Courtesy Kenn Kaufman
Formerly widespread from Mexico to Kansas, this vireo is now confined as a breeder to 38 counties in Texas, three counties in Oklahoma, and three states in Mexico, from Coahuila, through Nuevo Leon, into southwestern Tamaulipas, where a small but significant breeding population was reported in 2005. The highest concentrations in summer appear to occur in Texas, probably on Fort Hood. The wintering range is a narrow band along Mexico's Pacific Coast. It starts in far southern Sonora State and extends south into coastal Oaxaca, with the widest distribution from Nayarit to Michoacan.
Global-level Important Bird Areas in Mexico that have been identified as supporting breeding populations of the species include Sierra Maderas del Carmen IBA, Sierra del Burro IBA, Nacimiento Rio Sabinas/ SE Sierra de Santa Rosa IBA, and Presa el Tulillo IBA.
In the breeding season, the Black-capped Vireo prefers arid scrub and brush lands with a complex mixture of vegetation close to the ground, both short and tall shrubs, and open spaces. This habitat is rare and localized in gullies, ravines and eroded slopes. Breeding territories range between 2.5 and 25 acres and can overlap with White-eyed Vireo (V. griseus) territories. The White-eyeds seem to prefer the brushier gullies and the Black-cappeds prefer the more open areas on slopes and hilltops. Typical plants in this habitat include dogwood, junipers, various scrubby oaks like the shin oak, sumac, and Texas persimmon. Frequent natural fire produces the best conditions. Wintering and migrating habitat may be taller and more moist, but always has thick plant growth.
Black-capped Vireos mostly eat insects, which they find by actively hopping and fluttering about thick vegetation. Spotting prey visually, the vireo plucks larvae and adult insects from plant surfaces. The summer diet concentrates on the larvae of moths, butterflies, and beetles, but includes many flies, spiders, leafhoppers, katydids, and many other arthropods. Some seeds are consumed in summer, and they become more important in winter. One study found aster seeds comprised 55% of bulk stomach contents. Young vireos eat insect larvae.
Shortly after males establish breeding territories in March or April, females arrive. The male attends or leads a female through his territory, as she evaluates it and he fends off rivals with songs, displays, and fights. Males may build nest platforms, which they show off during their tours. The pair bond lasts for at least one brood. Under thick cover, the pair builds a nest in a forked branch, 1.5 to 6.5 feet above the ground. The open nest is formed by building a flexible platform of silks and plant fibers, which is then depressed to form a cup. Grasses, strips of plant stems, and leaves are then added to deepen it.
The female almost always lays four whitish eggs, which the pair incubate for 14-17 days. Hatchlings are naked, blind and helpless. Initially both parents tend the young, but males provide most of the food, up to 80% in the early stages. The young fledge in about 11 days and receive care for another four to eight weeks. After the young fledge, females may start a new nest in the male's territory, depart to find another mate, stay to rear the young, or take part of the brood by herself. Juvenile Black-capped Vireos disperse singly or in groups, wandering throughout the surrounding area.
Migration routes are largely unknown, but birds are thought to avoid the high elevations of the Sierra Occidental and to follow shrubby habitats over plateaus and through valleys. After breeding, southbound migrants may depart in July, and early arrivals appear on the wintering grounds in late August. Young vireos depart first, followed by females and then mature males. Spring migrants arrive between March and April. This species probably travels at night.