Acoustic analyses support the recognition of a distinct southwestern subspecies.
Classifying birds into species and subspecies can be a tricky exercise. Different criteria – such as morphology, genetics or behavior – can result in drastically different taxonomic arrangements. The incongruence between these criteria is expected when you understand that speciation is a gradual process. As two populations follow their own evolutionary trajectories, different traits will evolve at different rates. For example, populations might become genetically distinct while they do not develop morphological differences (or vice versa). When it comes to tyrannid flycatchers (genus Empidonax), song and plumage are evolving at contrasting speeds. These small songbirds show very little differentiation in plumage patterns, but tend to sing distinct songs. But how to classify these birds? A recent study in the Journal of Avian Biology took a closer look at one species: the Willow Flycatcher (Empidonax traillii).
Despite the lack of clear plumage differences, the Willow Flycatcher has been divided into four subspecies that can be found in different parts of the USA: trailii in the east, brewsteri in the northwest, adastus in the interior west and extimus in the southwest. The studies underlying this subdivision have been criticized because they lacked rigorous statistical analyses, had small sample sizes and used wild birds that were released and could thus not be re-analyzed (see this paper for an overview of the critique). In a recent study, Sean Mahoney and his colleagues addressed these shortcomings by measuring museum specimens and analyzing publicly available song recordings. Moreover, they used unsupervised clustering algorithms that do not take into account the current taxonomic arrangement and are thus more objective.
The clustering based on plumage coloration suggested two main groups that did not follow the subspecies classification. This result highlights the lack of plumage differentiation in these flycatchers. The song analyses, however, did align with the taxonomy of the Willow Flycatcher. The algorithm pointed to two clusters that correspond to the subspecies extimus and the remaining three subspecies (trailii, brewsteri, and adastus). Hence, the authors conclude that “our song data support the recognition of the southwestern population as a distinct subspecies.” This finding is relevant for the conservation of the Willow Flycatchers, because this subspecies is currently listed as endangered by the US Fish and Wildlife Service. Uncertainty around its taxonomic status would have complicated its protection.
Mahoney, S. M., Reudink, M. W., Pasch, B., & Theimer, T. C. (2020). Song but not plumage varies geographically among willow flycatcher Empidonax traillii subspecies. Journal of Avian Biology, 51(12).
Featured image: Willow Flycatcher (Empidonax traillii) © VJ Anderson | Wikimedia Commons