The Hawaiian Duck (Anas wyvilliana) is a species of the bird Anas. It is endemic to the large islands of Hawaii. Some authorities treat it as an island subspecies of the Mallard, based on their capacity to produce fertile hybrids, but it appears well distinct and capability of hybridization is meaningless in dabbling duck taxonomy. The native Hawaiian name for this duck is koloa maoli.

The former range of the Hawaiian Duck included all of the main Hawaiian islands except the island of Lanai. Now the Hawaiian Duck only exists on the island of Kauai. The Hawaiian Duck was extirpated on all other islands, but was subsequently reestablished on Oahu, Hawaii, and Maui through release of captive-reared birds. However, all the Hawaiian Ducks in the reestablished populations have bred with feral Mallard ducks and have produced hybrid offspring (Griffin et al. 1989); consequently, "pure" Hawaiian Ducks are still only found on Kauai.

Males, 19-20" (some 50 cm) long, are bigger than females, (16-17" or about 45 cm). Both sexes are mottled brown in color, resembling a female mallard. The speculum feathers are greenish-blue, bordered on both sides by white. The tail is dark overall, unlike the black-and-white tail of a mallard. The adults male has a darker head and neck which is also sometimes green. A first-year male koloa maoli looks like an eclipse-plumaged male Mallard. The feet and legs are orange. The bill is olive green in the male and dull orange with dark markings in the female. Another difference between the Hawaiian Duck and the Mallard is their vocalizations: The koloa maoli quacks like a mallard, however not as harsh and vocal. Instead, the voice is softer than a mallards.

The Hawaiian Duck is a very wary bird often found in pairs instead of large groups. They occur in lowland wetlands, river valleys, and mountain streams, not adapting too well to human-modified habitat. The koloa maoli's diet consists of freshwater vegetation, mollusks, insects, and other aquatic invertebrates. Some pairs nest year round, but the primary breeding season is from December to May. During the breeding season, pairs are often engaged in spectacular nuptial flights. Two to ten eggs are laid in a well-concealed nest lined with down and breast feathers. Soon after hatching, the young can take to the water, but cannot fly for nine weeks.

Threats to the koloa maoli include feral cats, rats, and mongoose which eat the eggs and young. Interbreeding with feral mallards is also a major problem, as the hybrids seem to be less well-adapted to the local ecosystem but still rather common due to the high numbers of feral mallards. Several attempted reintroductions have already failed due to the hybrid ducks produced in captivity faring badly in the wild.(Rhymer & Simberloff 1996)

1. ^ This means "native mallard". Often, it is shortened to just koloa but this term denotes the Mallard if applied strictly, and all mallard-like dabbling ducks if applied loosely.