During breeding season, Warbling Vireos occur in mature deciduous woodlands from sea level to an elevation of about 10,500 feet—especially along streams, ponds, marshes, and lakes, but sometimes in upland areas away from water. They also take up residence in young deciduous stands that emerge after clearcutting. They are rarely found in purely coniferous forests. Warbling Vireos often nest in around people, including in neighborhoods, urban parks, orchards, and campgrounds. Their winter range, which extends through western Mexico and northern Central America, is much smaller than their breeding range. It includes diverse habitats, from shade-coffee plantations to thorn forests to pine-oak woodland. During the winter in western Mexico, this bird almost always hangs out with mixed-species feeding flocks.Back to top
Warbling Vireos eat mainly caterpillars, pupae, and adult moths and butterflies. They also eat ladybugs, beetles, bugs, bees, ants, wasps, and spiders. In fall and winter they add elderberries, poison oak berries, and other fruit to their diet. They forage mainly in treetops, gleaning insects from leaves and sometimes twigs; they also hunt by hovering, stalking, hawking, and flycatching. To subdue caterpillars and other larger prey, a Warbling Vireo whacks victims forcefully against its perch. Breeding pairs forage alone during the breeding season; at other times individual Warbling Vireos forage in mixed-species flocks. Back to top
Warbling Vireos nest in the outer portions of deciduous trees and tall shrubs from 3 to 140 feet above the ground. The female selects the site, sometimes placing nesting material in several locations before making a final choice.
Warbling Vireos weave a rough, slightly rounded hanging cup, usually suspending the nest from a horizontally forked twig. The nest may consist of plant matter, cobwebs, lichen, animal hair, and rarely feathers. Nests may contain willow down, dry grass, leaves, rootlets, horsehair, cow hair, spider silk, cocoons, cotton, birch bark, paper, thread, and string. Females do most of the building, sometimes stealing material from the nests of neighbors. The nest is about 3 inches across and 2 to 3 inches deep, with an inner cup about 2 inches across and 1.5 inches deep.
|Number of Broods:||1-2 broods|
|Egg Length:||0.7-0.8 in (1.8-2 cm)|
|Egg Width:||0.5-0.6 in (1.3-1.5 cm)|
|Incubation Period:||12-14 days|
|Nestling Period:||13-14 days|
|Egg Description:||White with a few scattered dots of reddish or dark brown.|
|Condition at Hatching:||Helpless, naked, with dark-yellow skin except for tufts of light-brown down, eyes closed.|
Warbling Vireos spend most of their time in the treetops of deciduous woods. Males are highly territorial and spend much of their time during the breeding season singing. They usually arrive on their breeding grounds before females, immediately commencing a singing-and-patrolling campaign to establish and defend territory. During courtship, a male approaches his prospective mate head-on, rhythmically weaving his body from side to side. With quivering wings, he closes the gap between them to about an inch, whereupon the female strikes repeatedly at his open bill with her closed one. While their nest is under construction, a male Warbling Vireo spends about a third of his time guarding the female. During incubation, the female stays on the nest at night while her mate sleeps in a nearby tree. Both sexes help raise their young to fledging stage, but females do the lion’s share. When parents are feeding young, one adult often waits at the nest until the returning partner signals with a call—ensuring that one parent is always with the nestlings. As hatchlings mature, feedings become more frequent. At one nest where young were close to fledging, an observer recorded 29 feeding visits within one hour. Both sexes ferociously mob jays, grackles, and other birds that approach their nests. Other probable nest predators include red and western gray squirrels.Back to top
Warbling Vireos are numerous and their numbers experienced a slight overall increase between 1966 and 2014, according to the North American Breeding Bird Survey. Partners in Flight estimates the global breeding population at 51 million, with 44% breeding in Canada, 53% spending part of the year in the U.S., and 87% of the population spending part of the year in Mexico. Warbling Vireos rate an 8 out of 20 on the Continental Concern Score and are not on the 2014 State of the Birds Watch List. Population gains may result from this species’ increasing adaptation to suburban environments. Warbling Vireos may also benefit from new habitat created when coniferous forests are cleared, leaving large deciduous trees near open spaces. Population declines recorded in the mid-twentieth century have been attributed to the spraying of shade trees with chemical pesticides. One study found severe declines in herbicide-thinned deciduous forests, while numbers increased in plots thinned by hand and in control plots. Degradation of streamside habitat causes populations to decline. Warbling Vireos also die from collisions with communications towers and other tall structures during nighttime migration. Because Warbling Vireos crowd into a winter range disproportionately smaller than their breeding range, habitat conservation in their wintering areas is important. Back to top
Gardali, Thomas and Grant Ballard. (2000). Warbling Vireo (Vireo gilvus), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.
Lutmerding, J. A. and A. S. Love. Longevity records of North American birds. Version 2015.2. Patuxent Wildlife Research Center, Bird Banding Laboratory 2015.
North American Bird Conservation Initiative. (2014). The State of the Birds 2014 Report. US Department of Interior, Washington, DC, USA.
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 Patuxtent 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, USA.