The Hawaiian Duck (Anas wyvilliana) is a species of bird in the family Anatidae. It is endemic to the large islands of Hawaiʻi. 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 Lānaʻi. Its range is now restricted to the island of Kauaʻi. The Hawaiian Duck was extirpated on all other islands, but was subsequently reestablished on Oʻahu, Hawaiʻi, 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 that is fully fertile (Griffin et al. 1989); consequently, "pure" Hawaiian Ducks are still only found on Kauaʻi. Kauai has experienced a loss of lowland wetland habitat, in turn affecting the Hawaiian ducks.
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 adult 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 its call: the koloa maoli quacks like a Mallard, however not as harsh and vocal. Instead, the vocalization is softer than a Mallard's.
The Hawaiian Duck is a very wary bird often found in pairs instead of large groups. They are very secretive birds and do not associate with other animals much. They occur in lowland wetlands, river valleys, and mountain streams, not adapting too well to human-modified habitat. Its 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 which 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 until about nine weeks of age.
The Hawaiian duck's population is affected by habitat loss, modifications to wetland habitats for flood control, non-native invasive plants, diseases, environmental contaminants, and predation. Predation threats to the koloa maoli include feral cats, rats, and Small Asian Mongooses, 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. This interbreeding is 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). The "conversion of flooded taro fields to dry sugar-cane acreage" destroys parts of this species' habitat.
- Griffin, C. R.; Shallenberger, F. J. & Fefer, S. I. (1989): Hawaii's endangered waterbirds: a resource management challenge. In: Sharitz, R. R. & Gibbons, I. W. (eds.): Proceedings of Freshwater Wetlands and Wildlife Symposium: 155-169. Savannah River Ecology Lab, Aiken, South Carolina.
- Hawaii Audubon Society (2005): Hawaii's Birds (6th edition). ISBN 1-889708-00-3
- Rhymer, Judith M. & Simberloff, Daniel (1996): Extinction by hybridization and introgression. Annu. Rev. Ecol. Syst. 27: 83-109. doi:10.1146/annurev.ecolsys.27.1.83 (HTML abstract)
- BirdLife International (2013). "Anas wyvilliana". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
- 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.
- Ehrlich, Paul R.; Dobkin, David S.; Wheye, Darryl (1992). Birds in Jeopardy. Stanford, CA: Stanford University Press. p. 67. ISBN 0-8047-1967-5.