In the still of a mid-August afternoon, not many creatures stir. Cicadas buzz their shrill song; leaves refuse to rustle in the heaviness of the humid air. Suddenly, a noise breaks the silence. The clear, sweet phrases ring throughout the quiet forest. The sound weaves through trees and carries through the air as smooth as silk. While most birds have hushed for the day, this bird sings double – sending out questioning phrases and immediately replying with an answer. To many, the Red-eyed Vireo (Vireo olivaceus) is a much loved part of Eastern forests. It’s conspicuous song makes it familiar to those who frequent the woods while its secretive habits give the vireo an air of mystery.
Found across the Eastern US, Southern Canada, and parts of the American Northwest, the Red-eyed Vireo generally occupies deciduous forests. Specific forest habitat varies by region, and even more so during migration. While usually inhabiting the interior of woods during the breeding season, vireos will take advantage of forest edges and other habitats during migration. Once they arrive on wintering grounds in South America, they feel at home in forested edges, some agricultural lands, mangroves, and more.
Anybody who is persistent (or lucky) enough to get a view of these birds will find that they are aptly named. The vireo’s gray cap fades into an olivaceous back and stark white belly. Their scientific name, Vireo olivaceus, refers to the birds’ olive color. Their white eyebrow contrasts greatly with the cap and is outlined with a fine black line, a characteristic that gives the bird a serious demeanor. Immature birds have black eyes, but the adults’ turn into a striking red just like the name suggests. The vireo is roughly the size of a sparrow with a long, narrow body, blocky head, and hooked bill.
Despite it’s subtle yet beautiful appearance, the Red-eyed Vireo is perhaps best recognized for its song. Sometimes called “preacher birds,” they frequently sing throughout the day when most other bird song dies down in the summer slump. They send out a string of short, clear phrases punctuated with brief pauses in between. Each phrase seems to alternate between rising and falling inflection, which sounds like the bird is asking a question and then giving a reply. Males are incredible songsters, and individuals can produce up to 30 song variations. Over 12,000 song types have been recorded across the species. One male in Canada was recorded giving an astounding 22,197 songs over nearly 10 straight hours of singing!
Once males claim a territory and find a mate, the pair begins working on a nest. Vireo nests are truly fascinating. After the female finds a suitably forked tree branch within thick vegetation, she then begins construction with a variety of plant materials. The nest does not sit in the fork, but rather hangs below the fork. It looks like a miniature basket hanging in the tree. To keep all the plant materials together, the female glues the nest with spider webs. Adults are fiercely territorial. I have watched them scold and chase away birds twice their size. Blue jays, crows, other birds, and small mammals don’t stand a chance of intruding the nest when a vigilant adult is on the lookout.
The vireo feasts on a variety of food items. Their menu changes as the seasons roll by. In summer, their diet can be made up of nearly 50% caterpillars. As fall approaches, they tend to consume a mix of both berries and insects. By the time they reach their wintering grounds in South America, they will have transitioned to eating mostly fruits. The Red-eyed Vireo is difficult to see because it often forages in leafy treetops. They are foliage gleaners, moving methodically along branches and picking invertebrates off of leaves.
Despite its seemingly ubiquitous presence across eastern forests, they still face the same threats that afflict other migratory songbirds. Because they prefer large tracts of intact forest, forest fragmentation and habitat degradation is a potential threat to the species. Brown-headed Cowbirds (Molothrus ater) frequently parasitize vireo nests, and fragmentation is allowing cowbirds to infiltrate more Red-eyed Vireo habitat. They also are victims to increased predation at fragmented edges and building collisions during nocturnal migration. While still a common species, these threats to their populations are concerning.
The Red-eyed Vireo is a charming member of many forest communities. Their tireless song is a hallmark of nearly any forest. Go outside to a patch of woods near you and listen for the continuous question-and-answer song of the omnipresent vireo.