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Double-crested Cormorants are common inhabitants of seacoasts and inland waters, rarely observed out of sight of land. They may be seen swimming low in the water, often with little more than their heads and sinuous necks showing, but they are more evident at daytime resting places on rocks, pilings, or trees. Resting birds often hold their wings in a spread-wing posture, characteristic of many cormorants, which is thought to aid in drying wet feathers. Cormorants dive from the surface and hunt their prey underwater using powerful strokes of their totipalmate feet (in which all 4 toes are connected by web, as in other pelican-like birds). These prey may be schooling fish or bottom-dwelling fish and invertebrates; a great variety of species has been reported. Cormorants are gregarious birds, often nesting in large numbers at diverse sites—on the ground on islands free from predators, in trees, or on various artificial structures. These colonies are conspicuous, not only because of the visible whitewash but also, downwind, because of the powerful reek of guano and rotting fish.
The Double-crested Cormorant is the most numerous and most widely distributed species of the 6 North American cormorants. In the U.S. and Canada, it is the only cormorant to occur in large numbers in the interior as well as on the coasts, and it is more frequently cited than the others as conflicting with human interests in fisheries.
Cormorants have been persecuted throughout history; recent great increases in numbers have spurred renewed controversy and may soon make parts of this account out of date. These increases have been most notable in the north and east of the breeding range (subspecies P. a. auritus) and where these birds winter in the southern states, especially near catfish farms. Cormorants feed opportunistically on fishes that are readily available and often congregate where these fishes are most easily caught. In natural environments, fish species of direct interest to recreational or commercial fishermen rarely make up a large part of the cormorants’ diet. Exceptions may occur, especially after fish-stocking releases or at aquaculture facilities, but even here the magnitude of the economic impact is difficult to establish unambiguously.
A depredation order to allow some shooting without federal permits of cormorants at aquaculture facilities was issued by the U.S. Fish and Wildlife Service in March 1998. Recently, planned programs to reduce numbers breeding have been carried out in Quebec (1989–1993) and are proposed for New York and Vermont in 1999–2000. Two recent volumes, edited by Nettleship and Duffy (1995) and Tobin (in press), address such conflicts and other aspects of the species’ biology and history. An earlier species account, principally by R. Arbib, is found in Palmer 1962: 325–340; for extensive comparative information see del Hoyo et al. 1992: 326–353 and Johnsgard 1993 . Numerous observations from a blind are included in the large account by Mendall (1936); also, see Lewis 1929 . A recent bibliography (Weseloh et al. in press) includes more than 1,300 titles (through 1995). In Europe, the ecological counterpart of this species, the Great Cormorant (P. carbo), especially the subspecies P. c. sinensis, has undergone similar fluctuations in numbers and, like the Double-crested Cormorant, is also perceived to be detrimental to human fisheries and aquaculture (van Eerden et al. 1995).