Boreal Chickadee has less white in the face than the Siberian Tit and Chestnut-backed Chickadee. Chestnut-backed Chickadee also has more rufous flanks and back. Black-capped chickadee lacks brown in its plumage.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.