The Hawaiian duck is a shy and secretive duck native to the Hawaiian Islands. It is unique to these islands and is found nowhere else on Earth. The Hawaiian duck is closely allied with the mallard. These two species can interbreed and produce viable offspring, and the Hawaiian duck has previously been considered an island subspecies of the mallard. Both male and female Hawaiian ducks are mottled brown in color and resemble a female mallard. The males are usually bigger than the females. The speculum feathers of both sexes are green to blue, bordered on both sides by white. The tail is dark overall and the feet and legs are orange to yellow-orange. The bill is olive green in the male and dull orange with dark markings in the female. The adult male has a darker head and neck, which is also sometimes green. The female is generally lighter-colored than the male and has plainer back feathers. A first-year male Hawaiian duck looks like an eclipse-plumaged male mallard.
Hawaiian ducks were once found on all of the main Hawaiian islands except the island of Lānaʻi and Kaho'olawe. Today these birds are only found on Kauaʻi and Ni'ihau and were reintroduced on O'ahu, Big Island (Hawai’i), and Maui. Hawaiian ducks lived on the hottest coasts with suitable ponds as well as in the mountains. This includes low wetlands, river valleys, coastal ponds, lakes, swamps, flooded grasslands and streams in mountains.
Hawaiian ducks are very wary birds, often found in pairs instead of large groups. They are diurnal and most prominently found residing in the tall, wetland grasses and streams near the Kohala volcano on the main island of Hawai’i. They are very secretive birds and do not associate with other animals much. Hawaiian ducks are strong, agile fliers and move mainly between the islands. The birds usually are not very vocal; they quack like mallards, but their call is softer and they quack less frequently.
Hawaiian ducks are omnivorous birds. Their diet consists of freshwater vegetation, mollusks, insects, and other aquatic invertebrates. Specifically, they consume snails, insect larvae, earthworms, tadpoles, crayfish, mosquito larvae, mosquito fish, grass seeds, rice, and green algae.
Hawaiian ducks are serially monogamous. It means that pair bonds stay together during one breeding season. Some pairs nest year-round, but the main breeding season takes place from December to May. During this time pairs often engage in spectacular nuptial flights. The female lays 2 to 10 eggs in a well-concealed nest lined with down and breast feathers. Incubation lasts about 4 weeks. Ducklings are precocial and can take to the water soon after hatching, but cannot fly until about 9 weeks of age. They become reproductively mature enough to reproduce after a year of age.
Hybridization is considered to be one of the most serious but overlooked threats to the Hawaiian duck, specifically interbreeding between Hawaiian ducks and feral mallards. The Hawaiian duck's population is also affected by habitat loss, modifications to wetland habitats for flood control, non-native invasive plants, diseases, environmental contaminants, hunting, and predation. Predation threats to these birds include feral cats, rats, and small Asian mongooses, which eat the eggs and young. Hawaiian ducks are also threatened through the predation of dogs, introduced fish, and also other birds that are being introduced into their habitat. The previous hunting of water birds in these area from the 1800s to the 1900s also played a major role in the decline of this species. Urban development along with loss of their natural habitat due to the use of the land for local agriculture plays a major role in the decline of this now endangered species. Amongst the threat of urban development, certain problems arise such as human disturbance, through recreation or tourism.
According to the IUCN Red List, the total Hawaiian duck population size is around 2,200 individuals, including 1,500 mature individuals. Currently, this species is classified as Endangered (EN) on the IUCN Red List and its numbers today are decreasing.