New Zealand wren
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 Xenicus lyalli
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The New Zealand wrens, Acanthisittidae, are a family of tiny passerines endemic to New Zealand. They are represented by six or seven known species in four or five genera, although only two species survive 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 recent studies suggest that they form a third, most ancient, suborder Acanthisitti and have no living close relatives at all. They are called "wrens" due to their similar appearance and behaviour, but are not related to true wrens (Troglodytidae) of the northern hemisphere.
New Zealand wrens are mostly insectivorous foragers of New Zealand’s forests, with one species, the South Island 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 a bunting from the Canary Islands, the only passerines 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. Three species went extinct after the arrival of the Maori and the Polynesian Rat, and are known today only from fossil remains. Two species, the Stephen’s 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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[edit] Taxonomy and evolution
The taxonomy of the New Zealand wrens has been a subject of considerable debate since their discovery by science, 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 which suggested the family was a sister taxon to the subocines and the oscines. More recent studies have supported this, and the New Zealand wrens are now thought to be the survivors of a lineage of passerines that was isolated when New Zealand broke away from Gondwana 82-85 million years ago. The extant species are closely related and thought to be decedents of birds that survived a genetic bottleneck caused by the marine transgression during the Oligocene when most of New Zealand was underwater (Cooper & Cooper 1995).
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 South Island Wren (known locally as 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 (Worthy & Holdaway 2002). The final genus was Dendroscansor, which had one species, the Long-billed Wren.
[edit] Distribution, habitat and movements
The New Zealand wrens are endemic and restricted to the 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 widepread distribution across North, South and Stewart Island. The range of the Rifleman and Bush Wren included southern beech forest and and podocarp-broadleaf forest, with the range of the Bush Wren also including coastal forest and scrub, particularly the Stewart Island subspecies. The South Island Wren is specialised for the alpine environment, in areas of low scrub and scree from 900 m up to 2400 m. Contrary to its name it 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 (Fuller, 2002), but fossil evidence has shown the the species was once widepread on both North and South Island (Worthy & Holdaway 2002). The Stout-legged Wren was similarly found on both islands, but fossils of the Long-billed Wren have only been found on South Island.
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 South Island Wren is now restricted to South Island and is declining in numbers. 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.
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 South Island Wren is an ornithological mystery, as they are thought to live above the snow line where obtaing food during the winter would be extremely difficult. Searches have found no evidence that they move altitudinally during the winter, howvere 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 (Gill, 2004).
[edit] Morphology
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 (Higgins 2002, Gill 2004); the South Island 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 South Island Wren and the Rifleman also show sexual dimorphism in size, unusually for passerines it is the female that is larger than the male (Gill 2004). The female Rifleman also exibits other differtences 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-biled 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. Contempory accounts of the Stephens Island Wrens describe the species as scrurrying on the ground rather than flying.
[edit] Species
- Titipounamu or Rifleman, Acanthisitta chloris
- Bush Wren, Xenicus longipes (probably extinct)
- Rock Wren or South Island Wren, Xenicus gilviventris (vulnerable)
- Stephens Island Wren, Xenicus lyalli (extinct)
- Stout-legged Wren, Pachyplichas yaldwyni (extinct in prehistoric times)
- Long-billed Wren, Dendroscansor decurvirostris (extinct in prehistoric times)
[edit] References
- Cooper A. & Cooper R. (1995) "The Oligocene Bottleneck and New Zealand Biota: Genetic Record of a past Environmental Crisis" Proceedings of the Royal Society of London, Series B. 261(1362) 293-302.
- Fuller, E. (2002) Foreword; Extinct Birds Pp. 11-69 in del Hoyo J., Elliott A. & Christie D.A. (2004) Handbook of the Birds of the World. Volume 7. Jamacars to Woodpeckers Lynx Edicions, Barcelona ISBN 84-87334-37-7
- Gill, B.J. (2004) Family Acanthisittidae (New Zealand wrens) Pp. 464-474 in del Hoyo J., Elliott A. & Christie D.A. (2004) Handbook of the Birds of the World. Volume 9. Cotingas to Pipits and Wagtails Lynx Edicions, Barcelona ISBN 84-87334-69-5
- Higgins P.J., Peter J.M & Steele W.K. (Eds) (2001). Handbook of Australian, New Zealand and Antarctic Birds. Volume 5: Tyrant-flycatchers to Chats Oxford University press, Melbourne. ISBN 0-19-553244-9
- Worthy, T.H. & Holdaway, R.N. (2002) The Lost World of the Moa, Indiana University Press:Bloomington, ISBN 0-253-34034-9
