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. Alder and Willow are browner above and whiter below with less obvious eye rings. Least is smaller, grayer above and whiter below and has a smaller bill.
Pewees are larger, browner above, whiter below and have indistinct eye rings.