Swan Creek, Tacoma, Washington

Swans are birds of the family Anatidae within the genus ''Cygnus''. The swans' closest relatives include the geese and ducks. Swans are grouped with the closely related geese in the subfamily Anserinae where they form the tribe Cygnini. Sometimes, they are considered a distinct subfamily, Cygninae. There are six living and many extinct species of swan; in addition, there is a species known as the coscoroba swan which is no longer considered one of the true swans. Swans usually mate for life, although "divorce" sometimes occurs, particularly following nesting failure, and if a mate dies, the remaining swan will take up with another. The number of eggs in each clutch ranges from three to eight.

Etymology and terminology

The English word ''swan'', akin to the German , Dutch and Swedish , is derived from Indo-European root ' ('to sound, to sing'). Young swans are known as '' cygnets'' or as '' swanlings''; the former derives via Old French or (diminutive suffix et 'little') from the Latin word , a variant form of 'swan', itself from the Greek , a word of the same meaning. An adult male is a ''cob'', from Middle English (leader of a group); an adult female is a ''pen''.

Description

Swans are the largest extant members of the waterfowl family Anatidae, and are among the largest flying birds. The largest living species, including the mute swan, trumpeter swan, and whooper swan, can reach a length of over and weigh over . Their wingspans can be over . Compared to the closely related geese, they are much larger and have proportionally larger feet and necks. Adults also have a patch of unfeathered skin between the eyes and bill. The sexes are alike in plumage, but males are generally bigger and heavier than females. The biggest species of swan ever was the extinct ''Cygnus falconeri'', a flightless giant swan known from fossils found on the Mediterranean islands of Malta and Sicily. Its disappearance is thought to have resulted from extreme climate fluctuations or the arrival of superior predators and competitors.

The Northern Hemisphere species of swan have pure white plumage, but the Southern Hemisphere species are mixed black and white. The Australian black swan (''Cygnus atratus'') is completely black except for the white flight feathers on its wings; the chicks of black swans are light grey. The South American black-necked swan has a white body with a black neck.

Swans' legs are normally a dark blackish grey colour, except for the South American black-necked swan, which has pink legs. Bill colour varies: the four subarctic species have black bills with varying amounts of yellow, and all the others are patterned red and black. Although birds do not have teeth, swans, like other Anatidae, have beaks with serrated edges that look like small jagged 'teeth' as part of their beaks used for catching and eating aquatic plants and algae, but also molluscs, small fish, frogs, and worms. In the mute swan and black-necked swan, both sexes have a fleshy lump at the base of their bills on the upper mandible, known as knob, which is larger in males, and is condition dependent, changing seasonally.

Distribution and movements

Swans are generally found in temperate environments, rarely occurring in the tropics. A group of swans is called a bevy or a wedge in flight. Four (or five) species occur in the Northern Hemisphere, one species is found in Australia, one extinct species was found in New Zealand and the Chatham Islands, and one species is distributed in southern South America. They are absent from tropical Asia, Central America, northern South America and the entirety of Africa. One species, the mute swan, has been introduced to North America, Australia and New Zealand.

Several species are migratory, either wholly or partly so. The mute swan is a partial migrant, being resident over areas of Western Europe but wholly migratory in Eastern Europe and Asia. The whooper swan and tundra swan are wholly migratory, and the trumpeter swans are almost entirely migratory. There is some evidence that the black-necked swan is migratory over part of its range, but detailed studies have not established whether these movements are long or short range migration.

Behaviour

Swans feed in water and on land. They are almost entirely herbivorous, although they may eat small amounts of aquatic animals. In the water, food is obtained by up-ending or dabbling, and their diet is composed of the roots, tubers, stems and leaves of aquatic and submerged plants.

Swans famously mate for life, and typically bond even before they reach sexual maturity. Trumpeter swans, for example, who can live as long as 24 years and only start breeding at the age of 4–7, form monogamous pair bonds as early as 20 months. "Divorce", though rare, does occur; one study of mute swans showing a 3% rate for pairs that breed successfully and 9% for pairs that do not. The pair bonds are maintained year-round, even in gregarious and migratory species like the tundra swan, which congregate in large flocks in the wintering grounds. Swans' nests are on the ground near water and about a metre across. Unlike many other ducks and geese, the male helps with the nest construction, and will also take turns incubating the eggs. Alongside the whistling ducks, swans are the only anatids that will do this. Average egg size (for the mute swan) is 113×74 mm, weighing 340 g, in a clutch size of 4 to 7, and an incubation period of 34–45 days."Mute Swan"
British Trust for Ornithology
Swans are highly protective of their nests. They will viciously attack anything that they perceive as a threat to their chicks, including humans. One man was suspected to have drowned in such an attack. Swans' intraspecific aggressive behaviour is shown more frequent than interspecific behaviour for food and shelter. The aggression with other species is shown more in Bewick's swans.

Systematics and evolution

Evidence suggests that the genus ''Cygnus'' evolved in Europe or western Eurasia during the Miocene, spreading all over the Northern Hemisphere until the Pliocene

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