Identification of Willow and Alder Flycatchers by Primary-Tip Spacing: The P6:7 Ratio

Discrimination of silent Alder (Empidonax alnorum) and Willow Flycatchers (E. trailli) has long been considered exceedingly difficult from photographs or under field conditions. Through extensive specimen and field research, we explored the applicability of primary-tip spacings on a folded wing to field identification. We found that the distance from the tip of primary 5 to primary 6 divided by the distance from the tip of primary 6 to primary 7, which we term the “P6:7 ratio,” is a nearly diagnostic field mark. In our sample of 217 Alder Flycatcher specimens, 95% have P6:7 < 1.02, whereas in our sample of 371 Willow Flycatcher specimens 95% have P6:7 > 1.05. Because the dividing line between the two species is close to a P6:7 ratio of 1, this field mark is discernible from photos and, with experience, in the field.