Found in humid coastal and interior forests from southeastern Alaska to southern California, this chickadee has expanded its geographic range during the past 5 decades by colonizing forest habitats in the central Sierra Nevada and suburban areas in eastern San Francisco Bay. Although plausible explanations have been proposed, the exact reasons for this geographic expansion remain unknown.
The Chestnut-backed Chickadee is apparently most closely related to other “brown-headed chickadees” such as the Boreal Chickadee (Poecile hudsonicus) and possibly the Grey-headed Chickadee (P. cinctus). Like its closest relatives (Mexican [P. sclateri] and Boreal Chickadees), this species lacks a whistled song; unlike them, it uses the Gargle call rarely. This reduction of repertoire diversity may be somewhat balanced by a particularly robust Chick-a-dee call complex.
Chestnut-backed Chickadees exhibit interesting patterns of foraging behavior by focusing on insects that live on Douglas fir (Pseudotsuga menziesii), and other conifer trees in the humid coastal regions and portions of the interior forest of the Pacific Northwest. They also make considerable use of insects and arthropod foods from broad-leaved trees. Like most chickadees and titmice, Chestnut-backed Chickadees nest in tree cavities and readily colonize available nest boxes. During winter, they forage in mixed-species flocks with other chickadees, primarily Black-capped (P. atricapillus) and Mountain (P. gambeli) chickadees, as well as other small birds such as kinglets (Regulus spp.) and nut-hatches (Sitta spp.).
Key studies of this species have been in Washington State on breeding habits and foraging ecology (Sturman Sturman 1968a , Sturman 1968b ), and in California on foraging and habitat use (Brennan et al. Brennan et al. 1999b , Brennan et al. 1999a ), foraging and nestling diet ( Kleintjes and Dahlsten 1994 ), and ecological interactions ( Root 1964 ). Grinnell ( Grinnell 1904b ) used this chickadee in his classic paper about how species evolve in relation to a narrow range of environmental conditions.