The unstreaked underparts, plain yellow (or orange) head and breast, and lack of wing bars separate this species from most other warblers. The female Yellow Warbler has yellowish, not blue-gray wings and yellow undertail coverts. Blue-winged Warbler has black eyeline and wing bars.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.