Fee-bee-o. This traditional description identifies the song of the Alder Flycatcher. Songs of Empidonax flycatchers, a notoriously difficult group to identify in the field, are the best, and sometimes only, means for determining species. This is especially true for separating the Alder Flycatcher from its sibling species, the Willow Flycatcher (E. traillii). Alder and Willow flycatchers cannot be identified reliably by sight alone, and even in-hand identification is not always certain. Alder (fee-bee-o song) and Willow (fitz-bew) flycatchers were considered one species called Traill's Flycatcher, a name still used to refer to the species pair, until 1973 when they were formally separated (Eisenmann 1973). Of these two sibling species, the Alder Flycatcher has the more northern and boreal distribution.
The Alder Flycatcher is inconspicuous in several ways. Besides having been hidden taxonomically and morphologically, this species is unassuming on its breeding grounds. Breeding Bird Atlas projects in Canada and the United States show it spread widely but at low density; its nests are hard to find, often “in shrubs growing in knee-deep water and muck” (Payne 1991a: 284). Few studies of Alder Flycatchers exist, and many observations of this species' biology are anecdotal. Most published studies of Traill's Flycatchers refer to the more southerly Willow Flycatcher, or are of uncertain specific identity. Many general statements about Alder Flycatcher biology can rely on reference to other Empidonax flycatchers, to which Alder Flycatcher is undoubtedly similar (e.g., Mccabe 1991, Sedgwick Sedgwick 1993a, Sedgwick 1994, Sedgwick 2000, Briskie 1994). Alder Flycatchers are late migrants in spring and leave early in fall; northern populations have a short 70–90 day breeding season. This flycatcher is single brooded, although individual pairs may renest after an early loss. Molt occurs primarily on wintering grounds.
In this account, references of pre-1973 literature to Traill's Flycatcher indicate possible Alder Flycatcher identity; reference to Alder Flycatcher indicates identification by song and/or observation of Traill's Flycatcher in breeding range separate from that of Willow Flycatcher.