The endangered blue duck Hymenolaimus malachorhynchos, also known as whio, belongs to the Anatidae family of waterfowl (ducks, geese and swans), of the Anseriformes order.
It is endemic to New Zealand, and has no close relative in the world, being the only species in its' genus Hymenolaimus.
The blue duck is New Zealand’s only true torrent duck, which live in fast-flowing rivers and streams. It is internationally, uniquely important because of the limited number of torrent species worldwide.
The blue duck’s closest relative is probably the torrent duck Merganetta armata which lives in fast-flowing rivers in the Andes of South America from Venezuela, Columbia, Ecuador and Peru, to Argentina and Tierra del Fuego in Chile. Another torrent duck is Salvadori's teal Salvadorina waigiuensis of New Guinea.
There is a striking similarity in the feeding and occupation habits of blue duck and torrent duck. Both are powerful paddlers and swimmers, but reluctant to fly long distances. Both hide well, but when found are unwary. Both are in decline because of competition for invertebrate food with trout. Both males have a shrill whistle.
The blue duck is one of New Zealand’s most important and distinctive birds. It appears on the back of the $10 note.
Blue duck were once commonly found on rivers and streams throughout New Zealand, however, the populations are now restricted to forested upland catchments in the Central North Island, Kahurangi National Park, south Westland, and Fiordland.
An accurate count is not available, but a 2004 Department of Conservation report says the number could be as low as 500 pairs.
Its' endangered listing, was lowered from ‘nationally endangered’ in 2005, to 'nationally vulnerable' on the 2008 New Zealand Threat Classification System. This does not reflect an improvement in blue duck populations, but is the result of reorganisation of the classification system.
Blue duck share this vulnerable status with other iconic threatened birds, North Island brown kiwi, great spotted kiwi, three tokoeka kiwi subspecies, Auckland Island teal, banded dotterel, New Zealand dotterel, Chatham Island snipe, bush falcon, North Island weka, kaka and yellow-eyed penguin.
Its' status was raised from ‘vulnerable’ in 2000 to ‘endangered’ on the 2008 IUCN Red List of Threatened Species.
In early 2012, 19 breeding pairs, 13 single ducks and 29 juveniles were found along 41km of waterway in the catchment of the Waiau River and Te Hoe River, in the 6000 ha Maungataniwha Native Forest in Hawkes Bay.
It was known there were some blue duck at Maungataniwha, but the density was more than expected. Hopefully more blue duck will be found in the many remote locations of New Zealand that have not been surveyed.
Like many New Zealand waterfowl that have evolved towards a more terrestrial way of living, blue duck can wander into the forest hundreds of metres above their stream territory. The forest is used for browsing. Fossils show that forest areas have been an extension of the stream territory.
New Zealand’s complex of streams, rivers and former wetlands evolved from reliable rainfall that sustained rainforest throughout a long history. Freshwater fauna of rivers and streams is particularly significant historically, because it depends on fresh, clear, cool, flowing water.
The blue duck is an integral part of these unique freshwater ecosystems, which have an abundantly rich diversity of invertebrate fauna (mayflies, stoneflies, caddisflies, and freshwater shellfish and crayfish), but are severely lacking in vertebrate fauna (fishes, frogs and birds).
Blue duck are dependent on the diverse invertebrate freshwater fauna, and clean, fast moving rock-bottomed streams in upper forested catchments, however, it also sometimes occupies larger flatter waterways.
New Zealand’s indigenous freshwater bird fauna was once well represented, however, 7 species of duck, 2 geese, a swan, and 7 rails, 17 species altogether, are extinct.
A strong indigenous tree canopy along the banks, sometimes covering the entire stream, is important as a source of food and as shelter for the entire fauna.
The blue duck's favorite food is cased caddisfly larvae, but also mayfly, Dobson fly or stonefly larvae from stone surfaces below and above water, drifting larvae from the water column, and adult forms from the water surface.
Whio have a sophisticated bill designed for feeding in rocky waterways. The bill is tapered with a pair of soft black flaps on each mandible which assist the collection of freshwater insects in the shallows, and protect the bony bill from damage when feeding around rocks.
They feed from emerging rocks and the water surface, and dive in deeper water, however, most foraging is in shallow rapids. Mandible develop through juvenile growth, and are ready for full use by the time of fledging.
Blue duck are also known to eat bottom-dwelling freshwater crayfish, Paranephrops planifrons, which is listed in ‘gradual decline’ in the 2005 NZ Threat Classification System
New Zealand’s indigenous freshwater bird fauna was once well represented, however, seven species of duck, two geese, a swan, and seven rails, 17 species altogether, are extinct partly because they were too tasty to eat and too easily caught by humans and predatory animals.
The remaining blue duck are an important part of the surviving New Zealand endemic freshwater vertebrate fauna that includes paradise shelduck, grey duck, South Island takahe, pukeko, black stilt, New Zealand dabchick, grey teal, and brown teal.
Two species of this group of nine are ‘nationally critical’ and three are ‘nationally endangered’.
Ducklings must be the cutest of cute bird chicks, but blue duck chicks are also the toughest and most instantly accomplished.
These fearless little swimmers jump into white water rapids the day after they hatch, can immediately swim underwater, and are incredibly fast paddlers. To see them do all of this so soon, is to witness one of the wonders of nature.
Chicks are seldom far from the watchful guidance of their parents, mom in front and dad behind. But despite this, and their toughness, chicks are extremely vulnerable to many hazards.
Clutches of up to nine eggs often produce just one or two juveniles. When blue ducks lose all of their eggs or chicks, they may come back for a second brood in the same breeding season.
Stoats incessantly prey upon eggs, chicks and ground-nesting females. When a family’s chicks are reduced to less than the usual four to nine, fatalities can normally be attributed to stoat predation.
Department of Conservation and University of Canterbury research (Whitehead, Edge, Smart, Hill and Willans, 2008) using a paired-catchment experiment as part of an adaptive management programme to assess the response of blue duck populations to stoat Mustela erminea control, identified stoats as the primary nest predator through video monitoring.
Year-round low-intensity stoat control (10 traps/km) significantly reduced stoat abundance in trapped sites. The success of nesting and productivity, and the number of fledglings produced per pair, increased significantly in the trapping area.
When a family’s chicks are reduced to less than the usual four to nine, fatalities can normally be attributed to stoat predation.
When rivers flood young blue duck may get separated from their parents. Lost chicks die from hypothermia, insufficient food, or predation.
In one breeding season on the Te Waiiti Stream in the Te Urewera, 57 juvenile blue duck were counted. After a torrential rain that caused the river to rise up to 2 metres, only 16 juveniles were found. In that season, only 12 known fledglings survived from 21 pairs in the territory, which is below the level needed for replacement.
It is very difficult to spot blue duck in the dimming light of dusk when they are feeding. They are not visible from 100 metres away, until their bright yellow eyes gives them away, and their movement is detected.
Despite the effective camouflage, before humans settled in New Zealand, blue duck were victims to indigenous avian predators, adzebill, goshawk, Eyles harrier and laughing owl that are now extinct. Today they are still preyed upon by morepork, New Zealand falcon, and shags.
A more recent predator from Australia is the Australasian harrier which preys upon ducklings in a similar way to morepork. If blue duck happen to see the predator, they may swim underwater to hide under a streambank or vegetation.
Blue duck are also vulnerable to predation from below as well as above, from native longfin eel Anguilla dieffenbachii, a prolific feeder, and shortfin eel A. australis.
Friendly, and even somewhat naively tame when not caring for chicks, whio ended up in many maori and pakeha cooking pots after being taken by hand or knocked down with sticks or stones. By some accounts they are not good eating.