Those scary Empidonax Flycatchers
If you were to walk into a room full of birders and ask them to point at the Empidonax expert, I can pretty much guarantee that none of them will be pointing at me. The Empidonax group is notoriously scary and even the most practiced of flycatcher watchers will stumble. Some members of this group are so closely related that fist fights break out over their taxonomy. Under some conditions of light or plumage or time of year, it may be genuinely impossible to satisfactorily sort them out. But the time for a birder to take his or her best shot is in the breeding season when the males are talking and the plumages are reasonably fresh.

The following is a beginners course in western Empidonax identification. Will it answer all your questions and solve all your problems regarding the identification of members of this group? Not even close, but it's a start.

Key to Breeding Western Empidonax Flycatchers
1a.Body color medium gray with little or no olive or brown. 
   Primary extension short; tail long..............GRAY FLYCATCHER
1b.Body color with substantial brown or olive....................2

        2a.Eyering absent or indistinct.........."Traill's" complex-3
        2b.Obvious eyering..........................................4

3a.Song "Fitz-bew"...............................WILLOW FLYCATCHER
3b.Song "Fee-bee-oh"(hypothetical in Oregon)......ALDER FLYCATCHER

        4a.Eyering elongated behind eye..........."Western" complex-5
        4b.Eyering more or less rounded.............................6

5a.Found west of the Cascades.Possibly darker and brighter.Call of          
   males on territory "peweat"............PACIFIC-SLOPE FLYCATCHER
5b.Found east of the Cascades.Possibly paler and more washed out.
   Call of males on territory "pit-peet"....CORDILLERAN FLYCATCHER

        6a.Back olive-gray to brown;breast dirty white with dusky 
           band.....................................................7
        6b.Throat and breast whitish.Small body,large head,short primary 
           extension,short tail.Song "che-bek".......LEAST FLYCATCHER

7a.Smallish.Primary extension long,tail short.Song is a low-
   pitched "chipit-brrk-grrip"................HAMMOND'S FLYCATCHER
7b.Primary extension short,tail long.Song a "Chirip-greep-pweet".
   Mostly east of the Cascades....................DUSKY FLYCATCHER
References

Kaufman,Kenn.1990. The Empidonax Flycatchers. Advanced Birding.
   Houghton-Mifflin. Boston.

Ramsey,Fred.1978. Empidonax Flycatchers. Birding Oregon. OSU
   Bookstores. Corvallis, OR.