The Gray Flycatcher is a common inhabitant of semi-arid woodlands and shrublands of the interior western United States in summer and northern Mexico in winter. From sagebrush (Artemisia spp.) shrub-steppe to pinyon-juniper (Pinus spp. – Juniperus spp.) woodland to yellow pine forests of the Great Basin and inter-mountain region, this flycatcher actively defends territories and forages for insects from shrubs or branches low in trees. It often nests atop branches against the trunk of a pine, in forks of branches, or hidden within shrubs. Females lay clutches of 3 to 4 eggs from May to early June, second clutches as late as July. Incubation lasts 14 to 16 days. Young fledge in about 16 days and may be fed by parents out of the nest for an additional 14 days.
Individuals begin fall migration in August and undergo complete feather molt on the wintering grounds. Although the diet and foraging behavior of this species have not been studied in detail, observed foraging behaviors include picking insects off the ground from a perch or in flight, as well as conventional flycatcher behavior of sallying to catch flying insects or hovering to pluck insects off vegetation.
The close similarity of the Gray Flycatcher to the Dusky Flycatcher (Empidonax oberholseri) has led to taxonomic confusion and subsequent name changes for both species. These problems were finally resolved by Allan R. Phillips ( Phillips 1939 ) when he discovered that the type specimen designated for the Dusky Flycatcher was in fact a Gray Flycatcher. Even after the Gray Flycatcher was recognized as a separate species in 1889, many studies did not accurately discriminate between Dusky and Gray flycatchers, sometimes including reports that were a composite of both. It was Allan Phillips again who provided a major breakthrough in identification that made the Gray Flycatcher perhaps the easiest species of Empidonax to identify in the field when he described its habit of wagging its tail in a gentle downward movement, similar to a slowed-down tail wag of a phoebe (Sayornis spp.), rather than flicking the tail up and then down as performed by other Empidonax ( Phillips 1944a ).
Because of difficulties in identification, early nomenclatural confusion, and the preference of this species for remote haunts in sparsely populated areas, ecological studies of the Gray Flycatcher have been scarce. Nevertheless, research in New Mexico, California, and Utah ( Russell and Woodbury 1941 , Johnson 1963b ) has provided important information about the phenology, nesting behavior, parental behavior, vocalizations, and ecological relationships of this species with other western Empidonax flycatchers. Additional research in the Southwest has informed Gray Flycatcher conservation, including responses to potential threats such as gas drilling, drought, and tree mortality (Schlossberg 2006, Francis et al. 2009).
Not until the early 20th century was the Gray Flycatcher known to breed in the United States. Before then it was presumed to breed in northern Mexico and to wander northward in fall. Improved characters for identification, such as the tail wag and others (e.g., see Whitney and Kaufman 1987 ), facilitated the discovery and documentation of Gray Flycatchers in the field and helped foster a more complete knowledge of the migration and breeding distribution of this species, including recent range expansions into central Washington and southern British Columbia ( Yaich and Larrison 1973 , Lavers 1975 , Cannings 1987b , Smith et al. 1997 ), and into southwestern California ( Johnson and Garrett 1974 ).