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Swan is a bird of an family in the genus Cygnus. Close relatives of swans include geese and ducks. The swans are separated from the closely related geese of the anserina family, where they form the Cygnus tribe. Sometimes, they are considered to be a unique subfamily, the swan family. There are six to seven species of swans in the genus Cygnus. In addition, there is another species called the koskoba swan, although it is no longer considered one of the real swans. Swans usually mate for life, although sometimes "divorces" occur, especially after unsuccessful nesting, and if a partner dies, the remaining swans are swallowed by another swan. There are three to eight eggs in each clutch.

Swan is the largest living member of waterbird family and one of the largest flying birds. The largest species, including the warty nosed swan, trumpet Swan and whooper swan, can reach a length of 1.5 m (59 inches) and weigh up to 15 kg (33 pounds). Their wingspan may exceed 3.1 m (10 ft). Compared with closely related geese, their geese are larger, and their feet and neck are larger. Adults also have a layer of unspoiled skin between their eyes and mouth. Androgynous, but males are usually larger and heavier than females.

Swans in the northern hemisphere have pure white feathers, but species in the southern hemisphere are black and white. The Black Swan (Cygnus atratus) in Australia is completely black, except for the white flying feathers on its wings. The chicken of black swan is light gray. South American black necked swans have white bodies and black necks.

The legs of swans are usually dark black gray, except for two species in South America, whose legs are pink. The color of the bill varies: the black bill for four sub Arctic species has different amounts of yellow, and the rest are all red and black. Although birds have no teeth, swan's beak has a serrated edge, which looks like serrated "teeth" and is part of its beak, used to catch and eat aquatic plants and algae, as well as mollusks, small fish, frogs and worms. There are lumps at the base of banknotes on mandible.