Anatra blu

Identificazione

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). 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. 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 .<p>The male's call is a high-pitched whistle. The female's call is a rattling growl or low-pitched grating notes.

Tassonomia

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 that 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".<p>The species has no close relatives. 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. It was formerly thought to be related to the shelduck tribe.<p>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. Other names may be known by are Mountain Duck or Blue Mountain Duck.<p>Two subspecies are recognised:<p>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.

Nome scientifico

Hymenolaimus malacorhynchos

Autorità

(Gmelin, JF, 1789)

Ordine

Anseriformi

Famiglia

Sconosciuto

Genere

Hymenolaimus

Areale e habitat

Migrazione

Resident species

Comportamento

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.<p>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. The blue duck on occasion take berries and the fruits of shrubs.<p>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.<p>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.

Conservazione

Tendenza della popolazione

declining