Taxonomy and Systematics
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), 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 is 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.
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. The final genus was Dendroscansor, which had one species, the Long-billed Wren.
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