Systematics

Figure 2. Numbers of White-breasted and Red-breasted Nuthatches

Geographic Variation

Complex geographic variation in body size (James 1970). In general, wing length correlates positively with latitude, excepting that Appalachian populations have the wings longer than those at similar latitudes in lowlands (James 1970, Wood 1992) and, west of the Great Plains, wing length increases somewhat from east to west. Along the Atlantic coast, wing and bill length vary as smooth clines, being longest in the north (Wood 1992); mantle may average paler to the north (Aldrich 1944). The proportion of females with the crown dark (blackish) increases toward the Southeast, reaching a peak in peninsular Florida (Wood 1992). Dorsal coloration is palest in East, darkest in the center of continent, and of medium saturation on the Pacific slope. Bill length peaks in central populations; the bill is tapered in Pacific and central birds, stout and blunt in Eastern birds.

Subspecies

The most recent revision (Phillips 1986) recognized 11 subspecies of White-breasted Nuthatch, a near doubling of the number in Ridgway (1904), but some of these subspecies are poorly defined. Subspecies fall into three distinct "call groups" (see Sibley 2000): (1) birds in the eastern United States and Canada utter a nasal yenk, (2) birds in the Great Basin, Rocky Mountains, and Mexico a rapid yijijijijiji or yiji-yiji-yiji-yiji, and (3) birds on the Pacific slope a high-pitched, drawn-out aaarn. Call groups correspond, respectively, to the Eastern clade, Eastern Sierra Nevada + Rocky Mountain, Great Basin, and Mexico clades, and Pacific clade of a detailed phylogeographic study (Spellman and Klicka 2007).

Carolinensis Group (Eastern):

S. c. carolinensis Latham, 1790. Includes S. c. atkinsi Scott, 1890, S. c. litorea Maynard, 1916, and S. c. cookei Oberholser, 1917. Phillips (1986) recognized the last subspecies, but Wood (1992) could not develop criteria that diagnosed more than 60% of specimens. Resident throughout eastern North America, from Maritime Provinces south to Florida and from southern Prairie Provinces south to north-central Texas. Bill stout, blunt; ventrum white; dorsum pale blue-gray.

Nelsoni Group (Central):

S. c. tenuissima Grinnell, 1918. Resident in Great Basin ranges from s.-central British Columbia south through mountains of w. and s. Nevada and e. California; vagrant to coastal California (Unitt 2004). Bill slender, very long; ventrum whitish but flanks dark; mantle dark blue-gray.

S. c. nelsoni Mearns, 1902. Includes S. c. uintaensis Twomey, 1942, and S. c. oberholseri Brandt, 1938. Resident in Rocky Mountains from w. Montana south to cen. Sonora, w. Oklahoma, and sw. Texas. Like S. c. tenuissima but ventrum grayish white, flanks darker, and bill shorter (no overlap; Phillips 1986:103).

S. c. mexicana Nelson and Palmer, 1894. Includes S. c. umbrosa van Rossem, 1939, and S. c. kinneari van Rossem, 1939. Resident in Mexico from sw. Chihuahua, central Nuevo León, and w. Tamaulipas south through central volcanic belt to s. Guerreo and Oaxaca. Like S. c. nelsoni but ventrum and flanks darker; bill and wing length average shorter, especially to the south.

S. c. lagunae Brewster, 1891. Resident in Sierra de Laguna, Baja California Sur. Bill stout; size like that of S. c. aculeata; darker dorsally and ventrally than S. c. tenuissima. Despite residing on the "Pacific coast," this subspecies groups with the central clade phylogeographically (Spellman and Klicka 2007). Voice undescribed?

Aculeata Group (Pacific):

S. c. aculeata Cassin, 1856. Resident on Pacific slope from nw. Washington south to nw. Baja California peninsula; vagrant to Mojave Desert (Patten et al. 2003). Bill slender, medium length; ventrum washed brownish; dorsum medium blue-gray.

S. c. alexandrae Grinnell, 1926. Resident in the Sierra San Pedro Mártir, Baja California. Like S. c. aculeata but bill longer (no overlap; Phillips 1986:102–103).

Related Species

Although long considered to be allied with the chickadees and tits (Paridae) and the creepers (Certhidae), molecular phylogenies (Sheldon and Gill 1996, Ericson and Johannson 2003) place the nuthatches in a group with the creepers, wrens (Trolodytidae), and gnatcatchers (Polioptilinae), in a radiation separate from the chickadees. Despite similarities in morphology and behavior, the Australian creepers (Daphoenosittidae) are not closely related to the nuthatches. By contrast, the Wallcreeper (Tichodroma muraria) of the southern Palearctic is the lone representative of a distinct subfamily (Tichodrominae) within the nuthatch family; all other species are in the subfamily Sittinae and, indeed, in the genus Sitta.

Relationships within the genus Sitta are unclear, as is the closest relative of the White-breasted Nuthatch (Mayr and Short 1970). Hybridization among the four North American species is unknown (McCarthy 2006); however, the Brown-headed (S. pusilla) and Pygmy (S. pygmaea) nuthatches are similar enough morphologically and ecologically that they have been treated as conspecific (e.g., Phillips et al. 1964, Mayr and Short 1970).