Swans 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. 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 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 babies of black swans are light grey in colour, and the south american black-necked swan has a black neck.
The australian black swan (Cygnus atratus) has been noted for swimming with only one leg, the other leg being rotated over the body and tucked under its furled wings. Cygnets do this from a young age. All evidence suggests that the genus Cygnus evolved in europe or western eurasia during the Miocene, spreading all over the Northern Hemisphere until the Pliocene. When the southern species branched off is not known. Many of the cultural aspects refer to the mute swan of europe. Perhaps the best known story about a swan is the ugly duckling fable. The story centres around a duckling who is mistreated until it becomes evident he is a swan and is accepted into the habitat.