[Photo © Josh Engel: Pacific-slope Flycatcher, Point Reyes National Seashore, California, 5 Sep 2011]
Empidonax flycatchers are hard to identify. Even dead ones, in the hand, when you can look feather by feather by feather. Without hearing them, Alder and Willow Flycatchers, which both pass through Chicago, are essentially identical. The same goes for Pacific-slope (shown above) and Cordilleran Flycatchers, which together used to be known as Western Flycatcher and tend to stick to the Western half of the continent [note that I use "Western Flycatcher" to refer to Pacific-slope and Cordilleran together]. So while Illinois birders are out finding vagrants that aren't so hard to identify, like Lark Bunting, Scissor-tailed Flycatcher, and Black-bellied Whistling-Duck, how many Dusky, Hammond's, and Western Flycatchers slip through the cracks? We don't know for sure, of course, but it's a safe bet that the answer is greater than zero. Now there's some proof, and here's how it happened.
As Matt Baumann and his colleagues at the University of New Mexico (UNM) set out to show that a new field mark they found for identifying Yellow-bellied and Western flycatchers was consistently reliable, they needed to know that the specimens that they were using to test this mark were correctly identified. How to go about that? With DNA, which opens up the opportunity for a whole new level of discovery.
To go about studying the plumage differences, they requested a loan of specimens from the Field Museum, where we have a large collection of Yellow-bellied Flycatchers because of collecting window-killed birds in Chicago by Field Museum staff and the Chicago Bird Collision Monitors. Yellow-bellied Flycatcher is one of the more distinctive Empidonax that passes through the Chicago area, where it is fairly common as it migrates through the area in May and September, but it is extremely similar to Western Flycatcher. Matt and his colleagues wanted to compare these Yellow-bellied specimens to those of Western Flycatchers that are housed at the Museum of Southwestern Biology at UNM, where they are based. Along with the specimens, they requested tissue samples--which we take from every specimen that we add to the bird collection--from which they could extract and sequence DNA and confirm the birds' identities. They had another reason to look at DNA, too. One of the Yellow-bellied Flycatchers from Chicago, according to their newly discovered field mark, was actually a Western Flycatcher, a bird never before found in Illinois. Would the DNA match?
When they compared the mitochondrial DNA sequence of the strange Field Museum specimen (FMNH 472922) with the DNA of other Empidonax flycatchers, they found it indistinguishable from Western (more specifically, Pacific-slope) Flycatcher, but very different than Yellow-bellied (see Figure 3 from their paper below). Not only was their new field mark confirmed to be accurate, but they had also just confirmed a new bird for Illinois! The specimen was collected at McCormick Place, Chicago, on 11 September 2010, which the authors point out is the peak time for Western Flycatcher migration in the western United States. However, most Western Flycatcher records from the eastern United States are from much later in the fall, when they are less likely to be overlooked because other Empidonax have departed by then for warmer climes.
The authors are ultimately cautious about the specific identification of the specimen. In their study, published online yesterday in the Journal of Field Ornithology, they write, "Considering that these two species recently diverged, overlap in distribution, and are geographically variable in genetics and vocalizations, it cannot be assumed that their [mitochondrial] DNA haplotypes provide certain species identification. We assigned [FMNH 472922] to the Western Flycatcher complex, but refrain from more specific identification due to current uncertainty regarding species limits in this group (Lowther 2000, Rush et al. 2009)." Now the record will be in the hands of the Illinois Ornithological Records Committee to decide if the record will be accepted as Pacific-slope Flycatcher or only as the broader Western Flycatcher.
I don't discuss here the new field mark that the authors describe in the paper, but it has to do with the extent of buffy edging on the secondaries. The abstract of the article can be found here. I also don't include a photo of the specimen, but I will add some next week [they have now been added].
CITATION: Baumann, M.J., S.C. Galen, N.D. Pederson, and C.C. Witt. 2014. Simple technique for distinguishing Yellow-bellied Flycatchers from Cordilleran and Pacific-slope flycatchers. Journal of Field Ornithology 85: 391-396. DOI: 10.1111/jofo.12078
LOWTHER, P. E. 2000. Pacific-slope Flycatcher (Empidonax difficilis) and Cordilleran Flycatcher (Empidonax occidentalis). In: The birds of North America online (A. Poole, ed.). Cornell Lab of Ornithology, Ithaca, NY.
RUSH, A. C., R. J. CANNINGS, AND D. E. IRWIN. 2009. Analysis of multilocus DNA reveals hybridization in a contact zone between Empidonax flycatchers. Journal of Avian Biology 40: 614–624.