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 Cordilleran Flycatcher (Empidonax occidentalis) and Pacific-slope Flycatcher (E. difficilis) 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 Cordilleran Flycatcher is primarily found in the Rocky Mountains, extending south in the Rockies through much of Mexico, while the Pacific-slope Flycatcher predominantly occurs in coastal ranges. Both species are commonly associated with cool, shady locations along waterways, where there is some openness under the canopy and opportunities for nest placement. Within their more easterly range, Cordilleran Flycatchers breed in cooler, more arid, and denser boreal forests of pine, fir, and spruce ( Small 1994 ). However, the distributions of Cordilleran and Pacific-slope flycatchers and species interactions are complex, and the 'story' is not a simple one. 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 ).
Most published studies of "Western Flycatcher" nesting biology refer to the Pacific-slope Flycatcher, and few observations have been made on breeding Cordilleran Flycatchers, although their biology is likely similar. Little is known about the migration or overwintering ecology of the Cordilleran Flycatcher. It is known that molt occurs primarily on the overwintering grounds (Johnson 1974b), but their overwintering distribution not fully resolved owing to the presence of resident Cordilleran Flycatchers (E. o. occidentalis) and overwintering Pacific-slope Flycatchers. In this species account, studies of the Western Flycatcher that were published prior to 1989 have been allocated to either the Cordilleran Flycatcher or Pacific-slope Flycatcher based on geography; use of the name "Western Flycatcher" in the account indicates uncertain identity.