Bell's Vireo Life History

Habitat

Look for Bell’s Vireos in many kinds of dense shrubby or scrubby habitat, including brushy fields, early successional growth, riverine scrub, coastal chaparral, scrub oak, mottes (isolated patches) of shrubs and trees in prairies, saltcedar stands, and mesquite bosques. Especially in arid regions, Bell’s Vireos are found along streams or in dry arroyos and gulches. Even when large trees such as cottonwoods and willows are present, the vireos tend to stay more in the low vegetation. They avoid open desert scrub, grasslands, and cultivated areas. Wintering birds in Mexico frequent both riparian and very arid areas, such as thorn forest, as well as dense scrub similar to their breeding habitats.

Food

Bell’s Vireos feed primarily on insects and spiders, and to a much lesser extent on small berries (such as black alder). Known prey items include beetles, weevils, stinkbugs, bees, wasps, flies, midges, ants, leafhoppers, grasshoppers, moths, and butterflies (especially caterpillars). They forage mostly by plucking (gleaning) insects from leaves and small branches, but they also hover-glean from leaves or hawk insects from midair. Most of their foraging is done below 12 feet in the vegetation, but they sometimes feed in treetops (up to 65 feet), if food is present.

Nesting

Nest Placement

Males and females select nest sites together, typically in a small fork in the branch of a tree or shrub, not far from the outer edge of the plant, roughly 3 feet above the ground, sometimes higher. Nests have been recorded in many dozens of plant species.

Nest Description

Usually the male and female construct the nest together. The male begins by making the supports, weaving material around the fork with spider silk. The female then fashions a cup, using grass, fine plant stems and fiber, small dry leaves, wool, moss, plant down, paper, and strips of bark. She then lines the nest and tightens the structure using spider silk. Finally, both sexes adorn the exterior of the nest, often using spider egg cases. Nest measurements average 2.8 inches wide by 3.3 inches deep, with interior dimensions 1.8 inches wide by 1.8 inches deep.

Nesting Facts

Behavior

Males chase females during courtship, sometimes colliding with them in flight, and perform elaborate displays. A “greeting” display, typically performed during nest construction, involves both male and female flicking wings and tails and reversing perch positions. As one puffs up its plumage, the other sleeks it. In the “pouncing” display, the female fans the tail and cocks it downward as the male responds in kind, then lunges at the female in what can appear to be a rather violent strike. The male also performs a “leap-flutter” display that involves jumping into the air several feet and hovering, then returning to the same perch. Like other vireos, Bell’s Vireos display before copulation. In this display, the male drops and fans the tail, puffs up his body plumage fully, throws back his head, and opens his bill, then sways mechanically from side to side in a deep arc as he sings. Males are equally remarkable in their pursuit of rivals, which they often attack in midair if threat gestures such as wing- and tail-flicking do not suffice. Pairs occupy territories as small as 0.25 acres and as large as 5 acres; males maintain territory mostly by song. Bell’s Vireos chase Brown-headed Cowbirds from their nests, in an attempt to keep the cowbirds from laying their own eggs in the vireos’ nest. Jays and thrashers, which likely eat eggs and nestlings, are also attacked or mobbed (surrounded and scolded) when near the nest.

Conservation

Bell's Vireo populations were stable between 1966 and 2015, according to the North American Breeding Bird Survey. Partners in Flight estimates a 38% gain since 1970. Partners in Flight estimates a global breeding population of Bell's Vireos of 5.9 million. The species rates an 11 out of 20 on the Continental Concern Score, indicating it is a species of low conservation concern. However, one subspecies, the Least Bell's Vireo of California and Baja California, is listed as federally endangered, primarily from loss of riparian habitat and cowbird brood parasitism. This subspecies continues to decline throughout its range. At the species level, agriculture, urbanization, wildfires, firewood cutting, grazing, flood control projects, and reservoir construction have reduced available habitat, especially in streamside habitats, and continue to do so.

Credits

Kus, B., S. L. Hopp, R. R. Johnson, and B. T. Brown (2020). Bell's Vireo (Vireo bellii), version 1.0. In Birds of the World (A. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.

Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.

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

Sauer, J. R., J. E. Hines, J. E. Fallon, K. L. Pardieck, Jr. Ziolkowski, D. J. and W. A. Link. The North American Breeding Bird Survey, results and analysis 1966-2013 (Version 1.30.15). USGS Patuxent Wildlife Research Center (2014b). Available from http://www.mbr-pwrc.usgs.gov/bbs/.

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

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