Systematics

Geographic Variation

Phenotypic variation is complex. Birds in the humid Pacific Northwest and (likewise humid) Newfoundland are darker gray, following Gloger’s rule. Another example of Gloger’s rule is evident in paler gray birds in the arid Great Basin, although in contrast to the rule the palest birds occur in Alaska. Variation is otherwise in the extent of white edgings to the remiges (generally increasing east to west, at least from the ne. Atlantic to the Rocky Mts.), the length of the tail (longest mid-continent), and extent of buff coloration on the flanks (grayest mid-continent and Alaska) or rump (Rocky Mts.).

Subspecies

Seven subspecies (see Duvall 1945, Am. Ornithol. Union 1957, Snow 1967, Phillips 1986, Smith 1991). The noted alpha taxonomist Allan R. Phillips (1986:84) remarked that “In view of apparent rapid foxing and seasonal color changes, I consider present subspecific distributions tentative. Much fresh material is needed.” For example, Phillips questioned the validity of Oberholser’s dark, Appalachian subspecies P. a. practicus after he discovered that the type specimen was dirty and its coloration thus misleading.

P. a. bartletti (Aldrich and Nutt, 1939). Resident on Newfoundland and Miquelon I. [type locality = Avalon Peninsula, Newfoundland]. Similar to P. a. atricapillus but upperparts and flanks dark gray, mantle tinged brown, and white edges to remiges less extensive. Presumably as a result of geographic isolation, this subspecies is the only subspecies whose mtDNA differs (Gill et. al. 1993).

P. a. atricapillus (Linnaeus, 1766). Includes P. a. frigoris (Sélys-Longchamps, 1843); P. a. practicus (Oberholser, 1937); P. a. anamesus (Todd, 1938); and P. a. aldrichi (Braund and McCullagh, 1940). Resident from se. Manitoba east through central Quebec to Maritime Provinces and south to e.-central Kansas and w.-central Missouri to central Ohio, central Pennsylvania, and n. New Jersey and south along Appalachian Mts. to w. North Carolina [type locality = Quebec City, Quebec]; in winter, sometimes wanders south to Mid-Atlantic region. Upperparts moderately dark, even gray; white edges to remiges (wing coverts and tail feathers) moderately extensive; flanks deep buff. Some clinal variation in color, with western populations slight paler, especially on the flanks. Size large (male wing = 60.0–68.6 mm), but tail short (58.0–66.0 mm).

P. a. septentrionalis (Harris, 1845). Includes P. a. albescens (Bairds, 1858) and P. a. fortuitus (Dawson and Bowles, 1909). Resident from s. Yukon and through most of British Columbia (except sw. corner) east to central Manitoba and south, east of the Cascades and excluding the Great Basin and Rocky Mts., to s. Oregon, central Idaho, n. Montana, and thence south, east of the Rocky Mts., to central Colorado and central Kansas [type locality = Yellowstone R. ~30 mi. north of Missouri R., Montana]; occasionally wanders south in winter, with records to n. Arizona and Oklahoma panhandle. Similar to P. a. atricapillus size and plumage, but tail longer (63.0–72.5 mm), upperparts paler gray, and white edges to remiges more extensive; wing averages longer.

P. a. garrinus (Behle, 1951). Resident through the Rocky Mts. from central and e. Idaho and s.-central Montana south to se. Utah and n. New Mexico [type locality = Merkley Park (10 mi. north of Vernal), Uintah Co., Utah]. Similar to P. a. septentrionalis but rump buff, contrasting mantle brownish gray, and flanks paler and pinker.

P. a. nevadensis (Linsdale, 1938). Resident in the Great Basin from se. Oregon and sw. Idaho south to ne. Nevada (local in small numbers; P. Lehman) and n.-central Utah [type locality = Salmon R. at Shoshone Creek, Elko Co., Nevada]. Similar to P. a. garrinus but lacks brown and buff tones (i.e., coloration purer gray).

P. a. occidentalis (Baird, 1858). Resident on Pacific slope from sw. British Columbia to nw. California [type locality = Shoalwater Bay, Washington]. Darkest, brownest, and smallest subspecies (male wing = 57.5–65.0 mm, tail 53.5–60.5 mm).

P. a. turneri (Ridgway, 1884). Resident in s. and central Alaska [type locality = St. Michael, Alaska]; vagrants recorded to Pt. Barrow and Nunivak I. The palest subspecies: like P. a. septentrionalis but mantle paler still and flanks pale buff or white; tail averages shorter (61.5–67.0 mm).

Related Species

Recent evidence from DNA-DNA hybridization (Slikas et al. 1996) raised to full generic status the subgenera Baeolophus (New World titmice) and Poecile (chickadees and related Old World “gray tits,” sensu Gill et al. 2005), which were split from the genus Parus, the genus that formerly included nearly all chickadees, tits, and titmice in the world. The recommended split was adopted by the American Ornithologists’ Union (1997) and was supported by cytochrome b sequences (Gill et al. 2005).

Relationships within Poecile are fairly clear. Before recent genetic phylogenies, P. atricapillus was thought to be conspecific with or sister to P. montanus, the Willow Tit of Eurasia. These species are broadly similar in phenotype and voice, but they now are widely considered to be distinct species given differences in behavior, plumage, and biochemistry (Gill et al. 1989, 2005, Smith 1991). On the basis of mtDNA, Gill et al. (1993, 2005) concluded that P. atricapillus and P. gambeli (Mountain Chickadee) are sister species and that P. montanus is sister to P. palustris, the Marsh Tit of Eurasia, although as a whole these species are close (Sheldon et al. 1992).

The Black-capped Chickadee is more distantly related to the Carolina Chickadee (P. carolinensis), yet the species hybridize across a broad front where their ranges meet (Tanner 1952, Braun and Robbins 1986, Robbins et al. 1986, Bronson et al. 2003a,b). Hybrids are relatively common but are restricted to zone 20–30 km wide (50 km wide in Pennsylvania; Reudink et al. 2007) from Kansas east to New Jersey (Sattler and Braun 2000, Bronson et al. 2005). Many hybrids are fertile (Reudink et al. 2006), although hybrid pairs have lower hatching success resulting in decreased reproductive success (Bronson et al. 2003a, 2005).

The hybrid zone is shifting north: it has moved ~20 km north from 1986 to 2003 in se. Pennsylvania (Reudink et al. 2007) and 100 km north from the 1930s to 1980s in Ohio (Bronson et al. 2005). Carolina Chickadees replace Black-cappeds as the hybrid zone shifts. Mate choice is implicated in the movement of the hybrid zone: Carolina males are dominant to Black-capped males, and females prefer dominant males in mate choice trials (Bronson et al. 2003b) and thus tend to choose Carolina-like males as extra-pair mates (Reudink et al. 2006). Within the hybrid zone, where most males are hybrids, males sing either a typical Black-capped song or are "bilingual" and also sing a typical Carolina song (Curry et al. 2007). Songs combining elements of both species' songs occur rarely; therefore, song is considered to be an unreliable indicator of hybridization (Sattler et al. 2007). Chick-a-dee calls exhibit the opposite pattern, with Carolina-type calls dominant in hybrid zones.

The Black-capped Chickadee also has hybridized with the Mountain Chickadee (P. gambeli; Hubbard 1978, Howe 1985, Martin and Martin 1996, cf. Banks 1970) and, apparently, with the Tufted Titmouse (Baeolophus bicolor; McCarthy 2006).