The small Bell's Vireo is dull even by vireo standards. It is best told from other vireos by its facial pattern. The white eye ring of the Bell's Vireo is broken in front of and behind the eye. The Hutton's Vireo's eye ring is thickest behind the eye and is broken at the top. The Gray Vireo has a complete eye ring. The White-eyed Vireo has yellow lores and white eyes (adults). The Warbling Vireo is larger and has a bolder supercilium that extends further behind the eye. Warblers have thinner, unhooked bills.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.