish each night, presumably one load carried by each parent bird. Differences occur from year to year in the availability of food, and so the growth rates of chicks will vary accordingly.³⁵¹ On Protection Island, more than 90 percent of the prey fed to the young is sand lance and herring.⁶⁶²

The Rhinoceros Auklet emits various wheezy sounds. When alarmed or caught it utters guttural growls. The calls associated with mating, aggression, and social displays include nasal wailing sounds uttered with varying pitch, stress, and number of wheezes in rapid succession. In some displays, the calls are made while vibrating partly-opened wings, with the head tilted up and back, and the mouth open in an ecstatic display, which is probably an agonistic or aggressive posture in this species.⁶⁰¹ During an all-night vigil on Protection Island, one bird carrying a load of sand lance landed less than an arm's length from me. A second bird landed between the first bird and me, almost at my elbow, and this bird stood tall, turned its bill upward, and with its bill partly opened uttered a series of short huffs. At this, the bird with the fish retreated quickly into a nearby burrow. The display of the second bird may have been a warning to the other one that it was beyond the limits of its own territory.

A Rhinoceros Auklet runs in a crouched position with its head forward, a pose which must be maintained while entering its burrow. Like other alcids, it also frequently stretches its wings, by standing tall and flapping them, a habit which adjusts the feathers and serves as a sign of relaxation after other activity.

The nestling period is calculated to last between 42 and 56 days. Some investigators have recorded shorter and longer nestling periods, perhaps due to variation in the supply of food. On Protection Island the young bird is ready to flutter out to sea when it has reached about 75 percent of the adult weight at an average age of 50 days.⁶⁶² Parent birds may keep returning to empty burrows for up to three weeks after their young has fledged.⁵⁰⁴ Young birds and adults may be seen feeding together in the Strait of Juan de Fuca and among the San Juan Islands throughout the winter, although it is generally believed that most winter in the open ocean. Many are also seen in winter off the southern coast of California.

The adult begins its postnuptial molt after breeding. It is thought that the flight feathers are shed synchronously, rendering the birds flightless until new feathers grow. Flightless birds have been seen early in August but these may have been yearlings.⁶⁶² The horn of the bill is also shed, and only a small knob remains. A young bird can be identified by the size and shape of the bill, and a year old bird apparently does not molt during the first spring; rather, it experiences an early postnuptial molt in mid-June. In the second year it gets its first prenuptial plumage by a molt beginning in March. Although we do not know at what age a young Rhinoceros Auklet is reproductively mature, we may guess that it is not different from other puffins in that it probably does not breed until after three or four winters.

Breeding success undoubtedly varies from year to year and from one location to another due to fluctuations in availability of food, weather, and other environmental influences. Higher breeding success is noted on Protection Island, a sheltered low rainfall location, than is recorded on Destruction Island off the open Washington coast. On the sheltered islands, chicks which hatch earlier in the season also tend to grow faster than those that hatch late.⁶⁶²

Foraging Rhinoceros Auklets in the Strait of Juan de Fuca, Washington may be inadvertently helpful to surface-feeding species by forcing prey fishes to the surface of the sea. Schools of sand lance of over 500 individuals may be compressed by the auklets into tight shoals near the surface making it easier for gulls to feed from above. Fish may tend to "ball" in this way to avoid the underwater predator but then fall prey to surface feeding birds.²³³

Auklets at sea, when not feeding, often assemble into rafts of a few to hundreds of resting birds. Most often, however, the birds are observed at sea as scattered individuals or pairs. When large numbers are seen together, they are usually in sheltered bays or in calm areas and upwellings between tidal currents. Auklets in the San Juan Island area suffer their largest losses in gill nets set for salmon.