Cormorants or shags are long-necked, generally black or dark grey, aquatic birds in the family Phalacrocoracidae. These birds occur in most temperate and tropical marine coasts, and on many large lakes. There are 29 species of cormorants with fewer species occurring at higher latitudes.
The plumage of cormorants is not completely waterproof, since these birds lack an oil gland for preening, so their feathers get waterlogged when they swim under water. As a result, after swimmming, cormorants spend time drying their feathers by standing with their wings spread to the sun and breeze.
The diet of cormorants is mostly small- to medium- sized species of fish. Cormorants dive for their prey, which they catch underwater in their bills. Cormorants power their swimming using their webbed feet, employing their wings and tails to assist with steering.
Cormorants are colonial breeders. They usually build their rather bulky nests of twigs and other debris in trees, and sometimes on artificial platforms such as old pilings. The young birds are initially without feathers and are fed by their parents by regurgitation. Cormorant colonies are loud, raucous places. These birds commonly kill the stand of trees that they nest in, mostly through the caustic influence of their copious defecations.
The most widespread species is the common or great cormorant (Phalacrocorax carbo), which occurs in North America, Eurasia, and Australia. This species and the double-crested cormorant (P. auritus) are the only cormorants on the coast of eastern North America. The double-crested cormorant also breeds abundantly on large, inland lakes.
The west coast of North America also has Brandt's cormorant (P. penicillatus) and the pelagic cormorant (P. pelagicus). The olivaceous cormorant (P. olivaceous) occurs on the southern coast of the Gulf of Mexico and off western Mexico, while the red-faced cormorant (P. urile) is a Eurasian species that occurs in the Aleutian islands of western Alaska.
The Peruvian cormorant (P. bougainville) breeds in enormous colonies on offshore islands of Chile and Peru, where its guano has long been mined as a source of phosphorus-rich fertilizer. This species is subject to occasional mass die-offs, caused by starvation resulting from periodic collapses of the stocks of its most important prey, the Peruvian anchovy. One unusual species, the Galápagos cormorant (Nannopterum harrisi), which lives on the remote and predator-free Galápagos Islands is flightless.
Captive cormorants in Japan and coastal China have been trained for fishing. When used for this purpose, the birds are tethered by tying a line to one of their feet, and their neck is constricted by a ring, so the cormorant can catch fish but not swallow them.
Cormorants are considered to be a pest in many places, because they may eat species of fish that are also sought by human fishers. In some cases, cormorants are killed in large numbers for this reason. Sometimes, they are also considered to be pests because they kill vegetation in their nesting colonies.
Double-crested cormorants breeding on some of the Great Lakes of North America (eg., Lake Michigan) have rather large concentrations of polychlorinated biphenyls (PCBs) and other chlorinated hydrocarbons in their body fat and eggs. This effect of pollution has been blamed for apparent increases in the incidence of developmental deformities in some colonies of cormorants, especially the crossed-bill syndrome. However, in spite of this toxic stress, colonies of double-crested cormorants have been increasing rapidly on the Great Lakes during the past decade or so.
One species of cormorant, P. perspicillatus, bred on Bering Island in the Bering Sea, but was rendered extinct by humans.
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