Vireos and Peppershrikes: Vireonidae

Physical characteristics: Black-capped vireos are small vireos with olive-colored upperparts. Males have white under parts, yellow wash beneath the wings, yellowish-white wing-bars, reddish eyes, a blackish bill, a glossy black head, and blue-gray legs and feet. They also have white eye-rings that look like broken eyeglasses. Females are similar except for a slate gray to bluish gray head, white eye-rings, pale lemon-yellow wing bars, buffy white under parts, and yellowish wash on the sides and flanks. Juvenile females have plumage that is more buff-colored. Adults are about 4.5 inches (12 centimeters) long, with a wingspan of about 8 inches (20.3 centimeters) and a weight of about 0.3 ounces (8.5 grams).

Geographic range: During cold months, black-capped vireos are found on the west coast of Mexico. During the warm months when the birds breed, they are located in parts of Texas, New Mexico, Oklahoma, Missouri, and north-central Mexico.

Habitat: Black-capped vireos inhabit open, grassy woodlands that contain clumps of shrubs and trees, especially oak scrublands and dense low thickets. Within that environment, they are usually found around low-lying vegetation.

Diet: They feed mostly on invertebrates such as insects, their larvae (LAR-vee; active immature insects), and eggs, taken from the deep cover among leaves of trees and shrubs. Other food sources are small spiders, small fruits, and berries.

Behavior and reproduction: Black-capped vireos are solitary birds. They migrate short distances between breeding and nonbreeding seasons, often going southwest, wintering along the western coast of Mexico. The birds defend breeding territories. Their song is a hurried string of husky-sounding two- or three-note phrases that is repeated slowly, such as "grrtzeepididid, prididzeegrrt . . . " Their call is a "ji-dit" or "tsidik."

The birds are monogamous (muh-NAH-guh-mus; having one mate). Males court females with fluttering display flights. The male then sings a courtship song, often with the spreading of his wings. The mating pair builds a cup-shaped nest that is made of twigs, bark, and leaves, surrounded with silk and lined with fine grasses. The nest hangs down from a branch fork of a shrub or low tree, about 1 to 15 feet (0.3 to 4.6 meters) off the ground in scrub oak or other short deciduous trees. Females lay three to four white, unmarked eggs. The incubation period is fourteen to nineteen days, which is shared by male (alternating with female during the day) and female (during the night). Both birds feed the young. Two broods are produced each year.

Black-capped vireos and people: There is no known significant relationship between people and black-capped vireos.

Conservation status: Black-capped vireos are not listed as threatened internationally, however, some populations face habitat loss from mining, agriculture, flood-control projects, and reservoir construction. Because of these problems, the U.S. Fish and Wildlife Service placed the bird on the U.S. Endangered Species List. Black-capped vireos are also hurt by cowbirds, which often threaten the birds especially during the breeding season. Efforts in Texas and Oklahoma are underway to trap and remove cowbirds and restrict human activities from areas where black-capped vireos have been most hurt. ∎