| Acanthisittidae | |
|---|---|
| Xenicus lyalli | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Passeriformes |
| Suborder: | Acanthisitti |
| Family: | Acanthisittidae Sundevall, 1872 |
| Genera | |
The New Zealand wrens, Acanthisittidae, are a family of tiny passerines endemic to New Zealand. They were represented by six known species in four or five genera, although only two species survive in two genera today. They are understood to form a distinct lineage within the passerines, but authorities differ on their assignment to the oscines or suboscines (the two suborders that between them make up the Passeriformes). More recentstudies suggest that they form a third, most ancient, suborder Acanthisitti and have no living close relatives at all. They are called "wrens" due to similarities in appearance and behaviour to the true wrens (Troglodytidae), but are not members of that family.
New Zealand wrens are mostly insectivorous foragers of New Zealand’s forests, with one species, the Rock Wren being restricted to alpine areas. Both the remaining species are poor fliers and four of the five extinct species are known to or are suspected of having been flightless (based on observations of living birds and the size of their sternum); along with the Long-legged Bunting from the Canary Islands they are the only passerines known to have lost the ability to fly. Of the species for which the plumage is known they are drab coloured birds with brown-green plumage. They form monogamous pair bonds to raise their young laying their eggs in small nests in trees or amongst rocks. They are diurnal and like all New Zealand passerines for the most part sedentary.
New Zealand wrens, like many New Zealand birds, suffered several extinctions after the arrival of humans in New Zealand. Two species went extinct after the arrival of the Māori and the Polynesian Rat, and are known today only from fossil remains; a third, the Stephen's Island Wren went extinct on the main islands, surviving only as a relict population on Stephens Island in the Cook Strait. Two species, the Stephens Island Wren and the Bush Wren, became extinct after the arrival of Europeans, with the Bush Wren surviving until 1972. Of the two remaining species the Rifleman is still common on both North and South Island, while the South Island Wren is restricted to the alpine areas of South Island and is considered vulnerable.
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The taxonomy of the New Zealand wrens has been a subject of considerable debate since their discovery, although it has long been known that they are an unusual family. In the 1880s Forbes assigned the New Zealand wrens to the subocines related to the cotingas and pittas (and gave the family the name Xenicidae). Later they were thought to be closer to the ovenbirds and antbirds. Sibley’s 1970 study comparing egg-white proteins moved them to the oscines, but later studies including the 1982 DNA-DNA hybridization study suggested the family was a sister taxon to the subocines and the oscines. This theory has proven most robust since then, and the New Zealand wrens might be the survivors of a lineage of passerines that was isolated when New Zealand broke away from Gondwana 82-85 mya (million years ago),[1] though a pre-Paleogene origin of passerines is highly disputed and tends to be rejected in more recent studies.
It must be remarked that Ericson et al.'s study used an entirely unreliable molecular clock. The Cretaceous date it suggested it generally not taken seriously by the majority of researchers today.
As there is no reason to believe that passerines were flightless when they arrived on New Zealand (that apomorphy is extremely rare and unevenly distributed in Passeriformes), they are not required by present theories to have been distinct in the Mesozoic. As unequivocal Passeriformes are known from Australia some 55 mya, it is likely that the acanthisittids' ancestors arrived in the Late Paleocene from Australia or the then-temperate Antarctic coasts. Plate tectonics indicates that the shortest distance between New Zealand and those two continents was roughly 1,500 km (not quite 1,000 miles) at that time. New Zealand's minimum distance from Australia is a bit more today - some 1,700 km/1,100 miles -, whereas it is now at least c.2,500 km (1,550 miles) from Antactica.
The extant species are closely related and thought to be descendents of birds that survived a genetic bottleneck caused by the marine transgression during the Oligocene when most of New Zealand was underwater.[2]
The relationships between the genera and species are poorly understood. The extant genus Acanthisitta has one species, the Rifleman, and the other surviving genus, Xenicus includes the Rock Wren and the recently extinct Bush Wren. Some authorities have retained the Stephens Island Wren in Xenicus as well, but it is often afforded its own monotpic genus, Traversia. The Stout-legged Wren (genus Pachyplichas) was originally split into two species but more recent research disputes this.[3] The final genus was Dendroscansor, which had one species, the Long-billed Wren.[4]
The New Zealand wrens are endemic and restricted to the main islands of New Zealand and their offshore islands; they have not been found on any of the outer islands of New Zealand (such as the Chathams or the Kermadec Islands). Prior to the arrival of humans in New Zealand they had a widespread distribution across North, South and Stewart Island/Rakiura. The range of the Rifleman and Bush Wren included southern beech forest and podocarp-broadleaf forest, with the range of the Bush Wren also including coastal forest and scrub, particularly the Stewart Island subspecies. The New Zealand Rockwren is specialised for the alpine environment, in areas of low scrub and scree from 900 m up to 2,400 m. Contrary to its other common name (the South Island Wren) fossil evidence shows it was more widespread in the past and lived on North Island. The Stephens Island Wren was once thought to have been restricted to the tiny Stephens Island in the Cook Strait,[5] but fossil evidence has shown the species was once widespread on both North and South Island.[3] The Stout-legged Wren was similarly found on both islands, but fossils of the Long-billed Wren have only been found on South Island. Fossils of the Long-billed Wren are far less common than those of the other species, in fact its bones are the rarest fossil finds in New Zealand.[3]
After the wave of extinctions and range contractions caused by the arrival of mammals in New Zealand the New Zealand wrens have a much reduced range. The New Zealand Rockwren is now restricted to South Island and is declining in numbers.[6] The range of the Rifleman initially contracted with the felling of forests for agriculture but it has also expanded its range of habitats by moving into plantations of introduced exotic pines, principally the Monterey Pine. It also enters other human-modified habitat when it adjoins native forest.[7]
Like all New Zealand passerines the New Zealand wrens are sedentary, and are not thought to undertake any migrations. It is not known if the extinct species migrated but it is considered highly unlikely as three of the extinct species were flightless. The situation with the Rock Wren is an ornithological mystery, as they are thought to live above the snow line where obtaining food during the winter would be extremely difficult. Searches have found no evidence that they move altitudinally during the winter, however they are also absent from their normal territories. It is suspected that they may enter a state of torpor (like the hummingbirds of the Americas or a number of Australian passerines) during at least part of the winter but this has not yet been proved.[7]
New Zealand wrens are tiny birds; the Rifleman being the smallest of New Zealand's birds. Their length ranges from 7 cm to 10 cm, and their weight from as little as 5-7g for the Rifleman to an estimated 50g for the extinct Stout-legged Wren. The South Island Wren (and probably the Bush Wren) weighs between 14-22g, and the extinct Long-billed Wren around 30g.
The plumage of the New Zealand wrens is only known for the four species seen by European scientists. All these species have dull green and brown plumage, and all except the Stephens Island Wren have a prominent supercilium above the eye. The plumages of males and females were alike in the Stephens Island Wren and the Bush Wren;[7][8] the Rock Wren shows slight sexual dimorphism in its plumage and differences between the plumage of Riflemen are prononced, with the male having bright green upperparts and the female being duller and browner.
Both the Rock Wren and the Rifleman also show sexual dimorphism in size, unusually for passerines it is the female that is larger than the male.[7] The female Rifleman also exhibits other differences from the male, having a slightly more upturned bill than the male and a larger hind claw.
The New Zealand wrens evolved in the absence of mammals for many millions of years, and the family was losing the ability to fly. Three species are thought to have lost the power of flight, the Stout-legged Wren, the Long-billed Wren and the Stephens Island Wren. The skeletons of these species have massively reduced keels in the sternum, and the flight feathers of the Stephens Island Wren also indicate flightlessness. Contemporary accounts of the Stephens Island Wrens describe the species as scurrying on the ground rather than flying.