The Black-capped Vireo is an active, little, gleaning songbird that breeds in some of the hottest areas in North America where suitable amounts of low scrubby deciduous vegetation, often oaks, are available. Unlike other vireos, this species shows distinct plumage differences in cap color between the sexes. In addition, males exhibit delayed-plumage maturation (first-year males have gray napes). During the breeding season, male Black-capped Vireos sing persistently well into the heat of the day, the intensity of their singing seeming to increase after singing by other local species has waned. This species' songs with alternating phrases are typical of those of many other vireo species, but they are unusual in being derived from a large syllable repertoire—an order of magnitude greater than that of other vireos.
The Black-capped Vireo breeds in a relatively narrow area of the south-central United States and north-central Mexico. It was likely extirpated from Kansas by the 1930s and is now gravely endangered in Oklahoma and much of the northern, eastern, and central portions of its range in Texas. Among the most influential factors contributing to its decline are nest parasitism by the Brown-headed Cowbird (Molothrus ater) and habitat deterioration through destruction and natural successional changes resulting from fire suppression. The Black-capped Vireo has been designated an Endangered Species by the U.S. Fish and Wildlife Service.