The Willow Flycatcher is a common migratory species that breeds in a variety of usually shrubby, often wet habitats from Maine to British Columbia and as far south as southern Arizona and southern California. It winters from southern Mexico to northern South America in habitats similar to those occupied on the breeding grounds. Formerly considered conspecific with the Alder Flycatcher (Empidonax alnorum), the 2 together were referred to as Traill's Flycatcher until 1973, when they were recognized as separate species ( American Ornithologists' Union 1957 , Eisenmann 1973 ). As are most members of the genus Empidonax, Willow Flycatcher is difficult to identify in the field, and without vocal cues is nearly impossible to distinguish from Alder Flycatcher, whose habitats often overlap those of the Willow.
The Willow Flycatcher has been a much-studied species, partly because of its convoluted taxonomic history and similarity to the Alder Flycatcher and, more recently, because of the listing of the southwestern subspecies (E. traillii extimus) as Endangered ( U.S. Fish and Wildlife Service 1995c ). Studies of its morphology and plumage, behavior, nesting ecology, and song were all at least in part stimulated by the close similarity of the 2 vocal types (fee-bee-o and fitz-bew) of the (then) Traill's Flycatcher. Detailed investigations prompted by declining populations in the southwestern United States include those on the costs of cowbird parasitism, population dynamics, habitat preferences, and vocal and genetic differentiation across subspecific ranges.
Willow Flycatchers are late spring migrants and have a short, 70- to 90-day breeding season. This flycatcher is nearly always single-brooded, laying a clutch of 3 or 4 eggs in late May-late June; the incubation period is 13-14 days, and young fledge about 13-15 days after hatching, usually in mid-July, or somewhat earlier in the Southwest. Both adults feed nestlings and fledglings, but nearly always it is the female that incubates the eggs and broods the young. The Willow Flycatcher is primarily an aerial forager, capturing most of its insect diet on the wing, but it may hover-glean extensively from leaf surfaces or occasionally take insects from the ground.
Because the Willow Flycatcher is restricted to river corridors (at least in the arid parts of the West), it is vulnerable to a variety of human activities that may alter or degrade such habitats, activities including river dewatering, channelization, overgrazing, dam construction, and urbanization. Breeding Bird Survey (BBS) data show this species decreasing in number in both the United States and the North American continent during the period 1966-1996 ( Sauer et al. 1997 ).
In this account, most citations of studies published before 1973 (of "Traill's," "Western Traill's," "Little," or "Alder" Flycatcher; American Ornithologists' Union 1931 , American Ornithologists' Union 1957 ) refer to the Willow Flycatcher only, based on identity by vocalizations or range. In instances where species identity is in doubt, the superspecies name (Traill's Flycatcher) is used. See also Lowther 1999 .