Males are said to have more red at the base of the bill, but I cannot see any obvious difference. Although this species is the largest of the waterfowl in South America, it is the smallest member of the Cygnus genus. These swans range throughout most of southern South America with the exception of the higher Andes.
The species was formerly placed in the monospecific genus Sthenelides. The specific name is sometimes misspelled melanocorypha, e.g. by HBW, Sibley & Monroe and others. The change to melancoryphus was necessitated for gender agreement when the species was moved to Cygnus but melanocoryphus was used by Clements until the 2007 revision of their sixth edition and that misspelling is common in older publications.
Digiscoped with Panasonic DMC-LX5 | Nikon FieldScope 3 | 30X WA | hand-held (no adapter)