Titmouse, traditional name for members of a family of songbirds. The only birds still called titmice are the crested North American species, such as the tufted titmouse (Parus bicolor).  It is a gray bird with rusty flanks and measures about 15 cm (about 6 in) long. The noncrested American species are called chickadees; all are small birds with black or brown caps and bibs. Titmice are gregarious most of the year and forage in flocks with other species. When the breeding season begins, pairs leave the flock and set up territories. Titmice nest in holes in trees or in nest boxes. Scientific classification: Titmice belong to the genus Parus of the family Paridae, order Passeriformes.