White-breasted Nuthatch is four species, study says
Nuthatches are small, short-tailed, sharp-billed songbirds widely recognized for their ability to hitch headfirst down tree trunks and upside-down along limbs. The family has representatives throughout the forests North America, Eurasia (including North Africa), and Indomalaya. Nuthatches are related to the Wallcreeper, treecreepers (Certhiidae), gnatcatchers, and wrens.
In North America, we have, traditionally at least, four species, the most familiar of which is the White-breasted Nuthatch (Sitta carolinensis).
White-breasted Nuthatch photographed in Ontario by Matt MacGillivray
But two studies suggest that White-breasted Nuthatches actually represent four distinctive and largely isolated populations that may deserve full species status.
In 2007, Garth M. Spellman and John Klicka published a paper, Phylogeography of the white-breasted nuthatch (Sitta carolinensis): diversification in North American pine and oak woodlands (full-text PDF), in which they revealed the existence of four distinctive populations based on studies of a single gene. They identified the populations as follows: Eastern clade; Pacific clade; Eastern Sierra Nevada clade; and Rocky Mountain, Great Basin, and Mexico clade.
Interestingly, the Eastern and Pacific clades are more closely related to each other than to the two populations in between them geographically. Spellman and Klicka propose that as patches of pine and oak forest were isolated by changes in the North American climate and uplift of mountain ranges in the west, populations were isolated and diverged.
Then, late last year, a graduate student named Woody Walstrom published a new paper, with Spellman and Klicka, that studied 20 different sections of DNA (instead of just one) and found strong support for the same four populations identified in the earlier study.
Noting these results in combination with long-recognized (if subtle) morphological and vocal differences, the 2011 paper proposes that all four populations could be given full species status.
So far, only the IOC World Bird Names project has made any move to consider this proposal, noting “potential splits” of Pacific Nuthatch, Great Basin Nuthatch, and Mexican Nuthatch from White-breasted Nuthatch.
These names sparked a predictably snarky discussion in the BirdForum taxonomy forum, leading ornithologist Frank Gill (of the World Bird Names project) to comment: “These initial names are just placeholders. Recommended English names for these candidates await discussion and decisions on species taxonomy.”
It’s not clear when the AOU might take this up, but it’s certainly interesting to consider, and it fits with a general pattern observed in many North American species groups that have been split and lumped repeatedly over the years.
What do you think — are you ready for seven North American nuthatch species? What should we call them?