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 Alder flycatcher and was once considered the same species. It often has a less noticeable eye ring and browner, less olive, upperparts; however, most individuals cannot be identified with certainty by sight. Acadian and Yellow-bellied flycatchers tend to be greener above and yellower below than the Willow Flycatcher. Least Flycatcher is grayer above, smaller, shorter-tailed and has a more distinct eye ring. Western United States empidonax flycatchers have darker lower mandibles and more obvious eye rings. Gray is grayer above and rangier. Hammond's is smaller, darker-chested and has a grayer throat. Pewees are quite similar but are larger, have different vocalizations and typically forage higher in the canopy.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.