Since the genus Empidonax species look strikingly similar, what vital identification tool is emphasized for distinguishing the Willow Flycatcher?

Vocalizations, specifically the distinct "fee-bee" song.

Distinguishing between species within the genus Empidonax presents significant identification challenges because many species exhibit striking visual similarity in their plumage and general morphology. Because subtle visual cues like eye-ring size or wing-bar appearance are often insufficient for reliable separation, field observers must frequently rely on acoustic cues. The distinct song, described specifically as the 'fee-bee' song for the Willow Flycatcher, serves as a vital identification tool. This reliance on auditory signals suggests that reproductive isolation between these closely related species is primarily achieved through acoustic distinction rather than visual differentiation.