This led to the next available name proposed by French ornithologist Louis Jean Pierre Vieillot in 1816 from a Paraguayan specimen as "Anas bicolor". The whistling ducks were moved to their current genus, "Dendrocygna", by British ornithologist William John Swainson in recognition of their differences from other ducks. The genus name is derived from the Ancient Greek "dendron", "tree", and Latin "cygnus", "swan", and " bicolor" is Latin for "two-coloured". "Fulvous" means reddish-yellow, and is derived from the Latin equivalent "fulvus". Old and regional names include large whistling teal, brown tree duck, Mexican duck, pichiguila, squealer and Spanish cavalier. The fulvous whistling duck is long; the male weighs , and the female averages marginally lighter at . It is a long-legged duck, mainly different shades of brown; head, neck and breast are particularly rich buff (fulvous) with a darker back. The mantle is more darker shade of brown with buff-tipped feathers, the flight feathers and tail are dark brown, and a dark brown to black stripe runs through the center of the crown down the back of the neck to the base of the mantle. It has whitish stripes on its flanks, a long grey bill and grey legs. In flight, the wings are brown above and black below, with no white markings, and a white crescent on the rump contrasts with the black tail. All plumages are fairly similar, but the female is slightly smaller and duller-plumaged than the male. The juvenile has paler underparts, and appears generally duller, especially on the flanks. There is a complete wing moult after breeding, and birds then seek the cover of dense wetland vegetation while they are flightless. Body feathers may be moulted throughout the year; each feather is replaced only once annually. These are noisy birds with a clear whistling "kee-wee-ooo" call given on the ground or in flight, frequently heard at night. Quarrelling birds also have a harsh repeated "kee". In flight, the beating wings produce a dull sound. The calls of males and females show differences in structure and an acoustic analysis on 59 captive birds demonstrated 100% accuracy in sexing when compared with molecular methods. Adult birds in Asia can be confused with the similar lesser whistling duck, which is smaller, has a blackish crown and lacks an obvious dark stripe down the back of the neck.