Blue duck
Blue duck at Staglands, Akatarawa Valley
Conservation status
Endangered  (IUCN 3.1)[1]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Anseriformes
Family:	Anatidae
Genus:	Hymenolaimus G.R. Gray, 1843
Species:	H. malacorhynchos
Binomial name
Hymenolaimus malacorhynchos (Gmelin, JF, 1789)
Subspecies
See text
Synonyms
Anas malacorhynchus (protonym)

The blue duck or whio (Hymenolaimus malacorhynchos) is a member of the duck, goose and swan family Anatidae endemic to New Zealand. It is the only member of the genus Hymenolaimus. Its exact taxonomic status is still unresolved, but it appears to be most closely related to the tribe Anatini, the dabbling ducks.

The whio is depicted on the reverse side of the New Zealand $10 banknote.

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.^[2][3] 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".^[4] 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.^[5] The blue duck is now the only species placed in the genus Hymenolaimus that was introduced specifically for the species by George Robert Gray in 1843.^[6][7] 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".^[8]

The species has no close relatives.^[9] Its taxonomic relationships with other waterfowl species remains uncertain; DNA analysis has placed it as a sister to the South American dabbling ducks (Anatini), but with no close relative. As of 2013, it was commonly listed as incertae sedis but likely within the Anatinae and allied to the Anatini.^[10] It was formerly thought to be related to the shelduck tribe.^[11][12]

It is commonly known in New Zealand English by its Māori name Whio, pronounced /ˈfiɔː/ FEE-oh, which is an onomatopoeic rendition of the males' call.^[13][14] Other names it may be known by are Mountain Duck or Blue Mountain Duck.^[14]

Two subspecies are recognised:^[7]

• 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.^[15] However, the populations were defined as distinct subspecies by the International Ornithological Congress in 2022, based on strong genetic divergence and some plumage differences.^[7]

The blue duck is 53 cm (21 in) long and varies in weight by sex. Females are smaller than males, weighing 680–870 g (24–31 oz), whereas the males weigh 820–970 grams (29–34 oz).^[16] The plumage is a dark slate-grey with a greenish sheen on the head, a chestnut-flecked breast. The outer secondaries 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.^[17] The pinkish-white bill has fleshy flaps of skin hanging from the sides of its tip. The beak is green at hatching and develops its final colour eight hours later .

The male's call is a high-pitched whistle.^[13][14] The female's call is a rattling growl or low-pitched grating notes.^[13][14][18]

Blue duck recording

Recording of adult pair in a stream communicating and answering tape recorded calls.

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This species is an endemic resident breeder in New Zealand, nesting in hollow logs, small caves and other sheltered spots. It is a rare duck, holding territories on fast flowing mountain rivers. It is a powerful swimmer even in strong currents, but is reluctant to fly. It is difficult to find, but not particularly wary when located.

The blue duck feeds almost entirely on aquatic invertebrate larvae. A study of blue ducks on the Manganuiateao 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.^[19] The blue duck on occasion take berries and the fruits of shrubs.^[1]

The foraging behaviour of the blue duck is characterized by its reliance on benthic invertebrates in fast-flowing riverine environments. Blue ducks primarily engage in dabbling behaviour where they feed by scouring rocks rather than up-ending or dabbling at the water's surface. Observations indicate that territorial males and females dabble approximately 96% of the time, with juvenile ducks dabbling 86% of the time. 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 dabbling during midday corresponds with an increase in diving behaviour in deeper pools. This pattern suggests that blue ducks capitalize on invertebrate resources that vary in availability throughout the day.^[20] Diving behaviour was seen most frequent in March and July when water levels are higher and prey living on stones and boulders above the water-surface would have been inaccessible using alternative foraging methods.^[21] Blue ducks’ foraging activities peak during early morning and late afternoon, coinciding with the diurnal activity patterns of invertebrate drift. The reliance on these temporal patterns allows blue ducks to exploit a resource that is continuously recolonizing denuded areas in the river.^[22] Ducks can be seen feeding at other times but often the day and night is spent in hiding or resting.^[23] Feeding behavior 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, typical of diving ducks, some attribute the evolution of this feature to the special absence of predators.^[23] Blue ducks also utilize tactile cues to scrape small chironomid larvae from submerged surfaces. ^[24] Changes in water clarity can therefore significantly affect foraging efficiency, particularly for mobile prey. Territoriality in blue ducks is closely linked to the availability of food resources. Although blue ducks occupy large territories, the size is not primarily determined by food abundance; rather, it reflects the overlapping life cycles of benthic invertebrates, which exhibit minimal seasonal variation. ^[22] This indicates that while food resources are critical for blue duck distribution and population structure, they do not necessarily dictate territorial size.^[25]

Blue ducks display territorial behaviours, they are known for their year-round territoriality, with territories typically ranging from 0.5 to 1.5 kilometres along river habitats characterized by alternating pools and rapids. The size and boundaries of these territories depend on resource availability, environmental conditions, and competition with other blue ducks.^[22][20] During the breeding season, territory defence becomes critical as breeding pairs protect their territories from intruders. This behaviour secures access to essential resources and optimal breeding sites, enhancing reproductive success. Both male and female blue ducks cooperate to defend their territories, using a variety of behaviours to deter intruders. Defensive behaviours include head bobbing, vocalizations, and aggressive confrontations. These behaviours communicate their presence and resolve conflicts over territory.^{[20] [26]} The intensity of these interactions varies based on several factors, including the presence of a female during confrontations. When females are present, aggressive behaviours may escalate, highlighting the significance of mate guarding in territory defence. This interaction shows social and environmental factors shaping territorial behaviour.^[20] Territoriality in blue ducks is linked to habitat quality. Research indicates that pairs in higher-quality environments expend less energy on territorial defence, allowing for greater foraging opportunities and improved reproductive fitness.^[25] While pairs defend their territories, extensive areas between them may remain unprotected. Juvenile and unpaired blue ducks exploit these undefended spaces for foraging, indicating that territoriality does not completely limit resource availability for non-breeding individuals.^[20] After the breeding period, blue ducks may adjust their territorial size in response to resource availability, often reducing their territory to focus on the most productive areas. As the breeding season progresses, interactions between neighbouring pairs can intensify, particularly with increasing competition for shared resources. Most aggressive confrontations involve males defending territories against foraging intruders, highlighting male-male competition's role in shaping blue duck social dynamics.^[20][26]

Blue ducks exhibit a complex social structure characterized by strong pair bonds and monogamous behaviour, integral to their reproductive success and territory defence in riverine habitats. Typically, blue ducks maintain permanent year-round territories defended by mated pairs, emphasizing the significance of monogamy for the successful rearing of offspring. ^[22] Pair members rarely stray far from each other, with individuals often remaining in close proximity throughout the year.^[26] In blue ducks, synchronous behaviours further enhance the stability of pair bonds, as pairs coordinate their activities, particularly during periods of brood dispersal and moulting. Pairs often forage together, displaying high synchronization rates during various behavioural activities, which can range from 62% during moult to 81% in the pre-laying phase. ^[22] This synchronized behaviour is said to likely improve foraging efficiency and reduces vulnerability to predation, reinforcing the significance of the social bonds. During incubation, males shift their foraging habits to support female partners and their offspring, highlighting a collaborative approach to parental care. ^[26] Although blue ducks generally exhibit monogamous pair bonding, instances of extra-pair mating may occur, particularly when environmental pressures or territory dynamics shift. ^[22]

Blue ducks nest between August and October, laying 4–9 creamy white eggs. The female incubates the eggs for 31 to 32 days and chicks can fly when about 70 days old.^[16]

Nesting and egg incubation of four to seven eggs is undertaken by the female while the male stands guard. Nests are shallow, twig, grass and down-lined scrapes in caves, under river-side vegetation or in log-jams, and are therefore very prone to spring floods. For this, and other reasons, their breeding success is extremely variable from one year to the next.^[27]

Captive North Island whio are held and bred on both main islands of New Zealand, but the progeny are returned to their respective island. South Island whio are held and bred in captivity on the South Island only. All captives are kept by approved and permitted zoological and wildlife facilities as part of the national recovery plan. As part of this current ten-year plan (2009–2019) is the WHIONE programme which works with specially trained nose dogs to locate nests. The eggs are removed, and the ducklings hatched and raised in captivity. Later they are conditioned for coordinated release.

Blue ducks were presented to the International Waterfowl Association in the UK in the 1970s along with New Zealand shovelers, New Zealand scaup, and brown teal by The Wildlife Service of New Zealand. The species was maintained in the UK until at least 2012^[28] before dying out; efforts to create the only captive breeding population outside of New Zealand with these ducks ultimately failed when the last two male ducks formed a same-sex relationship with each other instead of with the female that was assigned to them.^[29] They have not been known to be exported and maintained anywhere else internationally.^[30]

The blue duck is classified as Endangered by the IUCN due to its highly fragmented and shrinking population, and it is listed as Nationally Endangered in the New Zealand Threat Classification System. A 2010 census estimated a total population size of 2,500–3,000 individuals, with a maximum of 1,200 pairs.^[1]

The blue duck is a very localised species now threatened by predation from introduced mammals such as stoats, competition for its invertebrate food with introduced trout, and damming of mountain rivers for hydroelectric schemes. Early recovery efforts by scientists, field workers and volunteers have been summarised in a project sponsored by Genesis Energy, the Central North Island Blue Duck Charitable Conservation Trust and the Royal Forest and Bird Protection Society in 2006.^[31] In 2009 the New Zealand Department of Conservation started a ten-year recovery programme to protect the species at eight sites using predator control and then re-establish populations throughout their entire former range.^[32] Female whio are especially vulnerable to stoats while nesting, and some populations are now 70 percent male.^[33] In one study area, clutches of eggs lasted an average of nine days before being destroyed by stoats, and the one brood that hatched was killed the next day.^[33]

In 2011 the New Zealand Department of Conservation and Genesis Energy started the Whio Forever Project, a five-year management programme for whio. It will enable the implementation of a national recovery plan that will double the number of fully operational secure blue duck breeding sites throughout New Zealand, and boost pest control efforts.

Wikimedia Commons has media related to Hymenolaimus malacorhynchos.

Wikispecies has information related to Hymenolaimus malacorhynchos.

• ARKive: Images and movies of the Blue Duck (Hymenolaimus malacorhynchos)
• BirdLife Species Factsheet.
• Blue duck/Whio at the Department of Conservation
• TerraNature | New Zealand ecology – Blue duck (Whio)
• Blue Duck Project Charitable Trust
• Whio Forever Project
• Central North Island Blue Duck Trust Archived 16 May 2013 at the Wayback Machine