The only New World representative of the family of long-tailed tits (Aegithalidae), the Bushtit is a very small, drab-gray bird with a long tail and a social nature. Throughout its range, which is primarily limited to the western parts of North America and the highlands of Central America, it is a common resident year-round, and often observed in flocks of 3 to more than 40 conspecifics or in mixed-species flocks. It prefers oak (Quercus spp.)-pine (Pinus spp.) forests in the mountains and shrub vegetation in the coastal region of its range and forages by gleaning insects from foliage, often upside down in the manner of chickadees (Poecile spp.).
Appearance is variable, with some individuals possessing black ear coverts. For many years, the black-eared and the plain-eared forms of the Bushtit were separated into 2 distinct species (Psaltriparus melanotis and P. minimus, respectively). However, it is now clear that the amount of black in the face is a polymorphism, the frequency of which varies geographically. In general, it occurs commonly in the extreme southern populations, is absent in the north, and is more common in males than in females ( Raitt 1967 ).
One of the first birds ever described as having “helpers at the nest” (Guatemala; Skutch 1935 ), the Bushtit varies considerably in this behavior, geographically and from year to year. In coastal areas of California, nest supernumeraries, individuals above and beyond the primary breeding pair, are rare ( Addicott 1938 , Ervin 1974 ). However, in the Chiricahua Mountains of southeastern Arizona, an average of 37% of nests have supernumeraries (Sloane Sloane 1992 , Sloane 1996 ). These extra individuals can be male or female, juvenile or adult. The majority are adult males that either are unmated or have lost a nest. Some of these individuals may be contributing genetically to the nest, making Bushtits potentially polygynous, polyandrous, or polygynandrous in most years (Sloane Sloane 1992 , Sloane 1996 ; but see Bruce et al. 1996 ). The key to this unusual social system may lie in Bushtit physiology and the behavioral mechanisms, such as huddling and the building of heavily insulated nests, that they use to cope with cold (Sloane Sloane 1992 , Sloane 1996 ).