Swans, genus Cygnus, are birds of the family Anatidae, which also includes 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 to seven species of swan in the genus Cygnus; in addition there is another species known as the Coscoroba Swan, although this species is no longer considered related to the true swans. Swans usually mate for life, though 'divorce' does sometimes occur, particularly following nesting failure. The number of eggs in each clutch ranges from three to eight.
The swans are the largest members of the duck family Anatidae, and are among the largest flying birds. The largest species, including the mute swan, trumpeter swan, and whooper swan, can reach length of over 1.5 m (60 inches) and weigh over 15 kg (33 pounds). Their wingspans can be almost 3 m (10 ft). Compared to the closely related geese they are much larger in size and have proportionally larger feet and necks. They also have a patch of unfeathered skin between the eyes and bill in adults. The sexes are alike in plumage, but males are generally bigger and heavier than females.
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 in colour, and the South American Black-necked Swan has a black neck.
The legs of swans are normally a dark blackish grey colour, except for the two South American species, which have 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 most birds generally do not have teeth, swans are known to be an exception to this, having small jagged 'teeth' as part of their beaks used for catching and eating fish. The Mute Swan and Black-necked Swan have a lump at the base of the bill on the upper mandible.
