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. The Least Flycatcher is the smallest empidonax with a small bill that makes the head look big. Yellow-bellied Flycatcher is similar in size and shape but is greener above and yellower below. Willow and Alder have less obvious eye rings, longer tails and browner upperparts. Acadian is larger, bigger-billed, greener above, and yellower below. In the west, Hammond's is typically darker on the breast. Dusky and Gray are larger and Pacific-slope and Cordilleran have oval eye rings, are larger, and yellower underneath.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.