Rhinoceros Auklet vs Cassin's Auklet

Rhinoceros Auklet

Cerorhinca monocerata

How to recognise a rhinoceros auklet?

General appearance

The Rhinoceros Auklet is a medium-sized seabird primarily characterized by a robust body and a distinct horn-like structure on its beak during breeding seasons. Its coloration features a dark, slate-colored plumage with a contrasting white belly. The bird's head is slightly darker, and it possesses small white facial plumes that give the appearance of whiskers. It has a stout, orange-yellow bill with a prominent bright orange horn in mature individuals during the breeding period, which is less prominent in non-breeding seasons.

Male vs female

Males and females of the Rhinoceros Auklet species are quite similar in appearance, making it challenging to distinguish between sexes based solely on visual observation. Both sexes exhibit the same plumage and bill characteristics. However, males might appear slightly larger and more robust than females, a difference observable mainly when both sexes are in proximity.

Juvenile

Juvenile Rhinoceros Auklets differ from adults primarily in plumage and bill structure. Young birds display duller and more uniform gray tones, lacking the defined contrast between their upper bodies and bellies seen in adults. Additionally, juveniles do not have the distinctive horn on their bills. The bill appears shorter and less colorful. As they mature, juveniles gradually acquire adult plumage and the characteristic bill horn that defines mature Rhinoceros Auklets.

Size

12.6 - 15.0 in/14.1 - 24.7 oz

Habitat

Primary habitat

Rhinoceros Auklets primarily inhabit the northern Pacific Ocean, spanning from Alaska to Northern California. They favor rocky coastlines and small islands for breeding, often returning to the same nesting sites each year. They thrive in cool, marine environments with rich food sources, such as schools of fish and squid that are abundant in these waters.

Additional range

Although commonly found in cooler climates, Rhinoceros Auklets can occasionally be spotted further south along the Pacific Coast. They may also adapt to different coastal conditions, including sandy shores or sheltered bays, depending on food availability and competition from other seabirds.

Range Map

What do rhinoceros auklets like to eat?

Natural diet

The Rhinoceros Auklet is a seabird with a diet primarily composed of fish and aquatic invertebrates. In the wild, they feast on small fish such as anchovies, sand lance, and herring. These birds are adept divers, able to plunge beneath the water to capture their prey efficiently. Depending on the season, their diet may vary slightly based on the availability of specific fish species. During the breeding season, they might carry multiple fish to their chicks, indicating a dietary focus on higher protein content to ensure the chicks' growth.

Bird feeders

Rhinoceros Auklets are not typically attracted to bird feeders due to their marine-based diet. They hunt and feed at sea, predominantly requiring a supply of fish that are unavailable in backyard settings. Thus, they do not consume the typical seed mixes, suet, or fruits found at feeders for land-based birds. For those interested in observing Rhinoceros Auklets, visiting coastal areas during their feeding times may provide a better view than attempting to attract them to a bird feeder.

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Cassin's Auklet

Ptychoramphus aleuticus

How to recognise a cassin's auklet?

General appearance

The Cassin's Auklet is a small, plump bird with a length ranging from 19 to 25 cm. This seabird exhibits a stocky build and short wings. Its plumage is predominantly gray with a darker tone on the upper side, while the underparts are lighter gray. The face is marked by a small white crescent above the eye, and the cheeks bear a slightly lighter patch. Its bill is short and stout, appearing black, and its legs and feet tend to exhibit a bluish-gray tone.

Male vs female

Cassin's Auklets show very little sexual dimorphism, meaning males and females look very similar. Both sexes sport identical coloration and size, which makes it challenging to differentiate them in the field. Observation of breeding behavior is generally necessary for accurate identification between males and females.

Juvenile

Juvenile Cassin's Auklets resemble adults but display a few key differences. Their plumage is slightly duller with a more uniform gray tone, lacking some of the sharp contrast seen in adults. The white crescent above the eye may not be as pronounced, and the bill is smaller and less robust, appearing paler in hue. As juveniles mature, they gradually acquire the definitive adult plumage.

Size

9.8 in/5.6 - 7.1 oz

Habitat

Primary Habitat

Cassin's Auklet primarily inhabits the cool, coastal waters off the Pacific coast of North America, ranging from Alaska to Baja California. It prefers open seas and offshore islands, nesting in burrows dug into grassy slopes or sandy soil near the shore. The bird thrives in cold water climates, often dependent on upwelling zones rich in marine food resources.

Additional Range

While less common, the bird occasionally enters more temperate waters further offshore during non-breeding seasons. It also exhibits a degree of adaptability by nesting on islands with varying vegetative cover, demonstrating its flexibility to thrive in diverse environmental conditions.

Range Map

What do cassin's auklets like to eat?

Natural Diet

Cassin's Auklet, scientifically recognized as Ptychoramphus aleuticus, has a diet that predominantly consists of small marine creatures. These adept seabirds primarily feast on zooplankton, including copepods, krill, and small fish. Their feeding patterns may demonstrate seasonal variation, with krill becoming more significant in their diet during periods of abundance, typically in spring and early summer. These seabirds rely on dense swarms of these small sea creatures that occur due to ocean currents and upwellings. During winter months, when food availability decreases, Cassin's Auklets may extend their range in search of sufficient nourishment.

Bird Feeders

Cassin's Auklets are not commonly seen at bird feeders and typically do not consume standard feeder offerings. These seabirds are highly adapted to their marine environment and their diet consists mainly of natural resources found in oceanic habitats. Thus, they do not show interest in foods such as seed mixes, suet, or fruits that are often provided by backyard bird feeders. As these birds are primarily pelagic, they rarely come into contact with typical feeder setups found on land. Encouraging the presence of Cassin’s Auklets is best achieved through conservation of their natural marine habitats rather than offering supplemental feeder food.

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Rhinoceros Auklet

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Cassin's Auklet

Rhinoceros Auklet

Cerorhinca monocerata

General appearance

The Rhinoceros Auklet is a medium-sized seabird primarily characterized by a robust body and a distinct horn-like structure on its beak during breeding seasons. Its coloration features a dark, slate-colored plumage with a contrasting white belly. The bird's head is slightly darker, and it possesses small white facial plumes that give the appearance of whiskers. It has a stout, orange-yellow bill with a prominent bright orange horn in mature individuals during the breeding period, which is less prominent in non-breeding seasons.

Male vs female

Males and females of the Rhinoceros Auklet species are quite similar in appearance, making it challenging to distinguish between sexes based solely on visual observation. Both sexes exhibit the same plumage and bill characteristics. However, males might appear slightly larger and more robust than females, a difference observable mainly when both sexes are in proximity.

Juvenile

Juvenile Rhinoceros Auklets differ from adults primarily in plumage and bill structure. Young birds display duller and more uniform gray tones, lacking the defined contrast between their upper bodies and bellies seen in adults. Additionally, juveniles do not have the distinctive horn on their bills. The bill appears shorter and less colorful. As they mature, juveniles gradually acquire adult plumage and the characteristic bill horn that defines mature Rhinoceros Auklets.

Size

12.6 - 15.0 in/14.1 - 24.7 oz

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Cassin's Auklet

Ptychoramphus aleuticus

General appearance

The Cassin's Auklet is a small, plump bird with a length ranging from 19 to 25 cm. This seabird exhibits a stocky build and short wings. Its plumage is predominantly gray with a darker tone on the upper side, while the underparts are lighter gray. The face is marked by a small white crescent above the eye, and the cheeks bear a slightly lighter patch. Its bill is short and stout, appearing black, and its legs and feet tend to exhibit a bluish-gray tone.

Male vs female

Cassin's Auklets show very little sexual dimorphism, meaning males and females look very similar. Both sexes sport identical coloration and size, which makes it challenging to differentiate them in the field. Observation of breeding behavior is generally necessary for accurate identification between males and females.

Juvenile

Juvenile Cassin's Auklets resemble adults but display a few key differences. Their plumage is slightly duller with a more uniform gray tone, lacking some of the sharp contrast seen in adults. The white crescent above the eye may not be as pronounced, and the bill is smaller and less robust, appearing paler in hue. As juveniles mature, they gradually acquire the definitive adult plumage.

Size

9.8 in/5.6 - 7.1 oz

There's more to learn about Cassin's Auklet

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How to tell the Rhinoceros Auklet from the Cassin's Auklet

Visual differences

The Rhinoceros Auklet is larger, weighing 14-25 oz (400-700g) and measuring 12.6-15 inches (32-38 cm), compared to Cassin's Auklet at 5.6-7 oz (160-200g) and 7.5-9.8 inches (19-25 cm). The Rhinoceros Auklet's slate-colored body features a distinctive horn-like structure on its orange-yellow bill during breeding. In contrast, Cassin's Auklet showcases a darker gray plumage with lighter underparts, a small white eye crescent, and a short, black bill. Rhino Auklet’s bright beak and horn contrast sharply with Cassin’s more uniform appearance.

Key differences and behavior

The Rhinoceros Auklet and Cassin's Auklet exhibit distinct behavioral and ecological differences. Rhinoceros Auklets, weighing 14-24 oz and measuring 12.6-15 in, are larger than the 5.6-7 oz, 7.5-9.8 in Cassin's Auklets. Both nest in burrows, but Rhinoceros Auklets prefer rocky coastlines and islands, while Cassin's opt for offshore islands with grassy slopes. Rhinoceros Auklets are fish-centric daytime foragers, whereas Cassin's focus on zooplankton like krill and copepods. The former engages in complex nocturnal courtship displays, while the latter uses synchronized swimming. Rhinoceros Auklets are monogamous, forming strong pair bonds, whereas Cassin's are socially monogamous, with slight seasonal flexibility. Both parents share incubation but Rhinoceros Auklets only lay one egg, while Cassin's lay up to two.