The Oak Titmouse is a common woodland parid that is narrowly distributed from southwestern Oregon through California to northwestern Baja California, with an outlying population in southern Baja California. Until somewhat recently, the Oak Titmouse and Juniper Titmouse were regarded as a single species in the genus Parus, the Plain Titmouse (Parus inornatus). A comprehensive analysis of geographic variation in the species complex ( Cicero 1996 ), along with genetic evidence of relationships within the family ( Sheldon et al. 1992 , Slikas et al. 1996 ), led to their reclassification as sibling species in the genus Baeolophus ( American Ornithologists' Union 1997 ). The two species are highly similar in appearance, but can be distinguished by a suite of morphologic, colorimetric, genetic, vocal, and ecologic traits ( Cicero 1996 ).
Oak Titmice are closely tied to open oak (Quercus) or oak–pine (Quercus–Pinus) woodlands. Indeed, the species is among the most common bird species in the oak woodlands of California, the “voice and soul of the oaks” ( Shuford 1993a ). They are highly vocal, and individuals are most commonly recognized by the chatter-like calls which males and females utter throughout the year. Males may sing infrequently during the nonbreeding season, with singing intensity increasing toward spring.
Pair bonds form during the first year and are usually permanent. Natural or woodpecker-excavated cavities are used for nesting, and clutches typically contain 6–7 unmarked eggs. The diet is varied, and the species is known to cache food. Oak Titmice are sedentary and defend territories throughout the year. Unlike many other parids, the Oak Titmouse generally does not form flocks during the nonbreeding period.