The coscoroba swan is the smallest swan, and it is recognized by its bright-red, flattened, ducklike bill and its red legs, which are the longest in relation to body size of any swan species. Coscoroba swans share some characteristics of whistling ducks …. Consequently, many authorities regard the coscoroba swan as a sister species to true swans; the coscoroba swan bears the closest affinities to the whistling ducks.