Probably no other pair of North American bird species poses greater field identification problems than the Rufous (Selasphorus rufus) and Allen's (S. sasin) hummingbirds. Observations of anything other than rufous-backed adult males (at least outside of known breeding ranges and breeding seasons) are usually lumped as "Rufous/Allen's," since even a small percentage of adult male Rufous are fully green-backed, matching the pattern of adult male Allen's (McKenzie and Robbins 1999).