The Blue Duck (Hymenolaimus malacorhynchos) is a member of the duck, goose and swan family Anatidae. It is the only member of the genus Hymenolaimus, placed in the shelduck subfamily Tadorninae after previously being considered part of the paraphyletic "perching duck" assemblage. The Māori name is whio, which is an onomatopoetic rendition of the males' call.
This 54 cm long species is an endemic resident breeder in New Zealand, nesting in hollow logs, small caves and other sheltered spots. It is a rare duck, holding territories on fast flowing mountain rivers.
It is a powerful swimmer even in white water, but is reluctant to fly. It is difficult to find, but not particularly wary when located.
The blue duck is a dark slate-grey with a chestnut-flecked breast and a paler bill and eye. The pinkish-white bill has fleshy flaps of skin hanging from the sides of its tip. The male's call is an aspirated whistle, and the female's is a rattling growl.
This is a very localised species now threatened by predation from introduced mammals, competition for its invertebrate food with introduced trout, and damming of mountain rivers for hydroelectric schemes. The New Zealand Department of Conservation is presently working on recovery programmes in habitats such as the Oparara River area of the West Coast.