Swans are birds of the familyAnatidae within the genus Cygnus. The swans' close relatives include the geese and ducks. Swans are grouped with the closely related geese in the subfamilyAnserinae where they form the tribeCygnini. Sometimes, they are considered a distinct subfamily, Cygninae. There are six or 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 one of 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 Anatidae are the biological family of birds that includes ducks, geese and swans. The family has a cosmopolitan distribution, occurring on all the world's continents except Antarctica and on most of the world's islands and island groups. These are birds that are adapted through evolution for swimming, floating on the water surface, and in some cases diving in at least shallow water. (The Magpie Goose is no longer considered to be part of the Anatidae, but is placed in its own family Anseranatidae.) The family contains around 146 species in 40 genera. They are generally herbivorous, and are monogamous breeders. A number of species undertake annual migrations. A few species have been domesticated for agriculture, and many others are hunted for food and recreation. Five species have become extinct since 1600, and many more are threatened with extinction.