I'm copying a reply of mine on the
Emberiza alcoveri thread that is also of interest here:
Xenicids or
Acanthisittidae, rather, are indeed generally considered suboscine passerines, In fact, they are considered to be the most primitive of all living passerines, representatives of an ancient lineage with no living relatives, supporting the theory of a Gondwanan origin for this order, already in the Cretaceous.
Nevertheless, the relationship of this family to other groups is far from clear:
- Sibley et al (1982), from the first DNA-DNA hybridization studies, indicated that the Acanthisittidae are the sister group to the suboscines.
- Raikow (1984), from analisys of the hind-limb myology concluded that they were outside from the suboscine radiation and he further stated that the syringeal anatomy excluded them also from the oscine group.
- Sibley & Ahlquist (1990), from a extensive DNA-DNA hybridization study, mentioned the possibility that Acanthisittidae should be asigned to a new, monotypic suborder.
- Barker et al (2001), in their phylogenetic hypothesis of Passerine birds, based on nuclear DNA data, support the possibility that Acanthisitta is the sister group to all other passerines (both oscines and suboscines included).
Data shows that this family radiated to at least seven species in five genera (
Acanthisitta, Xenicus, Traversia, Pachyplichas and
Dendroscansor, the last three extinct).
Traversia (merged at times with
Xenicus) correspond to the extinct Stephens Is Wren, which was just a relict population as it has been found in holocene deposits in both main islands.
Three species are known from late Pleistocene - Holocene caves and sand dune deposits, that probably died out after the Maori colonization in about 1250. Two belong to the genus
Pachyplichas, and were described in 1988: the North Island Stout-legged Wren (
P. jagmi) and the South Island Stout-legged Wren (
P. yaldwyni).
The last of the extinct species, the Long Billed Wren (
Dendroscansor decurvirostris), found only in the South Island, is the only one with long decurved bill and reduced sternum and therefore,
the only flightless species apart from Stephens Wren.
(Info extracted from HBW9)
That makes a total of three known flightless (to a certain degree) passerines. One oscine (
E. alcoveri) and two, let's say, "non-oscine" (
T. (X.) lyalli and
D. decurvirostris) passerines.
