The black-necked swan has occasionally been placed by itself in the genus Sthenelides. Its closest relatives are the black swan (C. atratus) and mute swan (C. olor).[4] It is monotypic.[5]
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 102 to 124 cm (40 to 49 in) long with a wingspan of 135 to 177 cm (53 to 70 in). Males weigh 4.6 to 8.7 kg (10 to 19 lb) and females 3.5 to 4.4 kg (7.7 to 9.7 lb). 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.[4][8]
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 elevations but non-breeding flocks can be found as high as 900 to 1,250 metres (3,000 to 4,100 ft) in the Andes of southern Argentina.[4]
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.[9] The population in this sanctuary has fluctuated widely over the years, reaching a low of 214 in January 2008 and a peak of 22,419 in May 2020 before plumbing to 2,782 in May 2022.[10][11]
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.[4]
The black-necked swan's breeding season varies geographically. In the far south of its range, it breeds from July to November but ends as early as September in the northern parts. In the Falklands, it breeds between August and November. The exact timing of breeding appears dependent on rainfall. The species is believed to form long-term pair bonds. Its nest is a mound of vegetation constructed by both members of a pair on a small islet or partially floating in a reedbed. The clutch size is four to eight eggs. Males guard females during the 34- to 36-day incubation period. Captive nestlings fledged about 100 days after hatch.[4]
The black-necked swan is mostly silent outside the breeding season. During that time, both sexes give a "soft, musical 'Whee-whee-whee' with the accent on the initial syllable", repeating it to challenge intruders. The call is also used to maintain contact between members of a pair. Males also give "a musical 'hooee-hoo-hoo'."[4]
The IUCN has assessed the black-necked swan as being of Least Concern. It has a very large range, and though its population size is not known, it is believed to be stable. No immediate threats have been identified.[1] It is widespread and generally common. Large-scale hunting in the 18th and 19th centuries extirpated it from much of Chile but it has recolonized those areas. Some egg collecting and hunting still occur, however. The species occurs in several protected areas in mainland Argentina, in which country the population is estimated at 50,000.[4]
^ abcdefghiCarboneras, C. and G. M. Kirwan (2020). Black-necked Swan (Cygnus melancoryphus), version 1.0. In Birds of the World (J. del Hoyo, A. Elliott, J. Sargatal, D. A. Christie, and E. de Juana, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.blnswa2.01 retrieved September 27, 2022
^ abGill, F.; Donsker, D.; Rasmussen, P., eds. (August 2022). "Screamers, ducks, geese, swans". IOC World Bird List. v 12.2. Retrieved August 9, 2022.
^Remsen, J. V., Jr., J. I. Areta, E. Bonaccorso, S. Claramunt, A. Jaramillo, D. F. Lane, J. F. Pacheco, M. B. Robbins, F. G. Stiles, and K. J. Zimmer. Version 24 July 2022. Species Lists of Birds for South American Countries and Territories. https://www.museum.lsu.edu/~Remsen/SACCCountryLists.htm retrieved July 24, 2022
^Ogilvie & Young, Wildfowl of the World. New Holland Publishers (2004), ISBN978-1-84330-328-2
^Ramirez, C., E. Carrasco, S. Mariani & N. Palacios. 2006. La desaparición del luchecillo (Egeria densa) del Santuario del Rio Cruces (Valdivia, Chile): una hipótesis plausible. Ciencia & Trabajo, 20: 79-86
