In alternate plumage, the only other warbler with rusty sides is the Chestnut-sided Warbler but it has a yellow crown and white throat. Basic (fall) and immature plumages are very dull and most similar to Blackpoll and Pine Warblers. The Blackpoll Warbler has yellow legs, white undertail coverts, and a streakier breast. Pine Warbler lacks black streaks on the back and is often yellower on the throat and breast. Often, Bay-breasted Warblers retain a trace of rusty sides in the fall.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.