These birds are called "chickadees" (onomatopoeic, derived from their distinctive "chick-a dee dee dee" communication or alarm call) or "titmice" in North America, and just "tits" in the rest of the English speaking world. The name titmouse is attested from the 14th century, composed of the Old English name for the bird, mase (Proto-Germanic *maison, German Meise) and tit, denoting something small. The spelling was influenced by mouse in the 16th century.
These birds are mainly small stocky woodland species with short stout bills. Some have crests. They range in length from 10 to 22 centimetres. They are adaptable birds, with a mixed diet including seeds and insects. Many species will live around human habitation and come readily to bird feeders for nuts or seed, and learn to take other foods. In Britain, Great Tits and Blue Tits famously learned to break open the foil caps sealing bottles of milk that had been delivered to homes to get at the cream floating on top.
These are hole-nesting birds, typically using trees, although some species build nests on the ground. They lay anything from three to nineteen speckled white eggs, depending on species.
More recently, the large Parus group has been gradually split into several genera (as indicated below), which has been pioneered by North American ornithological authorities and to a more limited degree (as of now) elsewhere. Whereas in the mid-1990s, only Pseudopodoces, Baeolophus, Melanochlora and Sylviparus were considered well-supported by the available data as distinct from Parus (Harrap & Quinn 1996). Today, this arrangement is considered paraphyletic as indicated by mtDNA cytochrome b sequence analysis and Parus is best restricted to the Parus major - Parus fasciiventer clade, and even the latter species' closest relatives might be considered a distinct genus (Gill et al. 2005).
In the Sibley-Ahlquist taxonomy, the Paridae family is much enlarged to include related groups such as the Penduline tits and Long-tailed tits, but while the former are quite close to the titmice indeed and could conceivably included in that family together with the stenostirid "warblers", the long-tailed tits are not. Indeed, the Yellow-browed Tit and the Sultan Tit are possibly more distant to the titmice than are the penduline tits (Gill et al. 2005 and Jønsson & Fjeldsa 2006). If the two current families are lumped into the Paridae, the titmice would be a subfamily Parinae.
Alternatively, all tits - save the 2 monotypic genera discussed in the preceding section and possibly Cyanistes, but including Hume's Ground Tit - could be lumped in Parus. In any case, 4 major clades of "typical" tits can be recognized: the dark-capped chickadees and their relatives (Poecile including Sittiparus), the long-crested Baeolophus and Lophophanes species, the usually tufted, white-cheeked Periparus (including Pardaliparus) with more subdued coloration and finally Parus sensu stricto (including Melaniparus and Macholophus). Still, the interrelationship of these as well as the relationships of many species within the clades are not well resolved at all; analysis of morphology and biogeography probably gives more a robust picture than the available molecular data (Gill et al., 2005).
Titmice have settled North America twice, probably at some time during the Early-Mid Pliocene. The first were the ancestors of Baeolophus; chickadees arrived somewhat later (Gill et al., 2005).