The Gray-headed Chickadee, formerly known as the Siberian Tit, is the only member of the family Paridae in common to the Old and New Worlds, occurring from Norway eastward across Eurasia into Alaska and arctic Canada. Ironically, although this is the rarest North American Poecile, many aspects of its general biology may be better known than those of any other American chickadee or titmouse because of extensive research in the Old World (where the name Siberian Tit is retained). This chickadee has been studied primarily in Scandinavia, most comprehensively by Haftorn ( Haftorn 1973 ) in Norway, by A. Järvinen (e.g., Järvinen 1982 ) and co-workers (e.g., Saari et al. 1994 ) in Finland, and by Pravosudov (e.g., Pravosudov 1987a ) in Russia. Most summary data in this account stem from studies in Fennoscandia and the former USSR; North American information is provided wherever possible, usually preceding information from elsewhere. This is a widely ranging species that appears to vary geographically in ecology, so results from one locality may not generalize everywhere.
The Gray-headed Chickadee closely resembles the Boreal Chickadee (Poecile hudsonicus) in morphology, behavior, ecology, and vocalizations and is of special interest as the candidate from whose ancestors may have given rise to the North American brown-capped and related species of chickadees. It is also of interest as one of the few cavity nesters ranging to tree line in subpolar habitats, for which environment it shows several apparent adaptations. Finally, although its vocal repertoire is generally organized like those of other parids of the subgenus Poecile (North American chickadees and Eurasian gray tits), the Gray-headed's vocalizations are remarkably complex and may hold a key to understanding the origin of song in songbirds.
Of all the tits, chickadees, and titmice in the northern hemisphere, the Gray-headed Chickadee, at least in the Old World, seems the most vulnerable to human disturbance (especially logging), with populations declining since at least the 1940s. (Trees in its habitat in Alaska are too small and scattered for commercial logging.) This chickadee lives in such harsh ecological conditions and is so sparsely distributed that its populations could decline rapidly.