Traill’s Flycatcher (sensu American Ornithologists’ Union [A.O.U.] 1957) is now recognized (A.O.U. 1973) as two species, Empidonax traillii (Audubon, 1828) (Willow Flycatcher) and E. alnorum Brewster, 1895 (Alder Flycatcher). The two species do not interbreed (Stein 1958, 1963, Gorski 1970), differ genetically (Zink and Johnson 1984, Seutin and Simon 1988), are usually though not always separated ecologically (Stein 1958, 1963, Barlow and McGillivray 1983, Zink and Fall 1981, Ewert 1981), and differ in song and calls (Stein 1958, LeGrand 1979, Lehman 1985).
Empidonax traillii vocalizes the “fitz-bew” song and breeds in swamps and willow thickets, usually along streams, in the conterminous United States and southern Canada. Empidonax alnorum vocalizes the “fee-bee-o” song and breeds in shrubs and alder thickets of the northeastern United States, Canada, and Alaska.