Grinnell and Swarth (1926) described a Baja California subspecies of the Mountain Chickadee as Penthestes (now Parus) gambeli atratus, separating it from P. g. baileyae of southern California on the basis of longer tail, darker general coloration, and restriction of the white supercilium. The range as then known was confined to the Sierra San Pedro Mártir, at altitudes from 6000 to 8500 feet. Later Grinnell (1928) added the more northerly Sierra Juárez to the range of atratus, citing localities at 4200 and 5200 feet. He reported that “some individuals seek somewhat lower levels adjacent in winter,” and listed November and December specimens from El Valle de Trinidad, 2500 feet; this locality is “the valley leading westward from the pass which separates the Sierra Juárez from the Sierra San Pedro Mártir, about lat. 31°20'.” He also quoted Anthony (1893), who stated of the Mountain Chickadee that “in winter it was seen about Valladares and along the lower valleys.” Valladares is at the base of the Sierra San Pedro Mártir at 30°54'N, 115°41'W, elevation (fide Grinnell 1928) 2700 feet.