The Black-necked Swan is the largest waterfowl native to South America. Males are 140 cm long and 7 kg in weight; females are 124 cm and 4.5 kg in weight. The wingspan is about 177 cm. The body plumage is white with a black neck, head and greyish bill. It has a red knob near the base of the bill and white stripe behind eye. The sexes are similar, with the female slightly smaller. The cygnet has a light grey plumage with black bill and feet.
It is found in freshwater marshes, lagoon and lake shores in southern South America. The Black-necked Swan breeds in Zona Sur, Patagonia, Tierra del Fuego and on the Falkland Islands. In the austral winter, this species migrates northwards to Paraguay and southern Brazil.
The Black-necked Swan, like its nearest relatives the Black and Mute Swan is relatively silent. Also, unlike most wildfowl, both parents regularly carry the cygnets on their backs. The female lays four to six eggs in a nest of vegetation mound. The diet consists mainly of vegetation, insects and fish spawn.