The yellow underparts, olive upperparts, and lack of wing bars of the Connecticut Warbler are similar to Mourning, MacGillivray's, Nashville Warblers, and Common Yellowthroats. Male Mourning and MacGillivray's Warblers have black markings on the upper breast. Male Nashville Warblers have yellow throats and male Common Yellowthroats have black masks. In female and immature plumages, the Connecticut Warbler has a complete white or buff eye ring. Mourning, MacGillivray's and yellowthroats have broken eye rings. Nashvilles also have complete eye rings but are smaller and have white bellies and yellow breasts. Connecticut Warblers are bigger and have heavier bills than the other species. Connecticuts spend much of their time walking on the ground and don't seem to forage as much in shrubs and trees as the other species.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.