Gray Vireo Life History

Habitat

Habitat Scrub

Gray Vireos nest in pinyon pine-juniper, mesquite scrub, oak scrub, and chaparral habitats of the Southwest, from lowlands into foothills and mountains as high as 7,800 feet elevation, including portions of the Mojave, Sonoran, and Chihuahuan Deserts. Their hot, arid habitats usually have dense brush from near the ground to about 6 feet high. Migrants and wintering birds (mostly in Mexico) select similar habitats, including stream areas with willow and cottonwood, as well as desert scrub adjacent to mangrove forest. In southern California and northern Baja California, they live in chaparral with chamise, redshanks, and Ceanothus shrubs. Farther east, habitats include scrubby oaks (including Graves and gray oaks), alligator juniper, madrones, mountain mahoganies, sumacs, ashes (including Gregg ash), mulberry, hackberry, big sagebrush, catclaw mimosa, mesquites, creosote bush, ocotillo, and various yucca and cactus species. In southwestern Arizona and neighboring California, Gray Vireos also winter in canyons with elephant trees, jojoba, ironwood, brittlebush, and cholla.

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Food

Food Insects

Gray Vireos eat mostly insects, which they capture by gleaning from leaves and twigs in dense vegetation. They spend most of their foraging time in the interior of small trees and shrubs, usually below 12 feet high (and often only 3 feet high). They occasionally take prey from the ground or in flight. They often subdue larger prey by battering it against a branch, and may remove the head or wings before eating. In summer they tend to forage higher in the vegetation than in winter and also tend to capture more prey in flight. In winter, some populations (such as in southern Arizona and adjacent Mexico) consume mostly small fruit. Animal prey includes grasshoppers, katydids, cicadas, butterflies, moths, caterpillars, stinkbugs, treehoppers, tree crickets, flies, beetles, dobsonflies, damselflies, and wasps.

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Nesting

Nest Placement

Nest Shrub

The female selects the nest site while accompanied by the male. It’s usually a fork in a branch of a small tree or shrub, about 6 feet off the ground.

Nest Description

The female builds a cup nest of grasses, bark, and plant fiber, festooned on the outside with leaves and spider cocoons, and lined with grass, hair, and plant down. Nests average about 2.7 inches across and 2.3 inches tall, with interior cup about 1.9 inches across and 1.6 inches deep.

Nesting Facts
Clutch Size:2-4 eggs
Egg Description:

White with variable amount of small spots.

Condition at Hatching:

Naked and pink, with eyes closed.

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Behavior

Behavior Foliage Gleaner

In early spring, male Gray Vireos begin singing to claim territory and attract females as soon as they return from wintering grounds. At this time, they can be very conspicuous, perched in treetops, unlike most vireo species, which sing mostly from concealed parts of trees. Gray Vireos appear to be monogamous. Courtship involves males flying after females through the vegetation, and a display usually precedes mating: males fluff out their body and crown feathers, fan and flick the wings and tail, and sway from side to side, as females open and vibrate their wings. Pairs also bond in selecting a nest site and building the nest together. Males sometimes build untidy “bachelor nests” with no lining while females build the actual nest. Males also often sing and make weaving motions as though building a nest. After mating and after the nest is built, the pair may perform a bill-touching display, in which the female moves the head from side to side. Both male and female chase away other birds that approach the nest too closely, often giving scolding calls, sometimes attacking intruders. Territory sizes vary greatly, from less than 5 acres to over 25. Both adults incubate the eggs and feed the nestlings. After young have fledged, family groups forage together, on occasion joining other species of birds in small flocks, but Gray Vireos are usually solitary during migration and winter. They begin southward migration in late summer, and on wintering grounds they defend individual feeding territories (about 2–20 acres). Some winter territories include a paired male and female.

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Conservation

Conservation Restricted Range

According to the North American Breeding Bird Survey, Gray Vireo populations were stable or rose slightly between 1968 and 2015. Partners in Flight estimates a global breeding population of 560,000 and ranks the species a 12 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. Destruction and degradation of its desert habitats are probably the chief conservation threats to the Gray Vireo.

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Credits

Barlow, Jon C., Sheridan N. Leckie and Colette T. Baril. (1999). Gray Vireo (Vireo vicinior), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

Partners in Flight (2017). Avian Conservation Assessment Database. 2017.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2017). The North American Breeding Bird Survey, Results and Analysis 1966–2015. Version 2.07.2017. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, USA.

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