The Western Flycatcher (Empidonax difficilis) is indeed difficult. This species, as recently reconstrued, occupies an enormous range, from the Isthmus of Tehuantepec in southern Mexico to the Canadian Rockies, and from Southeastern Alaska to the Los Cabos municipitality of Baja California del Sur.
Wherever there is a shady ledge on which to build a nest, and moss to build it with, this olive-drab Empidonax is likely to be found. The birds in each of those great north-south swathes behave like good species, but the complete range is now, in recent millennia at least, shaped like a horseshoe, not parallel bars. In the broad area of contact from northern California to Alberta, the unsuitable habitat of the intervening cold and hot deserts to the south is replaced by cooler mountain ranges; stepping stones whereby the products of two independent evolutionary experiments are able to meet and reconstitute their ancestral species, or not, as though the whole affair was arranged by Ernst Mayr to test his theory of speciation.
Recent research, as yet to be published, suggests that the eastern bird, formerly known as Cordilleran Flycatcher (Empidonax occidentalis) from 1989 until 2023, has invaded a large inland area formerly occupied by the erstwhile Pacific-coast Flycatcher (E. difficilis, sensu strictu). In that area, the preponderance of coastal mitochondrial DNA in birds with more-interior nuclear DNA suggests that the coastal population got there first, but that invading Cordilleran Flycatcher, which have been shown to be more aggressive, reduced the participation of Pacific-coast Flycatcher males in breeding, leaving the signature of their former mates in the maternally-inherited mitochondria of the hybrid offspring. No one knows how this will end. But for the persistence of the late Ned K. Johnson in proving the two forms had successfully completed Mayr’s path to species status, and the equal persistence of his successors at replicating his famously intensive studies, Western Flycatcher would remain what it was to the world in 1980, a common but inconspicuous little bird, best known as the easiest North American member of the genus Empidonax to identify by sight. The taxonomic split championed by Johnson made the two sister species impossible to identify by sight, and vocalizations turned out to be undependable because of a continuous spectrum of variation from the norm of one “species” to the other. The confusion led to research that elucidated a story as complex as those of the Yellow-rumped Warbler (Setophaga coronata) and Northern Flicker (Colaptes auratus). For now, all three are safely lumped, but each story shows that there is more in a name than meets the eye. Meanwhile, another genealogical question has arisen in southern Mexico that threatens the well-established split of Western Flycatcher from its sister-species, Yellowish Flycatcher (Empidonax flavescens) . Much more work is needed on this situation, which has major implications for historical biogeography.