Rhinoceros Auklets (sometimes called Horn-billed Puffins or Unicorn Puffins) are incredible birds that demonstrate how diverse nature can be. Since they tend to be nocturnal during breeding season or far out at sea the rest of the year, not a lot is known about these mysterious birds! Recent research is bringing new aspects of the species into the light and we are still learning more about them all the time.

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Natural History of the Rhinoceros Auklet

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What is a Rhinoceros Auklet?

A Rhinoceros Auklet is a medium-sized seabird with a heavy body and short neck. They are mostly dark grey in colour with a white underside. The bill is relatively large and yellowish-orange in colour, and they have large webbed feet attached to short little legs. Their wings are broad and pointed for lots of swimming and diving underwater. Females and males exhibit the same characteristics, except that the males are slightly larger (Addison, 2008). To put them in perspective, the average Rhinoceros Auklet is 28-29 cm long and weigh 350-620 g (Cornell University: All About Birds).

Depending on the season, Rhinoceros Auklets may look quite different. These differences are referred to as breeding and nonbreeding plumage. Rhinoceros Auklets don their breeding plumage in early spring by growing a white horn on top of their bill and two white tufts on their face. At the end of the season in late summer, they will shed their horn and lose their white tufts to take on their nonbreeding plumage. Juvenile Rhinoceros Auklets closely resemble nonbreeding adults except for a smaller and darker bill (Cornell University: All About Birds). Despite having a similar appearance to other species, a Rhinoceros Auklet in full breeding plumage is hard to miss!

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Rhinoceros Auklets are quiet at sea, but during breeding season on land they make strange noises. Near colonies, they make about ten low nasally mooing noises. They can also make short barking noises and groaning calls (Sibley, 2016).

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How do Rhinoceros Auklets survive in the middle of the ocean?

Rhinoceros Auklets survive quite successfully on the ocean, commonly living well into their 20’s (Hipfner, 2019). The oldest recorded Rhinoceros Auklet was banded in British Columbia in 1984, making it over 26 years old when it was found again in 2010 (Cornell University: All About Birds).

One of the most important parts of survival in the vast ocean is having an efficient way to feed. Rhinoceros Auklets primarily eat small fish, and will even occasionally snack on some zooplankton, crustaceans, or squid (Cornell University: All About Birds). They typically forage at underwater depths above 10 meters, but they are capable of diving below 30 meters. In order to catch lots of small slippery fish at once, Rhinoceros Auklets have developed spikes in the back of their upper jaw called palatal denticles (Cornell University: All About Birds). This adaptation is vital to bring back large meals for their growing nestlings.

Rhinoceros Auklets often hunt for food in large flocks, which may include other seabird species that are after similar meals, such as loons or gulls. A common strategy to catch fish is to approach the fish from below and drive them up to the surface. Research suggests that groups of Rhinoceros Auklets may work together to force schools of fish into “bait balls” by trapping them between the ocean surface and themselves from below (Cornell University: All About Birds).

Since they spend much of their life on the ocean, Rhinoceros Auklets are fantastic swimmers. Their sturdy wings act as flippers to propel them through the water and they use their webbed feet to steer (Cornell University: All About Birds). They can even stay underwater for over two minutes long (BirdWeb).

Flying is a little trickier for these seabirds. Takeoff requires a lot of energy and may look clumsy compared to lighter, more elegant birds. In fact, research suggests that they may prefer nesting on slopes towards the sea because it is easier for them to takeoff towards the water (Seattle Aquarium). Once the Rhinoceros Auklet is in the air though, they are fast fliers and can travel long distances. They steadily flap their wings without ever gliding and fly low to the water (Audubon).

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Where do Rhinoceros Auklets come from?

Raising a Rhinoceros Auklet into adulthood is a time consuming process for the parents, but is rewarding in the end. It all begins with finding a mate. Rhinoceros Auklets have a monogamous mating system, and although they will separate during migration they often repartner with the same mate from the previous year (Kubo, 2018). To form a pair bond, they will orchestrate courtship displays such as tapping their beaks together or marching in a hunched position. This may occur at sea or near their nest. They will also demonstrate ownership of their chosen nest site by widely opening their wings and pointing their bill up to hiss loudly (Audubon).

Rhinoceros Auklets breed in dense colonies on the edges of islands high above the tide line. They seek out areas with lots of soil or grass for their burrows, and may even choose sites in forests. Rhinoceros Auklet couples dig long burrows together that can reach up to 6 meters in length. At the end of the burrow they build a nest, which is a shallow depression lined with moss, grass, and sticks they have collected together. When the nest is deemed suitable, the female lays a single egg inside (Cornell University: All About Birds).

Both parents take turns incubating their beloved egg for about 45 days (BirdWeb). When the egg hatches, the newborn Rhinoceros Auklet is covered in down feathers and can already walk (Cornell University: All About Birds). The baby stays in the nest for another 50-55 days and is fed by its parents until its big enough to survive on its own (Hipfner, 2019).

While the baby sleeps soundly in its warm nest, its parents are out hunting for food. Researchers have observed Rhinoceros Auklets waiting in the water with a bill full of fish until dark to safely return to the nest. This may be a strategy to prevent other seabirds from stealing their hard earned meal (Cornell University: All About Birds).

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Where am I most likely to spot a Rhinoceros Auklet?

Rhinoceros Auklets are found in the Pacific Ocean from California all the way up to the Aleutian Islands in North America. They can even be found in parts of Japan, North Korea, and Russia. They live on open ocean but usually no more than 20 kilometers from shore. Whale-watching tours in Puget Sound and the Salish Sea often come across Rhinoceros Auklets and occasionally they can be spotted from shore if they are chasing an especially tasty school of fish (Cornell University: All About Birds). With enough patience and good binoculars, you might get lucky.

During breeding season, Rhinoceros Auklets stay closer to land. They are considered pelagic birds, meaning they live on open ocean and only return to land to breed. Nesting sites are chosen on islands in the North Pacific with suitable burrowing land. They prefer to forage close to their colony, especially in tidal currents that create upwellings which bring prey close to the surface (Cornell University: All About Birds). You may think this would be a great way to spot a Rhinoceros Auklet, but they aren’t reported much on islands during breeding season due to their nocturnal tendencies.

Rhinoceros Auklets have a decently sized migratory route, with some overlap between summer and winter ranges. They follow the trend of flying south for winter, and end up travelling all the way down to California (BirdWeb). In the summer they fly back up north to build a nest and start a family.

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Are Rhinoceros Auklets at risk?

Rhinoceros Auklets appear to be fairly common, but little is truly known about their population trends. In 2017, Partners in Flight estimated the global Rhinoceros Auklet population to be approximately 1.5 million birds (Cornell University: All About Birds). Numbers seem to vary year to year because the species is quite sensitive to disturbances during their nesting period, and adults will even abandon their nests during incubation if threatened. A large, well-studied colony on Protection Island, Washington has been declining in population since the 1990s, but there are some indications that populations may be increasing in specific areas on the West Coast as further research is being done (BirdWeb).

Overall, the Rhinoceros Auklet population appears to be declining but it is rated at “Least Concern”. The reasoning behind this is because the population decline rate is fairly slow, the population size is so large that they won’t become endangered any time soon, and they have a vast expanse of habitat to live on (BirdLife). While they may not be of much concern, they are still suffering from effects of a changing world. Humans have introduced numerous threats to the species, including invasive mammal predators, oil spills, climate change, fishing nets, and pesticides (Cornell University: All About Birds).

Some actions we can take to better conserve seabirds are cleaning the ocean, banning single-use plastics, promoting sustainable and safe fishing gear that won’t trap animals, and eradicating invasive species that may affect seabirds such as rats (NABCI Canada). Even taking small actions to help combat climate change will help seabirds and many other species on the planet.

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Recent Research on the Rhinoceros Auklet

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Why are Rhinoceros Auklets sometimes considered the fourth puffin species?

The Rhinoceros Auklet (Cerorhinca monocerata) is a member of the order Charadriiformes and family Alcidae, which includes species like the Parakeet Auklet and Tufted Puffin. “Rhinoceros Auklet” is actually a very misleading name. Rhinoceros Auklets were originally grouped with auklets because they share similar physical characteristics, but recent studies have determined they share a more recent common ancestor with puffins (Smith, 2011). Some scientists have even proposed to change their name to the Rhinoceros Puffin.

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Rhinoceros Auklets are the only living member of the genus Cercorhinca, but millions of years ago there were at least five more species of this genus that lived in the ocean as well (Cornell University: All About Birds). In today’s day their closest cousins are the three puffin species: the Atlantic Puffin, the Horned Puffin, and the Tufted Puffin.

To classify the order Charadriiformes, a detailed study was conducted in 2011. Extinct species were evaluated by direct fossil observation, while species that are alive today were examined more thoroughly. When available, five or more adult specimen were chosen for a more accurate evaluation. Species were also chosen from multiple locations across their range to account for geographical variation (Smith, 2011).

In order to classify these species in the most accurate way, a phylogenetic analysis was conducted. This is a systematic process that quantifies specific traits of each species and determines similarities and differences. 344 morphological characteristics were scored, including skeletal features, eye colour, feather structure, foot webbing, and egg shape (Smith, 2011).

It may be a strange concept to wrap your head around, but here is a simple example of how it works. Rhinoceros Auklets share the common trait of webbed feet with Marbled Murrelets (Brachyramphus marmoratus), but a type of plover called the Killdeer (Charadrius vociferus) does not have webbed feet. This suggests to us that Rhinoceros Auklets are more closely related to Marbled Murrelets than Killdeer because they have more characteristics in common.

After scoring the characteristics of all these species, a computer is used to calculate the most probable path that evolution took place based on using the least amount of evolutionary steps (Smith, 2011). This is because it is more likely that webbed feet evolved a single time and was passed down to all descendants than to have evolved several times independently. With the data collected, a cladogram was created to illustrate the evolution of birds in the order Charadriiformes. This cladogram is by no means a final draft, as scientists are continuously making new discoveries all the time that may challenge what we currently believe.

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Why have Rhinoceros Auklets evolved a horn?

Rhinoceros Auklets are named after the keratin-based horn they grow during breeding season at the base of their upper mandible. The true purpose of this horn has always been a mystery, but recent research gives us some useful clues.

In 2018, the Atlantic Puffin was discovered to have fluorescent properties in its bill (Wilkinson, 2019). This was an extraordinary discovery, so they wanted to test if similar species had the same properties. Rhinoceros Auklets were chosen for a study because of their close relation to puffins. This led to the discovery that Rhinoceros Auklets have fluorescent horns!

To determine if Rhinoceros Auklets displayed fluorescent properties, a study was conducted in 2019 off the coast of central California. Mist nets were set up around burrows nine nights in a row, and scientists patiently waited until night for Rhinoceros Auklets to return to their burrows with a bill full of fish. When one got caught in the net, it was carefully extracted and processed. Special care was taken to cover the birds’ eyes when exposed to the UV light. 35 Rhinoceros Auklets were caught and photographed in total, and they were quickly released back into the darkness to carry on with their lives (Wilkinson, 2019).

Every single Rhinoceros Auklet caught in the study displayed a blue/green glow in their horn when exposed to UV light with slight variation of location on the bill. The majority of the fluorescence appeared in the horn, indicating that the horn may have a greater purpose than scientists originally thought. The level of fluorescence didn’t seem to be dependent on sex, but it did vary among individuals (Wilkinson, 2019).

Now that we know the horn is fluorescent, that leaves us to wonder about the reasoning behind it. Birds are able to detect a much wider range of colour than humans, including the UV range, so Rhinoceros Auklets can likely see each others brightly coloured horns. This unique feature may help Rhinoceros Auklets see each other when they hunt together underwater, or at their breeding colonies in the middle of the night (Cornell University: All About Birds). Alternatively, the fluorescence may help in mate selection, especially since many social interactions take place during dawn or dusk when the UV spectrum is enhanced. Perhaps the fluorescent horn is instead used to identify individual birds in a colony since they are a social species and are able to recognize their mate after a long separation (Wilkinson, 2019). More research needs to be conducted on Rhinoceros Auklets to come up with a better explanation, but a glowing horn is an exciting discovery all on its own!

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References

Addison, B., Kitaysky, A.S., & Hipfner, J.M. (2008). Sex
allocation in a monomorphic seabird with a single-egg clutch: Test
of environment, mate quality, and female condition hypotheses.
Behavioral Ecology and Sociobiology, 63, 135–141. doi:10.1007/
s00265008-0643-z

Audubon. (n.d). Rhinoceros Auklet. Retrieved from https://www.audubon.org/field-guide/bird/rhinoceros-auklet.

Birdlife International. (2020). Species factsheet: Cerorhinca monocerata. Retrieved from http://datazone.birdlife.org/species/factsheet/22694924.

BirdWeb. (n.d.). Rhinoceros Auklet. Retrieved from http://www.birdweb.org/birdweb/bird/rhinoceros_auklet.

Cornell University: All About Birds. (n.d.). Rhinoceros Auklet. Retrieved from https://www.allaboutbirds.org/guide/Rhinoceros_Auklet/id.

Hipfer, J.M., Shervill, D., Domalik, A.D., Bertram, D.F., Lemon, M.J.F., Rodway, M.S., Smith, C., & Hudson, S.A. (2019). Longevity in the Rhinoceros Auklet Cerorhinca monocerata and a comparison with other species of alcidae. Marine Ornithology, 47, 225-227.

Kubo, A., Takahashi, A., Thiebot, J., & Watanuki, Y. (2018). Rhinoceros Auklet pair‐mates migrate independently but synchronize their foraging activity during the pre‐laying period. International Journal of Avian Science, 160 (4), 832-845. https://doi-org.ezproxy.viu.ca/10.1111/ibi.12583

NABCI Canada. (2019). The State of Canada’s Birds 2019. http://nabci.net/resources/state-of-canadas-birds-2019/.

Seattle Aquarium. (n.d.). Rhinoceros Auklet. Retrieved from https://www.seattleaquarium.org/animals/rhinoceros-auklet.

Sibley, D.A. (2016). Sibley Birds West: Field Guide to Birds of Western North America (2nd ed.). New York, NY: Alfred A. Knopf.

Smith, N.A. (2011). Taxonomic revision and phylogenetic analysis of the flightless Mancallinae (Aves, Pan-Alcidae). Zookeys, 91, 1-116. doi: 10.3897/zookeys.91.709

Wilkinson, B.P., Johns, M.E., & Warzybok, P. (2019). Fluorescent ornamentation in the Rhinoceros Auklet Cerorhinca monocerata. International Journal of Avian Science, 161 (3), 694-698. https://doi.org/10.1111/ibi.12715