About Polioptila nigriceps S.F.Baird, 1864
The black-capped gnatcatcher (Polioptila nigriceps S.F.Baird, 1864) measures 10 to 12 cm (3.9 to 4.7 in) long and weighs 5 to 8 g (0.18 to 0.28 oz). Adult individuals have blue-gray upperparts and white underparts. They possess a long slender bill and a long black tail, with mostly white outer tail feathers. Breeding (alternate plumage) males have a glossy black cap. Females have gray upperparts with less bluish tint than males, but are otherwise similar in appearance to non-breeding males. This species is very similar in appearance to the black-tailed gnatcatcher (P. melanura). Two subspecies occur in western Mexico: the nominate subspecies ranges from central Sinaloa south to Colima, while P. n. restricta mostly occurs from eastern Sonora south to northern Sinaloa. The species has occasionally nested just across the U.S. border, in the Madrean sky islands region of southernmost Arizona. In the northern portion of its range, it lives in mesquite thickets near riparian corridors. Further south, it inhabits thorn scrub and arid deciduous woodlands. Most of its population occurs at elevations below 500 m (1,600 ft). It is primarily non-migratory, though the northernmost populations may move south after breeding.
