Adult upperparts are dark olive, with a gray crown and no wing bars. Eye line is black, eyebrow is white, and there is a black line between the gray crown and the white eyebrow. Underparts are white, with some yellow on the flanks in fall. As with other vireos, the bill is relatively thick until the tip, and there is a small hook at the tip. Iris of the eyes are red in adults, but this can be hard to see. The song, very helpful for locating this hard-to-see bird, is a monotonous series of 2-, 3- or 4-syllable whistles, with alternating up- and down-slurred phrases. The call, like that of most vireos, is a burry vray or tjay. Except for indigo buntings, red-eyed vireos may often be the only bird calling on a hot, humid summer afternoon in an Ozark forest.
Similar species: Warbling vireos lack the black line above the eyebrow. The yellow-throated vireo has a similar song, but it is given at longer intervals and is not whistled as clearly. The Philadelphia vireo’s song is also similar, but it is slightly higher, less emphatic, and has longer pauses between phrases; it is less common in our state, smaller, has a shorter bill, and has yellower underparts. The Tennessee warbler is colored similarly, but it flits around more busily, and its bill is shorter, thinner, with a pointed (not slightly hooked) tip. Vireos, as a group, can be distinguished from warblers by their thickish, slightly hooked bill. Warblers’ bills are narrow and taper to a sharp point.