The black-necked swan (Cygnus melancoryphus) is a species of waterfowl in tribe Cygnini of subfamily Anserinae. It is found in Argentina, Brazil, Chile, Uruguay, and the Falkland Islands. Taxonomy and systematics The black-necked swan has occasionally been placed by itself in genus Sthenelides. Its closest relatives are the black swan (C. atratus) and mute swan (C. olor). It is monotypic. Description The black-necked swan is the only member of its genus that breeds in the neotropics, and is the largest waterfowl native to South America. Adults are long with a wingspan of . Males weigh and females . The sexes are alike. Adults' body plumage is white and the neck and head black; the latter usually has a white stripe behind the eye. They have a prominent red knob at the base of their bill. Juveniles are grayish rather than white and lack the knob until their third or fourth year. Distribution and habitat The black-necked swan is found in the southern tier of South America. It nests from Tierra del Fuego north to central Chile, Uruguay, and Rio Grande do Sul in extreme southern Brazil. It withdraws from the southern half of Argentina in winter and is then found as far north as Brazil's São Paulo state. It is a year-round resident of the Falkland Islands. Vagrants have been found on Juan Fernández Island, the South Orkney Islands, the South Shetland Islands, and the Antarctic Peninsula. The wetlands created in Chile by the 1960 Valdivia earthquake, such as Carlos Anwandter Nature Sanctuary on the Cruces River, have become important population centers for the black-necked swan. The black-necked swan inhabits freshwater marshes and swamps, shallow lakes, brackish lagoons, and sheltered coastal sites. On the mainland of South America it is often found near human habitation, but shuns built up areas in the Falklands. It is generally found at low elevation but non-breeding flocks can be found as high as in the Andes of southern Argentina. Behavior Feeding The black-necked swan's diet is almost entirely vegetarian. It feeds on aquatic plants like Chara, Potamogeton, Typha; algae such as Aphantotece and Rhyzoclonium; and presumably small numbers of aquatic invertebrates. In parts of Chile its principal food is Egeria densa. It forages mostly by immersing its head and neck and by surface feeding, but also upends to reach deeper. In times of drought it has been observed grazing in meadows and pastures. Breeding