Pelagic Cormorant - Range and Ecology

Range and Ecology

The Pelagic Cormorant inhabits the shores and the neritic zone of the North Pacific. Its North American range extends from Alaska to the Baja peninsula in Mexico. It furthermore is found on the Aleutian and other Bering Strait islands, and from the Russian Far East Chukchi Peninsula via Sakhalin south to Kamchatka, and ultimately Kyūshū (though not the rest of Japan). The subarctic populations are migratory, while the birds from temperate and subtropical regions only disperse locally after breeding, but even so Asian birds may reach China or Korea. Vagrants have been recorded in the Hawaiian Islands.

On land, Pelagic Cormorants are rather clumsy and walk with the high-stepped waddling gait typical for all Sulae except darters; after landing they often scratch the ground, as is typical for cormorants. When they feel threatened, they will dart the bills at the intruder, and shake their heads and make a gargling noise. This bird forages by swimming to locate prey, then diving and going after it underwater, propelled by its feet and steering with the wings. It can dive as deep as 100 ft (30 m) to feed on or near the seafloor. Typical hunting grounds are sheltered inlets and bays; especially outside the breeding season they can also be seen fishing out at sea. They prefer to hunt in the vicinity of kelp beds or among rocks. Typical prey are smallish, bottom-living non-schooling fishes, such as Ammodytes sand eels, sculpins (Cottidae), gunnels (Pholidae) and Sebastes rockfish. Apart from fish, small crustaceans – in particular shrimp – are also often eaten. It has been observed to join mixed-species feeding flocks going after schools of young Pacific Herrings (Clupea pallasii). Like in all cormorants, due to their vestigial uropygial gland their plumage is not waterproof. Thus, the birds return to a safe place after foraging to preen and dry their feathers, typically adopting a spread-winged posture.

The Pelagic Cormorant breeds on rocky shores and islands. They do not form large colonies, but smaller groups may nest together. In some cases these birds alternate between two or three nesting sites in a region from one year to another. The nest is built at the cliff face, usually on ledges, less often in crevices or caves. The nest is built from stringy plants, such as grasses or seaweed, and held together by the birds' own guano. Once the birds have found a nest site they like, they tend to remain faithful to it for the rest of their life. The nest is repaired and improved in each season if need be; it can thus grow up to 5 ft (1.5 m) deep.

Males searching for a mate or bonding with their partner give an elaborate courtship display, as is typical for Sulae. Like all cormorants, this includes stretching the gular sac with the hyoid bone and repeated "yawning"; as in many but not all cormorants, the Pelagic Cormorant's display furthermore includes arching the neck and hopping, lifting the folded wings and rapidly fluttering them to show the white thigh patches. During the "yawning" display, the head is thrown back and calls are given which differ between males and females; when the birds land, males and females give an identical call. Otherwise, the displays are given in silence.

The clutch is generally between 2 and 5 eggs, most often 3 or 4, but clutches of up to seven eggs have been recorded. Incubation lasts for 3 weeks to one month. At hatching, the young weigh somewhat more than an ounce (35 g) and are naked, but they soon grow sooty-grey down feathers. In normal years, all young of a typical clutch may be raised successfully, but less than 4 rarely fledge. On the other hand, during unfavorable phases of the Pacific decadal oscillation, most pairs manage to raise only two young at most. They reach sexual maturity at two years of age, and a maximum age of almost 18 years has been recorded in the wild.

This numerous and widespread species is not considered threatened by the IUCN. The bulk of its population is found in the relatively inaccessible waters of the Bering Sea region; about 50,000 each breed in the Kuril Islands, the Bering Sea islands, and the U.S. state of Alaska and its offshore islands (including the Aleuts). About 25,000 breed down the Pacific coast of North America, some 60% of which are found in California. Local populations may be all but wiped out temporarily by oil spills, and on a larger scale competition with gillnet fisheries and drowning in such nets is putting a limit to its stocks.

The acanthocephalan parasite Andracantha phalacrocoracis was described from this cormorant.