Long-term research on geographic variation among populations of the "Western Flycatcher" revealed the existence of 2 very similar flycatcher species divided roughly between coastal mountain ranges and the Rocky Mountains of western North America ( Johnson 1980b , Johnson and Marten 1988 ). Originally considered subspecies of the “Western Flycatcher", the Pacific-slope Flycatcher (Empidonax difficilis) and Cordilleran Flycatcher (E. occidentalis) were elevated to species status in 1989 based on differences in vocal, morphological, and allozyme characters ( Johnson and Marten 1988 , American Ornithologists' Union 1989 ).
The Pacific-slope Flycatcher predominantly occurs in coastal mountain ranges of western North America, whereas the Cordilleran Flycatcher is found in the Rocky Mountains from southwestern Canada to southern Mexico. Both species are commonly associated with cool, shady locations along waterways, where there is openness under the canopy and opportunities for nest placement. Pacific-slope Flycatchers breed in humid mixed-conifer forest, pine-oak forest, and second-growth woodland ( American Ornithologists' Union 1998a ). However, the distributions and interactions between Pacific-slope and Cordilleran flycatchers are complex and more study is needed. For example, the two species overlap in portions of the interior Northwest, and a genetic analysis of breeders has reported evidence of hybridization in southwestern Canada ( Rush et al. 2009c ). In addition, populations breeding on the Channel Islands off southern California, currently treated as the Pacific-slope Flycatcher subspecies E. d. insulicola, could actually be a distinct species (see Systematics).
In this species account, studies of the Western Flycatcher that were published prior to 1989 have been allocated to either the Pacific-slope Flycatcher or Cordilleran Flycatcher based on geography; use of the name "Western Flycatcher" in the account indicates uncertain identity. The few published studies of "Western Flycatcher" nesting biology refer to the Pacific-slope Flycatcher. Three major references of this species have included detailed observations of nesting pairs in Monterey County, California ( Davis et al. 1963 ); observations of the entire nesting cycle of 3 pairs in Humboldt County, California ( Sakai 1988b ); and studies of vocalization behavior and nesting of 7 males on Queen Charlotte Islands, British Columbia ( Ainsley 1992 ). Little is known about the migration or overwintering ecology of the Pacific-slope Flycatcher, although it is known that molt occurs primarily on the overwintering grounds ( Johnson 1974b ).