Many Northwest birders are aware that there are two discrete populations of White-breasted Nuthatch (Sitta carolinensis) in the Pacific Northwest: one found in the pine forests of the interior and the other in broad valleys west of the Cascades in oak and ash forests. These two populations have very different conservation statuses as well. The interior birds (subspecies tenuissima) are apparently either stable or increasing, while the westside birds (subspecies aculeata) also called Slender-billed Nuthatch) have shown a dramatic decline and range contraction over the last century (Wahl et al 2005). As a result of their decline in the Northwest, they are a Candidate species in Washington state and in Oregon are listed as Sensitive. Both Washington and Oregon include Sitta carolinensis aculeata in their State Wildlife Action Plans.
Recent genetic work and vocalizations work has found consistent and significant differences between these two subspecies groups, as well as a third group, the eastern deciduous woodland birds (subspecies carolinensis). From this work, there are proposals to recognize these three groups as separate species, although when the North American Classification Committee evaluated a proposal in 2013, they concluded that additional information was needed. The purpose of this note is to help birders understand how to separate the two populations in the field, for both conservation status monitoring and to be ready for the potential split.
Field Identification – The most consistent field marks are vocalizations (Pandolfino and Pieplow 2015). Fortunately, nuthatches tend to be fairly noisy birds throughout the year, making most individuals identifiable. The plumage differences are more subtle, but can be useful, particularly in backing up vocal identifications (Mlodinow 2014). Even with a good knowledge of the field marks, eBirders will find that some individuals cannot be identified to population, and are best entered as White-breasted Nuthatches without modifiers.
Vocalizations – Resources on the Web
Potential overlap in the Northwest
The high elevation coniferous forests of the Cascade Range appear to create a significant ecological barrier between the two populations. Pandolfino et al (2017) documented that the Sierra Nevada range separates the two populations throughout most of their ranges in California, but did find two areas of potential overlap at each end of the range. They documented apparent intergrades at the north end of the Sierra Nevada, in Modoc county just south of the Oregon/California border and below the south end of the Cascade Range.
Oregon birders should be alert to the possibility of overlap in the area between Klamath Falls and Ashland. Another area of potential overlap is the Columbia River Gorge, which provides a low elevation break in the Cascades. Numerous eBird reports show an almost continuous distribution through the Gorge, with a short gap between Cascade Locks and Cook. eBirders should make a particular effort to identify birds to subspecies in this area, and whenever possible add audio recordings to their checklists.
White-breasted Nuthatches, particularly birds from the Interior West, are well known to disperse sporadically but broadly in the non-breeding season, likely in response to poor food supply in their breeding range in some years. For that reason, identification of winter birds by range can be problematic, particularly for birds found outside of their regular ranges. Birds found on the west side of the Cascades, away from their restricted breeding ranges, may be more likely from interior populations than from the scarce and declining Pacific populations. Adding audio recordings of out of range winter birds is a useful addition to a checklist.
Article by: Bill Tweit, Washington Department of Fish and Wildlife
Mlodinow, S.G. 2014. Cryptic species: The White-breasted Nuthatch. Birding 46: 26-32.
Pandolfino, E.R. and N.D. Pieplow. 2015. Comparison of vocalizations of the four U.S. subspecies of the White-breasted Nuthatch. Western Birds 46:278-290.
Pandolfino, E.R., K.P. Able, J.L. Dunn, K.L. Garrett, D. Lasprugato. 2017. Ranges of the subspecies of the White-breasted Nuthatch in California. Western Birds 48: 26-34.
Wahl, T.R., B. Tweit and S.G. Mlodinow. 2005. Birds of Washington: Status and Distribution. OSU Press, Corvallis, Oregon. 436 p.