The swans are the largest extant members of the waterfowl family Anatidae, and they are among the largest flying birds. The largest, including the mute, trumpeter and whooper swan, can reach a length of over 1.5 meters and weight over 15 kg. Their wingspans can be over 3.1 meters. The swans mostly can be found in temperate environments, rarely occurring in the tropics. Swans feed in the water and on land. Evidence suggests that the genus Cygnus evolved in Europe or western Eurasia during the Miocene, spreading all over the Northern Hemisphere until the Pliocene. Mute swan or Cygnus olor is a Eurasian species that occurs at lower latitudes than whooper swan and Bewick's swan across Europe into southern Russia, China and the Russian Maritimes. Black swan, Cygnus atratus of Australia, and introduced in New Zealand. Their swan, Cygnus atratus sumnerensis, an extinct subspecies of the black swan from New Zealand and the Chatham Islands. Black-necked swan, Cygnus melancoryphus of South America. Whooper swan, Cygnus cygnus breeds in Iceland and subarctic Europe and Asia, migrating to temperate Europe and Asia in winter. Trumpeter swan, Cygnus buccinator is the largest North American swan. Tundra swan known as Cygnus columbianus is a small swan that breeds on the North American tundra, further north than the trumpeter swan. Bewick's swan, Cygnus (columbianus) bewickii is the Eurasian form that migrates from Arctic Russia to western Europe and eastern Asia (China, Japan) in winter. Whistling swan, Cygnus (columbianus) may be used to refer specifically to the North American form.