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 FLYCATCHERReferences
Kaufman,Kenn.1990. The Empidonax Flycatchers. Advanced
Birding.
Houghton-Mifflin. Boston.
Ramsey,Fred.1978. Empidonax Flycatchers. Birding Oregon.
OSU
Bookstores. Corvallis, OR.