The Blue duck is a native of New Zealand, where its Maori name is “whio”. It is the only member of the family Hymenolaimus. It inhabits fast-flowing inland streams, nesting along the riverbanks, where predation is a threat. It is a powerful swimmer and generally reluctant to fly. The Blue duck has dusky blue-grey plumage and chestnut markings on the chest. This species is rare and is listed as endangered.
The Blue duck lives only in the North and South Islands of New Zealand, in the west of the South Island, especially in Otago and Southland, as well as in the central North Island within national parks. It usually occurs in alpine river regions, favoring cascading, bush-fringed rivers, as well as streams within gorges. It is rarely seen on open or side creeks but lives only on fast-flowing rivers, which supply all its food.
Blue ducks feed while swimming, by dipping their head and neck beneath the surface of the water. They also dive well and dabble, upending, pecking in the shallow water, and taking insects from rocks. This bird can detect its prey by sight, due to its wide binocular field of vision, and it can also see directly forwards. It forages early in the morning and in the late afternoon, sometimes by night. This species is commonly seen in pairs, remaining in the same section of river and shore for the whole year and often for life. The birds are very terrestrial and will usually chase all intruders away from their area. Its blue-gray plumage blends well into its surroundings, and only its pale bill is visible when it moves its head. When disturbed, it dives into the fast-flowing current and swims rapidly away or it flattens onto rocks and waits, looking out for an intruder or predator. The male makes high-pitched whistling as advertising calls and when defending territory. With intruders, a short “whi” or “whio” is heard, and the female will give guttural, rasping growls when disturbed or threatened.
This species is monogamous. They are thought to remain in a pair for life and to defend the same territory during their nesting period. Pairs will defend small territories of a size of 0.7 to 1 km beside a river. These birds are solitary nesters and once the breeding season is over, they stay in family groups. From August to December is the breeding season. Birds nest near water, under overhanging rocks, on banks under thick vegetation, in a hollow log, a rock crevice or a small cave, no more than 30 m from the water, where there is a threat from spring floods. 4-7 creamy white eggs are laid, incubated by the female for 33-35 days while her mate remains in close vicinity. Soon after hatching, the young can swim and dive. They stay with their family group within the territory for 70-80 days, tended by both parents. The adults molt while their young are unable to fly. Most females mate when they are one year old but males do not breed until they are two years old.
Blue duck numbers have declined due to habitat loss and disturbance, predation, particularly by non-native mammals, competition with introduced trout, and population fragmentation. Human recreational activity and hydroelectric dams on rivers also disturb this species.
According to the IUCN Red List, the total Blue duck population size is 1,800 individuals, including 1,200 mature individuals. According to the New Zealand Birds Online resource, there are unlikely to be over 1,000 pairs in the total breeding population. Currently this species is classified as Endangered (EN) on the IUCN Red List and its numbers today are decreasing.
Blue ducks are important in their riverine habitats as predators of aquatic insects, thus controlling their populations.