The Ash-throated Flycatcher is a familiar sight and sound during spring and summer across much of western North America as far north as central Washington, Idaho, Wyoming, and western Colorado and south to central Mexico, occurring in a variety of lowland to mid-elevation habitats, including desert scrub; open deciduous, coniferous, and mixed woodlands; and riparian forest. Except for year-round presence in parts of extreme southeastern California and southern Arizona, Baja California, and Sonora, the species is completely migratory. Ash-throateds move south during late summer and early fall, generally departing the United States by mid-September. Most winter on the Pacific slope from Mexico to Honduras, with some found regularly on the northeastern coastal slope of Mexico and irregularly in the desert lowlands of the extreme southwestern United States. Throughout most of the breeding range, especially at high elevations and at more northerly latitudes, where breeders arrive relatively late in the spring (mid-April to May), only a single brood is raised. In more southerly low-elevation desert areas, breeders arrive much earlier (March to mid-April) and, at least locally, are able to raise 2 broods.
Because Ash-throated Flycatchers are tolerant of high temperatures, do not need to drink water, and can nest in relatively small cavities, they can breed even in sparse desert scrub as long as adequate food resources and nest sites are available. Territory size varies substantially depending on habitat structure and quality and nest-cavity availability. Highest densities and smallest territories tend to be associated with riparian corridors. A secondary cavity nester, this species will readily use human-made cavities such as nest boxes and hollow metal fence poles; this adaptability has probably resulted in population increases (or at least helps offset loss of natural habitat), and has allowed some local expansion into previously unoccupied areas.
The slightly shorter-winged, longer-billed, non-migratory population of central and southern Baja California is considered a separate subspecies. Otherwise, there is only minor geographic variation in size and practically no detectable variation in plumage coloration within the species. Over much of its breeding distribution in the western United States, this flycatcher is the only regularly occurring member of the difficult-to-identify genus Myiarchus . Identification can be more challenging, however, in more southerly and southeasterly breeding areas and in winter, where extensive distributional and habitat overlap can occur with other superficially similar species: Brown-crested (M. tyrannulus), Great Crested (M. crinitus), Dusky-capped (M. tuberculifer), and Nutting's (M. nuttingi) flycatchers.
As with many other common North American species, the Ash-throated Flycatcher is surprisingly understudied, despite being widespread in the American West and a somewhat more accessible subject than its more canopy-dwelling relatives, the Brown-crested and Great Crested flycatchers. Our knowledge of the species is generally anecdotal in nature, with the exception of thorough analyses of morphological and vocal systematics by Lanyon ( Lanyon 1961 , Lanyon 1963a , Lanyon 1963b ), community-oriented studies of nest-site selection, territory size, and nesting success ( Carothers et al. 1974a ; Stamp 1978 ; Brush Brush 1981 , Brush 1983 ; Seavey Seavey 1997 , Seavey 2000 ), and foraging ecology ( Landres and MacMahon 1980 , Rosenberg et al. 1982 ), and more specific studies of nestling growth and energetics ( Mock et al. 1991 ) and effects of high temperatures on nests in metal poles ( Dunning and Bowers 1990 ).