The males of this species can best be identified by the glossy black cap and and white spectacles.
Females have the same markings, but the head is slate gray instead of black.
Both are olive above and white below and are about 4.5 inches in length with the female slightly smaller than the male. They were once found as far north as central Kansas, but because of the clearing of brushlands for agriculture and subsequent overgrazing by livestock, today, the black-capped vireo breeds only as far north as Blaine County, Oklahoma. There are two small populations in west-central Oklahoma, with the largest populations on the Lampasas Cut Plains and the Edwards Plateau in Texas, where the numbers are estimated at 1500 pairs. The current population of vireos in Mexico is unknown. During the winter, the black-capped vireo can be found on the west coast of Mexico, from southern Sonora to Guerrero.