The empidonax flycatchers are very difficult to tell apart. The safest way to differentiate them is by habitat, range, and voice in the breeding season. Differences in plumage due to molt, wear and age make the plumage quite variable. This species is very similar to the Cordilleran flycatcher and can best be separated by call, two-syllabled in Cordilleran and upslurred single note in Pacific-slope. Other empidonax flycatchers have more rounded eye rings and less yellow underparts (except the smaller Yellow-bellied).
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.