The blue duck (Hymenolaimus malacorhynchos) or whio is a member of the duck, goose and swan family, Anatidae and is endemic to New Zealand. It is the only individual species belonging to the genus Hymenolaimus. Its closest relatives are the ducks in the genus Sarkidiornis.
First seen and recorded by the British explorer, James Cook in his second voyage to New Zealand, its scientific name was given by the German naturalist, Johann Friedrich Gmelin in 1789 and later George Robert Gray in 1843. There are two subspecies of the blue duck, one in North Island and the other in South Island. Common to both gender, its physical characteristics include yellow iris, a pale grey beak with semicircular flaps and lamellae, dark-slate grey plumage, chestnut flecked breast, dark grey webbed feet, and well developed claws. The male blue duck has a high-pitched whistle, which gave its Māori name Whio, while the female has a low-pitched rattle.
The European voyages led to more human settlement in New Zealand, causing a loss of forests, and which started the decline of Blue duck in the country. It was last commonly seen in New Zealand in the 1950s following which stoat predation caused its further population decline and limiting it to only North and South Island. Geographically, it is found in clean and stable rivers which have a medium to steep gradient. It roosts close to the undercut of river banks, and can do breeding and moulting in more wider area having tributaries and small streams.
Blue ducks have a territorial behaviour with paired ducks staying in its territory during and post-breeding season. Single males often attempt to intrude into paired blue duck territory with differing results while the juvenile ducks stay close to their catchment. It mostly feeds on aquatic invertebrate larvae such as Chironomidae and caddisfly larvae. Foraging is done by using both methods–dabbling and diving. Dabbling is more commonly used by the adult blue duck than the juvenile blue duck. Also, territorial blue duck forages on mornings and evenings, while non-territorial duck forages on midday. Paired ducks defend themselves using techniques for e.g., head-low flight, upright, head-bobbing, siphon-feeding and vocalisations. When ducklings learn to fly, they lose contact with their parent blue ducks which by then becomes flightless. In the breeding season, a clutch of 6 eggs are laid with an incubation period of 35 days. They build their nest between August and December, and during breeding season the male blue duck guards the territory while the female duck incubates the eggs.
The progeny of North Island blue duck subspecies are held and bred on both North and South Island, but are later returned to their respective island. In contrast, the South Island subspecies are held and bred only locally. The WHIONE programme, operating between 2009 and 2019, located the nest of these ducks for raising it in captivity. International efforts to raise the blue duck outside New Zealand was made in the United Kingdom in 1970s, but it lasted only until 2012. Its threats include weka, stoat, man-made actions and environmental hazards. The Central North Island Blue Duck Trust (CNIBDT) works in the conservation effort towards blue duck with projects such as the "Whio Forever Project". Predator control is done with aerial 1080 operations, which are run by TBfree New Zealand and the New Zealand Department of Conservation. The blue duck is depicted on the reverse side of the New Zealand $10 banknote.
Taxonomy Captain James Cook saw the blue duck in Dusky Sound, South Island, New Zealand, on his second voyage to the South Pacific. In 1777 both Cook and the naturalist Georg Forster mentioned the blue duck in their separate accounts of the voyage. A specimen was described in 1785 by the English ornithologist John Latham in his A General Synopsis of Birds. Latham used the English name, the "soft-billed duck". When in 1789 the German naturalist Johann Friedrich Gmelin revised and expanded Carl Linnaeus's Systema Naturae he included the blue duck and placed it with all the other ducks in the genus Anas. He coined the binomial name Anas malacorhynchos and cited the earlier works. The blue duck is now the only species placed in the genus Hymenolaimus, and it was introduced specifically for the species by George Robert Gray in 1843. The genus name combines the Ancient Greek humēn, humenos meaning "skin" or "membrane" with laimos meaning "throat". The specific epithet malacorhynchos is also from Ancient Greek and combines malakos meaning "soft" with rhunkhos meaning "bill".
The species has no very close relatives. Genetic evidence published in 2010 and 2018 has shown its relationship is closest to the knob-billed and comb ducks in the pantropical genus Sarkidiornis, and with them to the Australian wood duck Chenonetta jubata. This group is then allied to a larger group of diving ducks, including the widespread genera Aythya and Netta. It was formerly thought to be related to the shelduck tribe, but these are now known to be much more distant genetically.
It is commonly known in New Zealand English by its Māori name whio, pronounced , which is derived from male's high pitched, wheezy whistle. Other names are mountain duck and blue mountain duck.
Two subspecies are recognised: H. m. hymenolaimus Mathews, 1937 – central, south North Island (New Zealand) H. m. malacorhynchos (Gmelin, JF, 1789) – west South Island (New Zealand)
Prior to 2022, the North Island and South Island whio were considered distinct but were not distinguished as subspecies; they were, however, treated as separate management units. However, the populations were defined as distinct subspecies by the International Ornithological Congress in 2022, based on strong genetic divergence and some plumage differences. The South Island (nominate) subspecies is larger than the North Island subspecies.
Description
The blue duck is long and varies in weight by sex. Females are smaller than males, weighing , whereas the males weigh . The plumage is a dark slate-grey with a greenish sheen on the head, a chestnut-flecked breast. The outer are tipped with white and the inner ones have black margins. The plumage of the two sexes are mostly the same, although the female has slightly less chestnut in the chest.
It has webbed feet which gives it power to swim in fast-flowing current, and well-developed claws which are suited for rough terrain. Juvenile blue ducks have an oversized feet and legs that helps it to similarly swim in fast currents, and jump onto large rocks and logs. The legs and feet are both dark grey in colour. The upper bill in blue duck has a semicircular soft flap that protects it from abrasion while it engages in dabbling. The tiny comb like structures (or lamellae) around the edge of the bill helps it to feed by diving. During aggressive confrontations or when frightened, the blue duck's bill changes its pale grey color to a distinct pink. The bill is green at hatching and develops its final colour eight hours later. The juvenile blue ducks have a grey iris while the adults possess a yellow one. Their life expectancy is upto 12 years which gets reduced due to predation. The oldest blue duck recorded in the wild lived over 13 years.
Song The male's call is a high-pitched whistle. The female's call is a rattling growl or low-pitched grating notes.
Distribution and habitat In the 18th century when the European voyages to New Zealand became significant, the Blue duck was widespread and abundant in the country. After the human settlement when the forests were being cleared, its population started to decline. Charlie Douglas in 1891 surprised on the blue duck's quick disappearances during an exploration of Waiatoto River remarked that:
It was last commonly seen in New Zealand in the 1950s following which its population dwindled because of stoats predating on it. Historical evidence suggests that it once inhabited the offshore islands of New Zealand and not just the North Island and South Island.
The blue duck heavily relies on clean, stable rivers and stream catchments. They are currently located in medium to steep gradient rivers with partial forest cover overhead and vegetation to the riverside. Roosting (or resting) spots are close to the stable undercut of river bank formed by the root system of trees or in log accumulations. A more wider area having tributaries and small disjoint streams is also utilised by the blue duck for activities such as roosting, breeding and moulting. Adult moulting females have been spotted in small streams from their territory at above sea level.Moorcroft, G., Allerby, T., Baigent, D., Barsdell, J., Gebert, S., Glaser, A., ... & Thyne, C. (2010). Te Urewera Mainland Island . Department of Conservation, Rotorua (unpublished). p. 84.
Behaviour Blue ducks are found year-round as territorial pairs along a river. Single males occasionally try to take space between separated pairs as new territory. Once the territory is established, it is held by the single male for life. Juvenile or young ducks try to establish their territories near to their natal catchment which leads to high levels of genetic similarity between ducks in a territory. It rarely settles beyond its natal catchment.
Diet The blue duck feeds almost entirely on aquatic invertebrate larvae. A study of blue ducks on the Manganuioteao River in the central North Island found the most common prey items were Chironomidae (midge) and cased caddisfly larvae, although cased caddisfly were less preferred and were only consumed so much because of their abundance. Hydrobiosidae (free-living caddisfly) and Aphrophila neozelandica (crane fly) larvae were also frequently eaten. Other prey included mayfly, Aoteapysche (net-building caddis) and stonefly larvae. In addition to these insects, it also consumes silt, periphyton and berries from riverside plants.
Foraging Blue duck forages by dabbling on aquatic invertebrates in cold, fast, well oxygenated and clean mountain streams. It dabbles more by scouring rocks than dabbling at the water's surface. Studies have shown that adult territorial blue ducks dabble more than juvenile ones. The stream invertebrates are washed off the rocks in a downstream movement known as invertebrate drift. This drift keeps the food resource renewed for the duck in evenings. Another way to forage that is less commonly used by the blue duck is to dive in stream.
During the diurnal cycle, territorial birds exhibit higher foraging in the early morning and late evening while non–territorial birds tend to forage more at midday. The decrease in foraging by dabbling during midday is because of foraging by diving at that time. This pattern suggests that blue ducks capitalise on invertebrate resources that vary in availability during mid-day and morning or evenings.
Feeding behaviour is influenced by the type of prey available, requiring both tactile and visual foraging techniques. While foraging, blue ducks primarily glean invertebrates from rock surfaces using visual cues for mobile prey such as mayfly larvae. They have forward facing eyes that indicate this visual foraging use which is common of diving ducks. Predator absence had evolved the feature in it to use visual cues. Blue ducks also utilise more tactile cues than visual ones to scrape small chironomid larvae from submerged surfaces. Hence a disruption in water clarity in streams can significantly affect the duck's visual foraging patterns.