In breeding plumage, the brown and white plumage of the Kittlitz's Murrelet is similar only to the Marbled Murrelet but the Kittlitz's has a white belly and white outer tail feathers. In winter, the white scapulars dividing the dark back and wings are only shared by the Marbled Murrelet. However, it has a dark cap that extends below the eye and a less complete breast band. The white outer tail feathers of Kittlitz's Murrelet are diagnostic in all plumages. Juvenile Kittlitz's Murrelets might be confused with juvenile Pigeon Guillemots but have shorter necks, bills, and no white in the wing.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.