Hawaiian Duck (Anas wyvilliana)

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Image of a Hawaiian duck
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Introduction

The Hawaiian Duck, also known as the koloa or koloa maoli, is an endemic Hawaiian dabbling duck with the scientific name Anas wyvilliana. It is closely related to the introduced Mallard (Anas platyrhynchos) and is recognised as a distinct species, although hybridisation between the two is a major conservation concern. The Hawaiian Duck is non-migratory, but it may move between feeding and breeding wetlands and between Kauaʻi and nearby Niʻihau.

Physical Description

The Hawaiian Duck is a medium-sized duck. Adult males are approximately 48–51 centimetres long, while females are about 41–43 centimetres. Both sexes are predominantly mottled brown and resemble a female Mallard, but males are generally larger and often have darker head and neck feathers.

The bill is usually dark olive to dull orange, with females tending to have a more orange-toned bill. Both sexes have orange legs and feet, although the female’s legs may be duller. The folded wing shows a distinctive greenish-blue speculum bordered with white. Unlike the strongly contrasting plumage of male Mallards, male and female Hawaiian Ducks are relatively similar in appearance. Juveniles are generally duller and less distinctly marked than adults.

Range and Habitat

The Hawaiian Duck is endemic to the Hawaiian Islands. It historically occurred across several of the main islands and has since disappeared from, or declined greatly in, parts of its former range. Populations or established groups are associated with Kauaʻi, Niʻihau, Oʻahu, Maui and Hawaiʻi, although the extent of genetic integrity varies between islands. Kauaʻi supports the principal remaining population of birds regarded as relatively free from Mallard introgression.

The species uses a wide range of freshwater and wetland habitats, including mountain streams, lowland ponds, freshwater marshes, reservoirs, flooded grassland, taro fields, irrigation ditches, fishponds and the mouths of larger streams. It may use both natural and artificial wetlands where suitable cover, shallow water and nesting habitat remain available.

Behaviour and Diet

Hawaiian Ducks are primarily active during the day and are usually wary of people. They forage by dabbling in shallow water, tipping forward to reach submerged food, and searching along muddy margins and wet vegetation. Their diet includes aquatic plants, seeds and aquatic and terrestrial invertebrates. Food availability varies between wetlands and seasons.

The species is generally resident rather than migratory. Local movements may occur between feeding and breeding areas, and birds may move between Kauaʻi and Niʻihau. Hawaiian Ducks nest on or near the ground, often in dense vegetation close to wetlands. Nesting can occur throughout the year, although most nesting activity has been recorded from January to May.

Identification Tips

When identifying a Hawaiian Duck, look for the following combination of features:

  • Mottled brown plumage in both sexes, with little of the strong male Mallard pattern.
  • A relatively dark head and neck, particularly in adult males.
  • An olive, dark or dull orange bill rather than the bright yellow bill typical of many male Mallards.
  • Orange legs and feet.
  • A greenish-blue wing speculum edged with white.
  • A smaller, more compact appearance than a typical Mallard, although size varies and is difficult to judge at a distance.

Identification can be difficult because Hawaiian Ducks and Mallards interbreed. Birds with strongly Mallard-like features, unusual plumage combinations or mixed characteristics may be hybrids. Plumage alone is not always sufficient for a reliable identification, particularly on Oʻahu, Maui and Hawaiʻi, where hybridisation has been extensive. Genetic testing is required when certainty is important.

Conservation Status

The Hawaiian Duck is listed as Endangered under the United States Endangered Species Act and is also listed as Endangered in the IUCN Red List assessment cited by Hawaiʻi’s Division of Forestry and Wildlife. The principal threats are wetland loss and degradation, predation by introduced mammals, disease and hybridisation with introduced feral Mallards.

Habitat loss has resulted from drainage, agricultural conversion, urban development, altered water regimes and the spread of invasive plants. Introduced rats, cats, dogs and mongooses can prey on eggs, ducklings and adults. Avian diseases, including localised outbreaks of avian botulism, may also affect Hawaiian waterbirds.

Hybridisation is a particularly serious problem because it can reduce the genetic distinctiveness of the Hawaiian Duck. Genetic studies have found few hybrids in the Kauaʻi population, while sampled populations on several other main islands have included extensive mixed ancestry. Conservation measures include wetland protection and restoration, invasive-plant management, predator control, disease monitoring, population monitoring and efforts to reduce contact with feral Mallards.

Frequently Asked Questions

What is the Hawaiian Duck’s scientific name?

The scientific name of the Hawaiian Duck is Anas wyvilliana. It is also known in Hawaiʻi as the koloa or koloa maoli.

Where does the Hawaiian Duck live?

The species is endemic to the Hawaiian Islands. It uses freshwater marshes, streams, ponds, reservoirs, taro fields, fishponds, irrigation channels and other suitable wetlands. Kauaʻi is the principal stronghold for genetically distinctive birds.

How can a Hawaiian Duck be distinguished from a Mallard?

Hawaiian Ducks are usually smaller and more uniformly mottled brown, with relatively dark heads and necks, olive or dull orange bills and orange feet. However, hybridisation makes visual identification unreliable in some areas.

Does the Hawaiian Duck migrate?

No. It is generally a resident species, although individuals may make local movements between feeding and breeding habitats and may move between Kauaʻi and Niʻihau.

What does the Hawaiian Duck eat?

It feeds mainly on aquatic plants, seeds and invertebrates found in shallow water, mud and wetland vegetation.

Why is hybridisation with Mallards a threat?

Hawaiian Ducks and introduced Mallards can produce fertile offspring. Continued breeding can therefore replace the distinctive Hawaiian Duck gene pool with mixed ancestry and make genetically pure birds increasingly difficult to maintain.

References

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