The warbling vireo is a drab little bird with a colorful, brilliant song. Adult upperparts are gray with a white eyebrow and indistinct gray eye line. There is no black line above the eyebrow. There are no wing bars, and the underparts are white. In fall it is greenish above and whiter below with yellowish sides, flanks, and under tail feathers. The song is a melodious series, lasting about 3–5 seconds, of rapid warbling, bubbling notes that rise and fall throughout the series, often ending on a high note (see Human Connections, below).
Similar species: The red-eyed vireo has a black eye line above the white eyebrow. The Philadelphia vireo has a black eye line, extending in front of the eye to the bill, that contrasts strongly against the white eyebrow; it has a yellow throat; its song is different; and it is an uncommon migrant and not seen here in summer. Bell’s vireo has two white wing bars and a longer tail. Warblers are often confused with vireos; remember that warblers have slender, straight-pointed bills, lacking the slight hook that vireos have. Warblers are usually more active than vireos, making quicker, less methodical movements.
Length 5½ inches.
Habitat and Conservation
Occurs in forests, woodlands, and suburbs, especially in large trees near water. If you want to see this bird, go to suitable habitat in spring and summer and listen for the distinctive song, which will likely come from high in a tree. Use binoculars, and be glad that vireos tend to move rather slowly as they forage along branches.
Warbling vireos forage for insects, especially caterpillars, by searching rather slowly among branches and leaves, inspecting each area closely.
Common summer resident; accidental winter visitor. Populations appear stable; one factor may be that this species seems to be adapting to the presence of humans. Nests are often found in suburban areas, campgrounds, orchards, city parks, and so on. But because the wintering range is so small and compact, it is important to ensure the well-being of habitats in that area.
Present in Missouri in April through September; numbers are highest May through August. Cup nests are suspended from forked small branches in trees, sometimes near campgrounds and other places humans are present. Nests are built of leaves, lichens, and fibrous plant materials bound together with spider and insect silk and lined with animal hair, string, and long list of other soft materials. Clutches usually contain 4 eggs, which are incubated 12–14 days; after hatching, the young remain in the nest another 13–14 days. There can be 1 or 2 broods. They overwinter from western Mexico through the northern part of Central America — a remarkably small territory, considering their breeding range comprises nearly all the Lower 48 and much of western Canada. A warbling vireo can live to be at least 13 years old.
The warbling vireo’s song has been likened to the complex, ascending flourishes of the piano at the start of Chopin’s Fantaisie-Impromptu. Before it was easy to make field recordings of bird songs, many people attempted to transcribe birdsong to written musical notation. Now, recordings and sonograms are commonplace, making it easier for people to learn and talk about the sounds birds make.
Warbling vireo populations decline when their principal food source, caterpillars, are wiped out by widespread pesticide use. This species is frequently parasitized by brown-headed cowbirds, which lay their eggs in the nests of other species. Sometimes the vireo parent recognizes the interloper’s eggs and rejects them; other times, the vireo unwittingly raises the cowbird chicks, usually at the cost of its own young.