Because of its unique feeding strategy, the Pygmy Nuthatch is most likely to confused with other nuthatches. Both White-breasted and Red-breasted Nuthatches have black, not brown, crowns and lack the pale spot on the nape. Brown-headed Nuthatch is very similar but their ranges do not overlap.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.