The pale supercilium, brown upperparts, and white underparts of the Swainson's Warbler are more reminiscent of a wren than a warbler. But wrens tend to carry their tails upright and have a very different shape. The Worm-eating Warbler has black and buff head stripes and a buffier breast.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.