The great cormorant (Phalacrocorax carbo), also known as just cormorant in Britain, as black shag or kawau in New Zealand, formerly also known as the great black cormorant across the Northern Hemisphere, the black cormorant in Australia, and the large cormorant in India, is a widespread member of the cormorant family of seabirds. It breeds in much of the Old World, Australasia, and the Atlantic coast of North America.
The great cormorant was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae under the binomial name Pelecanus carbo. Linnaeus specified the type location as "Europe", but this was restricted to the "rock-nesting form of the north Atlantic Ocean" by the German ornithologist Ernst Hartert in 1920. The great cormorant is now one of 12 species placed in the genus Phalacrocorax that was introduced in 1760 by the French zoologist Mathurin Jacques Brisson. The genus name is Latinised Ancient Greek, from φαλακρός (phalakros, "bald") and κόραξ (korax, "raven"); the specific epithet carbo is Latin for "charcoal".
This is a very common and widespread bird species. It feeds at sea in inshore waters, in estuaries, and on freshwater lakes and rivers. Northern birds migrate south to escape waters that freeze in winter, moving to any coast or freshwater that is unfrozen and well-supplied with fish; in warmer areas, birds disperse locally. They only rarely cross larger bodies of water such as the North Sea.
The type subspecies, P. c. carbo, is found mainly in Atlantic waters and nearby inland areas, on northwestern European coasts, the Faroe Islands, Iceland and Greenland, and on the northeastern seaboard of North America. The widespread continental subspecies P. c. sinensis occurs on most of mainland Europe and east across the Palearctic to Siberia and southeastern Asia. The subspecies P. c. novaehollandiae is found in Australian and New Zealand waters.
The great cormorant nests in colonies near wetlands, rivers, and sheltered inshore waters. Pairs will use the same nest site to breed year after year. It builds its nest, which is made from sticks, in trees, on the ledges of cliffs, and on the ground on rocky islands that are free of predators. They lay a clutch of three to five eggs that measure 63 by 41 millimetres (2+1⁄2 by 1+5⁄8 in) on average. The eggs are a pale blue or green, and sometimes have a white chalky layer covering them. These eggs are incubated for a period of about 28 to 31 days.
The great cormorant feeds on fish caught through diving. Birds at the Chausey Islands feed primarily on wrasses, but also takes sand smelt, flathead and common soles. The average weight of fish taken by great cormorants increases with decreasing air and water temperature. Cormorants consume all fish of appropriate size that they are able to catch in summer and noticeably select for larger, mostly torpedo-shaped fish in winter. In winter, the foraging efficiency described for cormorants by various researchers is increased by capturing larger fish, not due to capturing more fish. In some freshwater systems, the take of fish by overwintering great cormorants was estimated to be up to 80 kg per ha per year (e.g. Vltava River, Czech Republic).
It forages by foot-propelled diving and capturing its prey in its beak. The duration of its dives varies from 1–3 minutes, with the bird diving to depths of up to a maximum of 9.5 metres (31 ft). About 60% of dives are to the benthic zone and about 10% are to the pelagic zone, with the rest of the dives being to zones in between the two. Studies suggest that their hearing has evolved for underwater usage, possibly aiding their detection of fish. These adaptations also have a cost on their hearing ability in air which is of lowered sensitivity.
Many fishermen see in the great cormorant a competitor for fish. Because of this, it was hunted nearly to extinction in the past. Due to conservation efforts, its numbers increased. At the moment, there are about 1.2 million birds in Europe (based on winter counts; late summer counts would show higher numbers). Increasing populations have once again brought the cormorant into conflict with fisheries.For example, in Britain, where inland breeding was once uncommon, there are now increasing numbers of birds breeding inland, and many inland fish farms and fisheries now claim to be suffering high losses due to these birds. In the UK each year, some licences are issued to cull specified numbers of cormorants in order to help reduce predation; it is, however, still illegal to kill a bird without such a licence.
