The Hutton's Vireo is very similar in plumage but has a thicker bill. The Golden-crowned Kinglet is similar in size to the Ruby-crowned Kinglet but has a white supercilium and yellow crown. Warblers are larger, don't flick their wings and lack the broken eye ring.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.