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 extremely similar to the Willow flycatcher and was once considered the same species. It often has a more noticeable eye ring and more olive upperparts, however, most individuals cannot be identified by sight. Acadian and Yellow-bellied flycatchers tend to be greener above and yellower below than the Alder Flycatcher. Least Flycatcher is grayer above, shorter-tailed and smaller. Western United States empidonax flycatchers (Dusky, Gray, Hammond's) have mostly dark lower mandibles and more obvious eye rings. Hammond's has a darker breast and grayer throat. Gray is rangier and grayer. Pacific-slope and Cordilleran Flycatchers have oval eye rings.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.