The Mountain Chickadee, a small, cavity-nesting songbird, is one of the most common birds of montane coniferous forests from southern Arizona and Baja California north to British Columbia and the Yukon Territory. The closest living relative of the more familiar Black-capped Chickadee (Poecile atricapilla), the Mountain Chickadee appears quite similar in biology to that better-studied species. Where the ranges of the 2 species overlap in the Rocky Mountains, they tend to segregate by habitat, but where both are sparse, most notably in the Rio Grande valley of New Mexico, they may hybridize extensively.
Mountain Chickadees cache conifer seeds as soon as they become available in autumn. The need to defend dispersed seeds promotes group territoriality and hierarchical social organization in this and other chickadees. Dominance hierarchies regulate social organization within groups, and many juveniles descend to the lowlands in seed-poor years. Migrants return to the same social groups of unrelated birds in the spring, and all members tend to mate within these social groups year after year.
Mountain Chickadees are monogamous and territorial for the breeding season. Parents show significant individuality in prey selection when feeding young. Mountain Chickadees abandon foraging in a location when the time to capture prey there significantly exceeds the time to capture prey in previous visits to that site.
Local populations experience occasional food-related crashes, forcing immigration among populations with different seed-crop schedules. Human-caused changes in the landscape, such as reduced availability of dispersal corridors and seasonally important food sources, could reduce the abundance of this species.
This species is easy to study, but most information on its natural history comes from 2 long-term studies. The longest was started in 1966 in Modoc National Forest, northeastern California, by D. L. Dahlsten and has expanded to study sites on the west side of the central Sierra Nevada (El Dorado County), and the Tehachapi Mountains (Kern County) of southern California. This descriptive study has focused on foraging ecology and breeding biology in nest boxes. D. A. McCallum has studied the species since 1978 at Cottonwood Gulch, in the Zuni Mountains of western New Mexico. This study was most intensive from 1983 to 1988, when nests in boxes and cavities were compared and subplots received supplemental food. Recently, the focus of the study has turned to vocal communication. In this account, these studies will be referred to by geographic location (e.g., central California for Dahlsten's El Dorado County site). Nest-box studies have also been conducted in the eastern Sierra Nevada (M. Reynolds unpubl.) and southern Wyoming (see Wachob 1996a ).