Yet another passerine 'family'

I discovered yesterday via Creagrus that the stitchbird (Notiomystis cincta) has been elevated to 'new monotypic family' status in a recent paper in the Australian Journal of Zoology (Driskell et al., 2007). This has been in the works a little while - a preliminary paper a year ago reported on its unexpected position (Ewen et al., 2006). Stitchbirds are a highly vulnerable species of passerine from the North Island of New Zealand - I say 'vulnerable' rather than 'endangered' because I believe populations are fairly healthy in the very few areas they are found, but they could easily fall victim if those areas were invaded by introduced predators - the only surviving natural population is on Little Barrier Island, but they have been re-introduced on Tiritiri Matangi Island and the Karori Wildlife Sanctuary in Wellington. I was fortunate enough to see both males and females at Karori last year while I was unsuccessfully trying to find harvestmen. The male is a quite attractive little bird with a black head with white streaks on the cheeks (the image here is by Michael Szabo, and comes via GrrlScientist. In the past, the stitchbird has been included in the family Meliphagidae (honeyeaters - superfamily Meliphagoidea), but the genetic data analysed by the two papers shows that it is actually sister-group to the endemic New Zealand family Callaeidae (wattlebirds - superfamily Corvoidea)*. The Callaeidae + Notiomystis clade makes for a nice little endemic New Zealand radiation - though only four genera and a maximum of six species** are known, all genera are quite distinct from each other.

*The wattlebirds are referred to as Callaeidae or Callaeatidae, and I must confess I haven't a clue which is correct. The type genus is Callaeas.

**I say "maximum of six" because authors differ whether the two taxa each in Callaeas (kokakos) and Philesturnus (saddlebacks) are species or subspecies - current opinion favours the former.

A little background here so you know what I'm talking about - the Passeriformes are the perching birds, and are the largest of the generally recognised bird orders. Traditionally, Passeriformes have been divided between Oscines (songbirds) and suboscines - it's the Oscines that concern us at the moment. In the past, Oscines were divided into a large number of families, but most were relatively small with the vast majority of species included in a few giant cosmopolitan families such as Muscicapidae (flycatchers) and Sylviidae (warblers). While it was long realised that this situation wasn't entirely satisfactory, it more or less persisted until the appearance of the DNA-DNA hybridisation studies of Sibley & Ahlquist (Sibley & Ahlquist, 1990) completely overturned the boat.

Sibley & Ahlquist identified a major split in the Oscines into two clades. The shocking part was that these clades were almost totally unexpected - rather than lining up with the previously recognised families, Sibley and Ahlquist recognised a division between mostly Australasian oscines (Corvida) and the mostly Holarctic Passerida. Genera that were once regarded as fairly closely related were placed on opposite sides of the divide, with massive morphological convergences implicated. Further testing has refined the idea slightly (Ericson et al., 2002; Barker et al., 2004), such that while the Passerida has remained monophyletic, the 'Corvida' are now regarded as paraphyletic with regard to the Passerida. As things currently stand, the Menurae (lyrebirds and scrub-birds) are the basalmost Oscines (as originally predicted by morphology), followed by a small clade of Climacteridae (Australian woodcreepers) + Ptilonorhynchidae (bowerbirds), then the Meliphagoidea (honeyeaters and fairy wrens), then the Passerida (most Holarctic songbirds) sister to the Corvoidea (mostly Australasian songbirds, as well as a number of non-Australasian taxa such as crows and shrikes). The upshot of all this is that to move the stitchbird from Meliphagoidea to Corvoidea is a fairly significant shift.

I do have one quibble with this paper, though, which effectively amounts to a quibble about the current state of passerine taxonomy as a whole. As more and more taxa are shifted about on the passerine tree, the general response has been to divide them into smaller and smaller families. Yes, the idea of a "family" rank doesn't really mean anything, and ultimately the recognition of such is entirely arbitrary, but there are still pragmatic implications to what gets recognised as a "family" and what doesn't, because this is a rank often used in assessing diversity. Why should Notiomystis get its own "family"? Recognising a monotypic family tells us nothing about its relationships. In my opinion, it would have been far more informative to expand the concept of Callaeidae and place Notiomystis as its basalmost member. But as I already said, the recognition of family rank is entirely arbitrary, and there is ultimately no obligation to accept my option over the authors'.

REFERENCES

Barker, F.K., A. Cibois, P. Schikler, J. Feinstein, and J. Cracraft. 2004. Phylogeny and diversification of the largest avian radiation. Proceedings of the National Academy of Sciences of the USA 101: 11040-11045.

Driskell, A., L. Christidis, B. J. Gill, W. E. Boles, F. K. Barker & N. W. Longmore. 2007. A new endemic family of New Zealand passerine birds: adding heat to a biodiversity hotspot. Australian Journal of Zoology 55: 73-78.

Ericson, P. G. P., L. Christidis, M. Irestedt & J. A. Norman. 2002. Systematic affinities of the lyrebirds (Passeriformes: Menura), with a novel classification of the major groups of passerine birds. Molecular Phylogenetics and Evolution 25: 53-62.

Ewen, J. G., I. Flux & P. G. P. Ericson. 2006. Systematic affinities of two enigmatic New Zealand passerines of high conservation priority, the hihi or stitchbird Notiomystis cincta and the kokako Callaeas cinerea. Molecular Phylogenetics and Evolution 40 (1): 281-284.

Sibley, C. G., & J. E. Ahlquist. 1990. Phylogeny and Classification of Birds: a Study of Molecular Evolution. Yale Univ. Press: New Haven (CT).