- 4.7–5.1 in
- 8.7 in
- 0.4–0.6 oz
- Slightly larger than a Black-capped Chickadee; smaller than a House Finch.
- Viréo mélodieux (French)
- Vireo gorjeador (Spanish)
- Warbling Vireos have a good name—the males sing a fast, up-and-down, rollicking song that suits the word “warbling.” The early twentieth century ornithologist William Dawson described the song this way: “fresh as apples and as sweet as apple blossoms comes that dear, homely song from the willows.” The highly variable song usually ends on a high note, leading the birder Pete Dunne to describe it as sounding “like a happy drunk making a conversational point at a party.”
- Across their wide range, Warbling Vireos differ from one population to another in several characteristics, including overall size, bill shape, plumage coloring, molt patterns, wintering areas, and vocalizations. The differences are significant enough to lead ornithologists to recognize six separate subspecies of Warbling Vireo, and at one time divided them into two species.
- Brown-headed Cowbirds frequently deposit their own eggs in the nests of Warbling Vireos. In some instances, the vireo pair incubates the alien egg and raises the young cowbird until it fledges. Female vireos in some eastern populations, however, tend to puncture and eject interlopers’ eggs.
- Researchers speculate that Warbling Vireo song is at least partially learned rather than hard-wired. They base this supposition in part on observations of one individual whose song more closely resembled that of a Red-eyed Vireo than that of its parents. The garbled song, they concluded, probably resulted from a flawed learning process during the bird’s development.
- The longest-lived Warbling Vireo on record—a male that was originally banded in July 1966—was at least 13 years, 1 month old when it was recaptured during banding operations in California.
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.
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.
- Clutch Size
- 4 eggs
- Number of Broods
- 1-2 broods
- Egg Length
- 0.7–0.8 in
- Egg Width
- 0.5–0.6 in
- 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 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.
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 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.
Warbling Vireos are numerous and their numbers have shown a slight overall increase (about 0.8 percent per year) over the last half-century, according to the North American Breeding Bird Survey. Partners in Flight estimates the global breeding population at 51 million, with 44 percent breeding in Canada, 53 percent breeding in the U.S., and 87 percent 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 2012 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.
Medium- to long-distance, nocturnal migrant. Some birds in Mexico may be nonmigratory.
Find This Bird
In the eastern part of their breeding range, you can likely find Warbling Vireos in almost any sizable deciduous woodland. They are fairly drab and tend to stay high in trees, so find them by listening for their loud, caroling song. Because of their habitually slow foraging speed, you can often track down the singer fairly easily. In the West, particularly in mountains where coniferous trees are common, Warbling Vireos are harder to find. Look for them in aspen forests and in woodland (often cottonwood) along streams, where they are a common breeder. Again, you will find them most readily by listening for their distinctive song.