The Warbling Vireo is most often located by its song, which it delivers for hours. The birds tend to like the mid and top parts of broad, leafy trees making them a hard bird to spot. We have a stable population of migrating warbling vireos that breed on Vancouver Island.
They are among the dullest of vireos, lacking wing bars and eye rings, they are gray with brownish to greenish tones to the upper parts. The face pattern is non-defined, with a dusky stripe and pale white eyebrow and lacking a dark upper border, underparts are typically a whitish color.
The warbling vireo breeds in deciduous woodlands, primarily in riparian areas. The western birds have a prolonged spring migration from early March to late May. Cowbird parasitism is implicated in the decline of some western populations.
The eastern and western forms differ slightly in size, bill shape, genetics, molt strategies, wintering areas, and possibly voice. Western birds are slightly smaller, have smaller, darker bills, are more olive green on the upper parts, and have a darker crown than the eastern birds.
The female warbling vireo selects the nest site and may place nesting material in several locations before beginning to build at the final location. The nest is almost always located in the outer portions of a tree or shrub, supported by two lateral branches. It is a rough and slightly rounded hanging cup and is usually suspended from forks or horizontal twigs. The nest may consist of plant matter, cobwebs, lichen, animal hair, and rarely feathers.
Warbling Vireos can be found singly, in small groups, or in mixed flocks in the treetops foraging for insects. They are also highly territorial during breeding.
The male usually arrives on the breeding grounds before the female and begins singing immediately. Males sing to establish and defend summer territories.