Noted for its tameness and ability to live near human habitations, the Boreal Chickadee has acquired a variety of local names, especially in Newfoundland, including Tom-tit, Chick Chick, and Fillady. Former common names for the species include Hudsonian Chickadee, Acadian Chickadee, and Brown-capped Chickadee.
The Boreal Chickadee is one of the few passerines with a range almost completely restricted to the boreal forests of Canada and adjacent portions of the United States, where it is often found in habitats with other coniferous species, such as Golden-crowned and Ruby-crowned kinglets (Regulus satrapa and R. calendula). Like most other chickadees, the Boreal Chickadee is territorial during the breeding season but flocks the rest of the year. Some behaviors of the Boreal Chickadee, such as timing of reproductive events, food storage of seeds and insects, and tendencies for irruptive movements, may reflect the uncertainties faced by a species living in a harsh environment.
On the basis of both morphology and recent molecular studies, it is apparent that the closest relatives of this species are the Gray-headed Chickadee (Poecile cinctus), found in boreal forests of both the Old World and New World (where it is rare), and the Chestnut-backed Chickadee (P. rufescens), a species usually associated with coastal coniferous forests of Canada and the United States. The three species belong to the "brown-capped" assemblage of chickadees, and molecular analysis indicates that another coniferous species, the Mexican Chickadee (P. sclateri), may be a member of this group ( Gill et al. 1993 ). All seem to lack a whistled song, which is typical of other North American chickadees, and some other vocalizations are more similar to the songs of other members of this group than to those of other North American chickadees also included in the subgenus Poecile (Black-capped Chickadee [Poecile atricapillus]; Carolina Chickadee [P. carolinensis], and Mountain Chickadee [P. gambeli]).
Present knowledge of this species includes basic breeding biology and vocal repertoire, distribution and habitat utilization, as well as phylogenetic relationships with other chickadees. Gaps in knowledge are many, however, including winter feeding and flocking. Even the better-studied aspects, such as vocalizations, require additional information to determine how sounds are used, especially with regard to territorial behavior.