Clay-colored Sparrows are similar to Brewer's Sparrows but have a white central crown stripe, dark borders to the cheeks, distinct supercilium, gray hindneck and a dark whisker. Field Sparrow has a pink bill, rusty crown and a white eye ring. In Fall and Winter, Chipping Sparrows are quite similar to Brewer's Sparrows. Often, they retain some rust in the crown and can be identified by that field mark. Brewer's Sparrows have pale lores, brown rumps, white eye ring and a plainer face pattern.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.