Andreas Jonsson's Weblog

From left to right: 1: Olive-sided Flycatcher; 2: Yellow-bellied Flycatcher; 3-6: Traill's Flycatcher; 7-8: Least Flycatcher. All spring/summer males.

At last week’s visit to the Royal Ontario Museum (ROM) Ornithological archives, we had the opportunity study some common groups of similar-looking bird species. One such complex is Tyrant Flycatchers within the genus Empidonax, often referred to as empids. These are small songbirds with olive upperparts, pale throats and bellies, and whitish wing-bars and eye-rings. In the field they are very difficult to distinguish visually, however, their songs and calls are distinct. So, to study skins of these species side by side was a useful exercise.

Same specimens as pictured above. From top to bottom: 1: Olive-sided Flycatcher; 2: Yellow-bellied Flycatcher; 3-6: Traill's Flycatcher; 7-8: Least Flycatcher. All spring/summer males.

The photos above show three Empidonax species: Yellow-bellied Flycatcher; Traill’s Flycatcher; and Least Flycatcher; and for comparison, another widespread North American flycatcher species, Olive-sided Flycatcher. Traill’s Flycatcher has since the 1970′s been considered to consist of two unique species, Alder Flycatcher and Willow Flycatcher (these two are the most difficult to distinguish), however, at the time when these specimens were collected this distinction was not made.

After some close looks at the skins a few conclusions could be drawn. Disregarding from the Olive-sided Flycatcher (which is larger and have darker chest sides that the other empids) it appeared that Yellow-bellied Flycatcher and Least Flycatcher can be uniquely identified by visual characteristics; Yellow-bellied is uniquely coloured bright olive and Least is significantly smaller that other empids. Furthermore, the skull of Least is proportionally larger than Traill’s, and Least has a more distinct eye ring, a character that cannot easily be seen in stuffed specimens, and certainly not in these images.

We also had detailed looks at specimens of Acadian Flycatcher (another empid, not shown), and concluded that they are indistinguishable from Traill’s. I looked especially at the primary projection, which according to Sibley’s “Guide to Birds” is supposed to be particularly long on Acadian, but there was no significant difference between the Acadian and Traill’s specimens we had at hand. I also noted that bill width does not appear to be a useful character in separating empids in the field.

So, where does that leave us. Well, essentially when encountering an Empidonax flycatcher in the field, I would only feel comfortable identifying Acadian, Willow and Alder Flycatchers by sound, whereas the other species should be identifiable by visual means.

Now, I know that there are some advanced birding guides out there, detailing differences between empids, stressing for example differences in bill shape, and I’ve just ordered a copy of the brand new “Field Guide to Advanced Birding” by Kenn Kaufmann, so after having read that I may have reason reassess my conclusions.