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). The swans are generally found in temperate environments, rarely occurring in the tropics. A group of swans is called a bevy or a wedge in flight. Four (or five) species occur in the Northern Hemisphere, one species is found in Australia and New Zealand and one species is distributed in southern South America. They are absent from tropical Asia, Central America, northern South America and the entirety of Africa. One species, the Mute Swan, has been introduced to North America, Australia and New Zealand. Swans form monogamous pair bonds that last for many years, and in some cases these can last for life.