Largest of the 6 cormorants in North America, the Great Cormorant is restricted as a breeder in the Western Hemisphere to the coasts of the northwestern Atlantic, from Maine north to West Greenland. In this area it is almost entirely a marine bird. The species is nearly cosmopolitan, also breeding in Europe, Asia, Africa, and Australasia, and is the most widely distributed of all the cormorants. Throughout much of this huge range, extending from arctic (74°N) to tropical to south temperate (47°S), it is generally a bird of freshwater lakes and rivers, rather than the sea cliffs and rocky islands characteristic of the distinctive North Atlantic subspecies. In western Europe, the numbers of coastal breeders have increased slowly but inland populations have grown enormously in recent decades, resulting in conflicts with fisheries similar to those involving the Double-crested Cormorant (P. auritus) in North America. In the western Atlantic, Double-crested Cormorants are more numerous and occur throughout the Great Cormorant's breeding range with the exception of Greenland. Both species may be found nesting in mixed colonies, typically numbering up to a few hundred pairs, and often feed and loaf nearby. In winter, the Great Cormorant has a more northern distribution than its smaller congener, which it then outnumbers along the coasts of Atlantic Canada and New England; it is now regular on the lower Hudson and Delaware Rivers and is seen occasionally farther inland and as far south as Florida. The 2 species can be readily confused at a distance, contributing to the early uncertainties of numbers and distribution. Widely persecuted in the past, and still disliked by fishermen, Great Cormorants have increased in range and numbers in the twentieth century, but these increases may have stopped early in the 1990s, having been much less conspicuous than those of the Double-crested Cormorant. Most of their prey are of no commercial interest, however, and no impacts of Great Cormorants on fisheries have been measured in North America.
Cormorants are neither effortless aerialists, like shearwaters and other tubenoses, nor enduring swimmers, like auks and loons, and are most characteristically seen standing upright on rocks, posts, or buoys in a familiar spread-wing posture. The function of this posture is uncertain. The partially wettable plumage serves to reduce the energetic costs of diving; it lowers buoyancy by replacing trapped air. The Great Cormorant feeds principally on bottom-living fish of many kinds that it catches by surface-diving to depths of 35 meters (although usually less than 10 meters). Daily foraging times are notably brief. In other parts of the world, it commonly occurs in flocks, feeding on schooling fish. Although most of its prey are small (less than 20 centimeters long), the ability to catch large fish (to 1.5 kilogram) has led to this species being used by people to catch fish commercially (notably in China) or, formerly, as a sport (in Europe).
This account summarizes the limited material available from North America and Greenland, and incorporates selected information from other parts of the world, but does not attempt to be a comprehensive account of the species. More attention is given to the North Atlantic subspecies, Phalacrocorax carbo carbo, than to others. In Europe, extensive research prompted by recent fisheries conflicts has focused principally on the widespread Eurasian subspecies, P. c. sinensis, which is smaller, often breeds and winters inland, and may show ecological differences from P. c. carbo . For detailed comparative information, including the other subspecies of P. carbo and other cormorants, see Johnsgard 1993; also del Hoyo et al. 1992; and the regional handbooks for the Palearctic (Cramp and Simmons 1977), Africa (Brown et al. 1982), and Australasia (Marchant and Higgins 1990). An earlier species account for North America is in Palmer 1962a .