Pacific-slope Flycatcher at home Temescal Canyon Southern California 051
"In 1989, the American Ornithologists’ Union split the Western Flycatcher into two species: Pacific-slope Flycatcher (Empidonax difficilis) and Cordilleran Flycatcher (Empidonax occidentalis), on the basis of vocal differences, differences in allozyme frequencies, and an area of sympatry in the Siskiyou region of northern California, where they were reported to mate assortatively.
Ever since then, these two species have been causing headaches for birders all across western North America. The conventional wisdom is that they are impossible to identify by plumage or structure, even in the hand. Voice is the only field mark."
Nathan Pieplow
earbirding.com
Pacific-slope Flycatcher at home Temescal Canyon Southern California 051
"In 1989, the American Ornithologists’ Union split the Western Flycatcher into two species: Pacific-slope Flycatcher (Empidonax difficilis) and Cordilleran Flycatcher (Empidonax occidentalis), on the basis of vocal differences, differences in allozyme frequencies, and an area of sympatry in the Siskiyou region of northern California, where they were reported to mate assortatively.
Ever since then, these two species have been causing headaches for birders all across western North America. The conventional wisdom is that they are impossible to identify by plumage or structure, even in the hand. Voice is the only field mark."
Nathan Pieplow
earbirding.com