The Botteri's Sparrow is very similar to the Cassin's Sparrow and is best identified by its different song. It has a somewhat larger bill and flatter head than the Cassin's. Cassin's has paler central tail feathers with dark barring, pale outer border to the tail, and faint dark streaks on the flanks. Bachman's Sparrow similar but has different range, song and favors more wooded habitats.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.