The Dusky Flycatcher, a common breeding species throughout much of mountainous western North America, inhabits open coniferous forest, mountain chaparral, aspen groves, streamside willow thickets and brushy open areas. It typically nests in deciduous trees and shrubs within a few meters of the ground, weaving its nest of plant fibers and animal hair into an upright crotch. Only females incubate, but their mates often feed them; both adults feed young, which fledge in about 18 days. Although little is known about the species' diet, its foraging tactics have been well studied. Primarily an aerial forager—a sit-and-wait predator—it sallies forth after flying insects or occasionally pounces on prey on the ground or low shrubbery.
Like other small temperate-zone insectivores, the Dusky Flycatcher may be particularly vulnerable to bad weather; severe spring rain and snow may cause mortality of entire local breeding populations. Likewise, weather may account for a significant proportion of nest failures, equal in many years to what is taken by predators. Nevertheless, long-term survey data suggest that this species is at least holding its own, if not growing in numbers, in most regions where it nests. The Dusky Flycatcher may benefit from forestry practices that thin dense coniferous stands or leave small openings.
The genus Empidonax is composed of a number of morphologically similar species that have long perplexed ornithologists. In western North America, geographical overlap of Dusky, Hammond's (E. hammondii), and Gray (E. wrightii) flycatchers, in particular, has led to confusion and disagreement as to the taxonomic status of these species. The convoluted taxonomic history of E. oberholseri attests to this fact. Until the late 19th century, Dusky and Gray flycatchers were disguised together as E. obscurus (Wright's Flycatcher). From that period until the publication of the Fifth Edition of the American Ornithologists' Union Check-list ( American Ornithologists' Union 1957 ), Dusky and Gray flycatchers were considered distinct species, known respectively as Wright's Flycatcher (E. wrightii) and Gray Flycatcher (E. griseus), although there was still much confusion as to the criteria of species identification. When Phillips ( Phillips 1939 ) considered the type specimen of E. wrightii to represent the Gray Flycatcher, and not the Dusky Flycatcher, the name oberholseri was proposed to cover the form Dusky Flycatcher and wrightii was assigned to the Gray Flycatcher. Phillips' identification was corroborated when Johnson ( Johnson 1963b ) verified identification of the type of the Gray Flycatcher by segregating age categories.