Great Cormorant Life History

Habitat

Great Cormorants in North America inhabit rocky sea coasts throughout the year. During breeding season, they nest on rocky islands or cliffs, usually on the ground but sometimes in stunted conifers. At this time of year, they avoid disturbance, usually perching as far from human activities as feasible and selecting nest areas without mammalian predators. Typically, they perch on rocks when not foraging, but they also use other elevated perches, including pilings, shipwrecks, buoys, channel markers, and jetties. Their preferred habitats during migration and winter are essentially identical, though small numbers now overwinter in major rivers of the mid-Atlantic states, where sometimes seen on sandy shorelines, sandbars, and edges of river ice. Increasingly often, single Great Cormorants have turned up far inland, to the Great Lakes, Finger Lakes of New York, and southward to reservoirs of the Southeast, almost always east of the Appalachian Mountains. In such contexts, they may perch on water control structures, dead trees, piers, and other sites used by the more common Double-crested Cormorant.

Food

In North America, Great Cormorants eat mostly bottom-dwelling fish. Their heavy, hooked bills are well suited for extracting fish from crevices in submerged rocks. When foraging, they dive from the surface with a small leap or lunge (unlike Double-crested Cormorant) and swim using their feet to reach the bottom of shallow inshore waters, normally less than 30 feet deep, though they can dive past 100 feet, deeper than the smaller Double-crested Cormorant. Underwater, they use their feet and tail to steer themselves. Prey include sculpins, rock gunnel, pollock, cunner, mummichog, Atlantic cod, winter flounder and other flatfishes, and tautog. They also take schooling fish such as sandlance and capelin, and small crustaceans such as crab (though these smaller items could be prey taken by the fish the cormorants have eaten). They swallow some prey while still submerged but bring larger fish to the surface to kill and soften before swallowing. In some areas, they forage more often in the morning or on ebb tides. They are excellent predators: careful studies have demonstrated that some individuals forage for as little as 20 minutes per day in summer.

Nesting

Nest Placement

Males choose flat ground on rocky islands and cliffs for nest sites, from near waterline to over 100 feet above the water level. Some nests are constructed on stunted trees or structures like ruined buildings or wrecked ships. The species nests colonially in some locations, with nests just a few feet apart from each other.

Nest Description

Nest is a mound of marine algae and sticks. Males bring the material to the female, which places it to make the nest. Nests measure on average about 19 inches across and 8.8 inches high.

Nesting Facts

Behavior

Great Cormorants form flocks year-round, even in nesting areas, but they seldom gather in large flocks like Double-crested. They spend most of the day quietly perched out of the water, preening, stretching, and resting. When foraging, they dive from the surface and swim underwater, but their plumage lacks the waterproofing of ducks and loons, so they come ashore between foraging sessions to warm and to dry the plumage, often holding the wings open for long periods. In spring, males establish very small territories, essentially just the nest site itself. Males display for females from the nest site by raising their wings repeatedly, exposing their bright-white flank patches. Females indicate interest by engaging males in elaborate courtship displays known as gargling, pointing, preflight, post-landing, hop, and kink-throat. These displays continue through nesting and chick-rearing, helping the pair to remain bonded. When another male encroaches on the nest, the male of the pair threatens him by leaning forward, raising the tail, opening the wings and bill, and wagging the head erratically. Males and females also react to intruders with what is called “nest worrying,” in which they grasp a piece of vegetation in the bill and shake it. Both male and female care for the young, and most pairs are monogamous, although extrapair copulations are not uncommon in European populations. Some pairs nest again together in the subsequent breeding season.

Conservation

Great Cormorant populations in northeastern North America appear to be stable, with the exception of Maine, where in 2016 the species was noted to be in steep decline. Partners in Flight estimates the global breeding population to be 1.4 million (most of these are outside North America) and rates the species a 9 out of 20 on the Continental Concern Score, indicating it is a species of low conservation concern. In 2002, the North American Waterbird Conservation Plan estimated a North American breeding population of 11,600 birds, consistent with more recent estimates by the Canadian Wildlife Service. Great Cormorants frequently drown when trapped in gill nets and other nets set for fish. They may also suffer from marine oil pollution. Although Great Cormorants are not hunted for food, the wanton killing of cormorants as a pest species was common until recent times, fueled by assumptions that the birds competed with fishermen for fish. Great Cormorants do not eat fish of commercial or recreational value but are occasionally still killed along with Double-crested Cormorants. In Maine, the species is thought to be in decline from Bald Eagle predation.

Credits

Government of Canada. (2014). Status of Birds in Canada: Great Cormorant (Phalacrocorax carbo).

Hatch, Jeremy J., Kevin M. Brown, Geoffrey G. Hogan and Ralph D. Morris. (2000). Great Cormorant (Phalacrocorax carbo), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

Kushlan, J. A., M. J. Steinkamp, K. C. Parsons, J. Capp, M. A. Cruz, M. Coulter, I. Davidson, L. Dickson, N. Edelson, R. Elliott, R. M. Erwin, S. Hatch, S. Kress, R. Milko, S. Miller, K. Mills, R. Paul, R. Phillips, J. E. Saliva, W. Sydeman, J. Trapp, J. Wheeler and K. Wohl (2002). Waterbird conservation for the Americas: The North American waterbird conservation plan, version 1. Washington, DC, USA.

Maine Department of Inland Fisheries and Wildlife. (2016). Maine 2015 Wildlife Action Plan Revision: Phalacrocorax carbo (Great Cormorant). Maine Department of Inland Fisheries and Wildlife, Augusta, Maine.

Partners in Flight (2017). Avian Conservation Assessment Database. 2017.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

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