Simple technique for distinguishing Yellow‐bellied Flycatchers from Cordilleran and Pacific‐slope flycatchers

Flycatchers of the genus Empidonax are readily misidentified in the field, in the hand, and even in museum collections. We describe a novel plumage feature that can be used to distinguish Yellow‐bellied Flycatchers (E. flaviventris) from the two species that comprise the Western Flycatcher complex, Cordilleran Flycatchers (E. occidentalis) and Pacific‐slope Flycatchers (E. difficilis). The length of the buffy fringing on the anterior edge of each secondary feather, visible on the folded wing, is significantly shorter in Yellow‐bellied Flycatchers than in Western flycatchers, with minimal overlap. A definitive identification can be made using a simple formula that includes measurements of wing chord and the length of the buffy fringing along the outer edge of the first secondary (S1). This method provides definitive in‐hand identification, and the difference in length of the buffy fringing on the secondaries is also a useful field mark for visual identification. Testing our method with 113 museum specimens that had been identified a priori based on locality, we correctly identified 112 specimens. The exception was a specimen from Illinois that had been assumed to be a Yellow‐bellied Flycatcher. However, based on our formula, it was a Western flycatcher and analysis of its mtDNA sequence confirmed this result, proving the utility of our method.