By Gail Cleveland

Although vireos are persistent singers during the breeding season, these rather plain birds seem to hide among the foliage of treetops and dense thickets, proving to be difficult to see. Consequently, they are one family that beginning bird watchers often overlook. Of this strictly New World family of birds, Northwest Montana has three fairly common species: the Red-eyed Vireo, Cassin’s Vireo and Warbling Vireo. Taking a closer look at my favorite, the Warbling Vireo, should send birders in hot pursuit of this seemingly-elusive little gray bird.

From their wintering grounds in Mexico and Central America, Warbling Vireos typically arrive in Northwest Montana in mid-May. Males arrive before females, immediately beginning to establish a territory by singing from a tall tree or bush. Warbling Vireos are mainly found in deciduous and mixed woods close to water. They avoid unbroken mature forests, concentrating on forest edges, breeding in broad-leaved trees of mountains, canyons and prairie groves.
Because of this preferred habitat, the Warbling Vireo’s range is widespread. The woodlands do not need to be extensive. They can be found in riparian corridors, tree-lined riverbanks, open city parks and cemeteries. In North America they breed in all states except the Southeast, and in western Canada their range extends to the Yukon and Northwest Territories. Their numbers are good, but there has been some decline in urban areas, possibly because of pesticide use.

Describing a Warbling Vireo is a drab task. In numerous field guides, they are described as pale, plain and muted. They have a dull, white eyebrow, no wing bars, tail spots or eye ring. In shape and behavior, they resemble a warbler, most closely resembling a Tennessee Warbler in coloration and habitat choice. The Tennessee’s bill is finer, and it has a grayish line through the eye. However, the best way to tell them apart is by song.
The song of the Warbling Vireo is infective and persistent. Pete Dunne in his Essential Field Guide Companion describes it as “zesty” and “a rapid, conversational, saucy-sounding, highly variable warble. . .In tone and structure, sounds like a happy drunk making a conversational point at a party.” According to Smithsonian writer Winsor Marrett Tyler, “The most suggestive verbal rendering of the vireo’s song, perhaps, is Wilson Flagg’s (1890): “‘Brig-a-dier, Brig-a-dier, Brigate,’ which, pronounced slowly, brings out the rhythm admirably.” The phrases of its song are often alternately inflected up and down, like a question and an answer. To listen to a local Warbling Vireo, Google, click on Search Tracks, search Tanneland, and the recording is at the end of the list.

One spring we had a rare look at the Warbling Vireo nest and nesting behavior outside our front window. The nest was a deep cup suspended by its rim from a forked twig in a serviceberry bush. This typical nest is built by both sexes. Both the male and the female sit on the nest. The male vireo sings from the nest, a fairly rare behavior for most birds, who do not want to draw attention to the eggs or young. Unfortunately, our nest was abandoned. The reason is unknown. Predation by other birds or a squirrel may be the answer. Warbling Vireos are also very susceptible to the parasitism of the Brown-headed Cowbird, who will destroy the vireo’s four or five eggs and replace them with its own. Eggs are incubated for approximately 12 days and chicks leave the nest after an average of 16 days. Both adults will feed the young.
Warbling Vireos dine almost exclusively on insects and larva, gleaning them off of leaves. With large prey items (such as caterpillars), the Warbling Vireo whacks its catch against its perch until the insect is subdued. Some early scientists were appalled to find a great many lady bugs in the stomachs of this species. In our area, my hope is that they have a taste for spruce budworms that are attacking the fir trees so voraciously. Like some warblers, they can also be seen darting out of a treetop to catch an insect and then returning. In the fall, they occasionally dine on fruits as well.

Starting at the end of May, in riparian woodlands, keep your ears tuned for this “zesty” songster, and then grab your binoculars to search for the Warbling Vireo among the leafy branches. If you are lucky, you may even find a male singing from a nest.