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Adults average 102 to 124 cm (40 to 49 in) and weigh 3.5-6.7 kg (7.7-14.8 lbs).
The wingspan ranges from 135 to 177 cm (53 to 70 in).
The body plumage is white with a black neck and 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. The black-necked swan was formerly placed in monotypic genus, Sthenelides.
The smallest member in its genus, it is found in freshwater marshes,
lagoon and lake shores in southern South America. The black-necked swan breeds
in Chilean Southern Zone, Patagonia, Tierra del Fuego and on the Falkland Islands.
In the austral winter, this species migrates northwards to Paraguay and southern Brazil.
 The wetlands created by the Great Chilean earthquake like Carlos Anwandter
Nature Sanctuary in Cruces River have become important population centers for the black-necked swan.
In 2004 and 2005 thousands of black-necked swans in the Carlos Anwandter Nature Sanctuary
in Chile died or migrated away following major contamination by Valdivia Pulp
Mill located on the Cruces River which feeds the wetlands. By August 2005
the birds in the Sanctuary had been "wiped out"; only four birds could be observed from
a population formerly estimated at 5,000 birds. Autopsies on dead swans attributed
the deaths to high levels of iron and other metals polluting the water.
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. Widespread and common throughout its habitat,
the black-necked swan is evaluated as Least Concern on the IUCN Red List
of Threatened Species. It is listed on Appendix II of CITES.