Philadelphia Vireos are slightly smaller, with a rounder head and shorter tail than Warbling Vireos. Philadelphia Vireos also typically have much brighter yellow underparts that are always brightest on the throat and breast, whereas Warbling Vireos tend to be brighter on the flanks or vent. Philadelphia Vireos also have a distinct line through the eye toward the bill (the lores), giving the bird a fiercer expression. This eyeline widens immediately in front of and behind the eye, giving Philadelphias the impression of having larger eyes than Warbling Vireos. Red-eyed Vireos are both larger and have a large, angular head and large bill. They also have a bright white superciliary that contrasts strongly with a wide eyeline and thin, blackish lateral borders of the crown. Red-eyed Vireos are brighter green on the back than Warbling Vireos. Bell’s Vireos are smaller than Warbling Vireos, with a rounder head and a longer tail. Bell’s Vireos also show at least one and usually two pale wingbars. Warblers such as Tennessee Warblers are more slender, with daintier, more pointed bills than Warbling Vireos, and they move from perch to perch much more quickly and restlessly.
Western birds tend to be a bit drabber than eastern birds, though plumage color varies more with season than from east to west. Western birds also have slightly longer bills, but this difference is only rarely useable in the field. The two forms have similar, but diagnosably different songs, western birds having higher-pitched, less “sing-songy,” less caroling, and slightly longer songs than those of eastern birds. The two forms meet near the western edge of the Great Plains, and both forms breed in, at least, Colorado.
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.