Plumages and wing spurs of torrent ducks Merganetta armata

Abstract

A study of nearly 200 museum skins of Torrent Ducks has made possible some generalizations on plumages and biology of this species which will facilitate future field work. Torrent ducks are distinctly and permanently sexually dimorphic, lacking any seasonal change in coloration. Juveniles lack sexual dimorphism and are characterized by a white belly and heavily barred sides. There is no evidence of a well-defined first non-nuptial plumage and the first nuptial plumage seemingly is acquired by the time the bird is 9-12 months old. Adults apparently have one complete annual moult in late summer but a partial spring moult is probable. Both sexes have prominent metacarpal spurs on their wings. These spurs increase in length, dark coloration and sharpness with age. However, most yearling males and some females can be distinguished from adults by their short, blunt yellowish spurs. Spurs are made up of a small and short bony core, a more extensive layer of yellow connective tissue, and a relatively thin outer layer of horn. No clear-cut growth rings are observable but the bony core, and probably the horn, increases in complexity with size.