Swans are the largest extant members of the waterfowl family Anatidae, and are among the largest flying birds. The largest living species, including the mute swan, trumpeter swan, and whooper swan, can reach a length of over 1.5 m (59 in) and weigh over 15 kg (33 lb). Their wingspans can be over 3.1 m (10 ft).[10] Compared to the closely related geese, they are much larger and have proportionally larger feet and necks.[11] Adults also have a patch of unfeathered skin between the eyes and bill. The sexes are alike in plumage, but males are generally bigger and heavier than females.[9] The biggest species of swan ever was the extinct Cygnus falconeri, a flightless giant swan known from fossils found on the Mediterranean islands of Malta and Sicily. Its disappearance is thought to have resulted from extreme climate fluctuations or the arrival of superior predators and competitors.[12]
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. The South American black-necked swan has a white body with a black neck.[13]
Swans' legs are normally a dark blackish grey colour, except for the South American black-necked swan, which has 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 birds do not have teeth, swans, like other Anatidae, have beaks with serrated edges that look like small jagged 'teeth' as part of their beaks used for catching and eating aquatic plants and algae, but also molluscs, small fish, frogs, and worms.[14] In the mute swan and black-necked swan, both sexes have a fleshy lump at the base of their bills on the upper mandible, known as knob, which is larger in males, and is condition dependent, changing seasonally.
