White-breasted Nuthatch at Palomar Mountain

White-breasted (Slender-billed) Nuthatch. Palomar Mountain, California. July 13, 2014. Greg Gillson.

The most recent (May/June 2014) issue of Birding magazine has an article on White-breasted Nuthatches as a cryptic species.

Definition of Cryptic Species: "a group of individual organisms which are virtually identical in appearance but nonetheless each satisfy the biological definition of a species - that is, they are reproductively isolated from each other." --A Cryptic Species Complex. Daniel H. Janzen, Winnie Hallwachs and John Burns.

Five recognized subspecies of White-breasted Nuthatches in North America are thought to perhaps fit the definition of full species to make 3 species. Thus last year the American Ornithologists' Union (AOU) voted on a proposal to split White-breasted Nuthatches. The proposal failed at this time. However, it is possible that such a split may happen in the future. Even if not, birders may want to be aware of the differences.

The Birding article discussed dividing White-breasted Nuthatch into 3 groupings: Carolina Nuthatch, Rocky Mountain Nuthatch, and Slender-billed Nuthatch.

Range
Carolina Nuthatch found east of the Rocky Mountains from extreme NE British Columbia to Prince Edward Island south from eastern Texas to Georgia. Rocky Mountain Nuthatch is found from south-central British Columbia to western Montana south from Arizona to western Texas, and south in Mexico to Oaxaca. A few outliers are found in the Black Hills of South Dakota and western Oklahoma. Slender-billed Nuthatch is found from SW Washington, western Oregon, most of California, to NW Baja.

Identification
The Rocky Mountain Nuthatch has a narrower dark crown stripe, thus a more extensively white face than the other two. The Carolina Nuthatch has an obvious dark line back from the eye. The main difference in the field is darker black greater coverts and tertials on otherwise pale gray wing on Carolina Nuthatch, some on Rocky Mountain Nuthatch, and virtually none on Slender-billed Nuthatch. The flanks are darker gray on Rocky Mountain Nuthatch. There is overlap in bill length, but the Rocky Mountain form averages longer, and has a maximum length longer than the others (see my photo of the long-billed Rocky Mountain Nuthatch below). Call notes are different, as well, as would be expected for different species.

White-breasted (Rocky Mountain) Nuthatch. Grant County, Oregon. May 29, 2007. Greg Gillson.