Cassin's Auklet

Cassin's Auklet is a small, unassuming seabird of the North Pacific, lacking the bold ornaments of many of its relatives and instead sporting a uniform sooty grey-brown plumage with a slightly paler belly. A small pale spot above the eye and a subtly pale-based bill are among the few distinguishing marks on an otherwise plain bird. Its stout body and short wings give it a compact, rounded silhouette both on the water and in flight.

Despite its plain appearance, Cassin's Auklet plays an outsized ecological role as one of the most numerous seabirds of the California Current and other North Pacific upwelling zones, where it forms large nesting colonies on offshore islands.

Key field marks

• Small, compact body with sooty grey-brown plumage
• Slightly paler grey belly
• Small pale spot above the eye
• Short, stubby bill with a pale spot near the base of the lower mandible

Similar species

• Rhinoceros Auklet is considerably larger with a distinctive bill horn in breeding season.
• Storm-petrels are superficially similar in size and dark coloring but have a very different flight style, with fluttering, erratic wingbeats compared to the auklet's direct, whirring flight.
• Juveniles closely resemble adults, making age generally difficult to determine in the field.

Cassin's Auklets breed in burrows on offshore islands along the North Pacific coast of North America, from Baja California north through California's Farallon Islands, British Columbia, and into the Aleutian Islands of Alaska. They forage over productive upwelling zones, particularly the California Current, where cold, nutrient-rich waters support dense concentrations of krill and other zooplankton.

Most populations remain within the North Pacific year-round, though some dispersal occurs outside the breeding season as birds track shifting concentrations of their planktonic prey.

Behavior

Like many small auks, Cassin's Auklets visit their nesting burrows mainly at night to reduce the risk of predation by gulls, and forage during the day by diving and swimming underwater in pursuit of tiny prey.

Voice

A harsh, creaking or growling call is given at night around burrows; the species is quiet while foraging at sea.

Feeding

Krill and other zooplankton dominate the diet, supplemented by small fish, gathered during pursuit dives close to the surface or at moderate depths.

Nesting and breeding

Pairs nest in burrows dug into soil on offshore islands, sometimes raising two broods in a single season when food is abundant, an unusual trait among auks; both parents share incubation and chick-provisioning duties, visiting the burrow under cover of darkness.