Drab vireo whose scratchy song is heard from dense, scrubby vegetation, often near water. Difficult to see through thick brush. Eastern birds have yellow flanks and olive-green back; are brighter than western birds. Note vague "spectacles" around eye and one white wingbar. Western birds vary from dull olive above to entirely plain gray; usually one wingbar. Look for long tail, often cocked. Very active, flitting around nervously.