I agree with Bob Archer, these are Western Flycatchers. I was corresponding
with Arch McCallum a couple of years ago over a bird I had on the Trout Creek
Hillside. Arch said it is questionable whether the Cordilleran/Pacific-slope
split was a good one and that the vast majority of birds north of the
Arizona-New Mexico/Utah-Colorado state lines are birds of mixed lineage. Most
birds in this region give mixed song/call types.
His published info online says “forget about identifying [the two “species”] by
voice as vocal variation from one extreme to the other is continuous.”
Sonograms are necessary. I know he is still working on this issue. His website
says, “I am currently in the process of revising it, and modifying slightly the
perspective. Go to
<http://www.appliedbioacoustics.com/Repertoires/Passeriformes/Tyrannidae/Empidonaxdifficilis>
Revised WEFL Site to see the work in progress. In particular, although the
"core" populations of the two "species" are distinguishable vocally,
geographically intermediate populations are also intermediate vocally. For the
time being, I recommend against trying to put a name on those birds. Andrew
Rush and I are currently seeking recordings of dawnsong from throughout the
range to determine if the variation in measurable song attributes is
continuous. We especially need recordings from Wyoming, western Colorado, Utah,
northern Arizona, and Nevada. We also need recordings of dawnsong from Mexico,
and recordings of the Yellowish Flycatcher (Empidonax flavescens), Western's
very similar-looking and closest relative, from Chiapas through Nicaragua.”
An article published in 2019 [Linck, E., K. Epperly, P. van Else, G. M..
Spellman, R. W. Bryson, Jr., J. E. McCormacks, R. Canales-Del Castillo, and J.
Klicka. 2019. Dense geographic and genomic sampling reveals paraphyly and a
cryptic lineage in a classic sibling species complex. Syst. Biol. 0(0):1–11,
2019, DOI:10.1093/sysbio/syz027.] posits that there anywhere between one and
four species involved in this complex. They favor either one species for all
of Western North America (including hybrids) or two species split with one for
the US and one for Mexico. Either would eliminate the
Pacific-slope/Cordilleran conundrum that we deal with here. The authors
acknowledge that a different definition of what is a species could lead to four
different species, but don’t resolve that question and say “a detailed analysis
of these data will be presented elsewhere.”
Here is a map showing the information their data shows the following. [Gray
dots are Pacific-slope, yellow are Cordilleran and the orange and green are the
Mexican forms. Where a dot has two colors it shows the intermixing of the
forms. Unfortunately they didn’t examine any Central Oregon birds.
It appears we haven’t heard the last word on Emidonax difficilis (difficilis
indeed!).
Tom Crabtree, Bend