Song but not plumage varies geographically among willow flycatcher Empidonax traillii subspecies

Plumage and song are important signals used by birds to attract mates and repel rivals. Divergence in sexual signals can lead to reproductive isolation among incipient species, but the relative importance of each modality may vary among taxa. Tyrannid flycatchers exhibit evolutionarily conservative plumage coloration but distinct song structure among subfamilies and species. Thus, tyrannids are an interesting group in which to study the relative role of plumage and song in contributing to population divergence. In this study, we assessed character divergence among four willow flycatcher Empidonax traillii subspecies by measuring spectral reflectance of plumage modeled in tetrahedral colorspace from museum specimens collected on putative breeding grounds. We also quantified differences in song structure based on publicly available and field-recorded songs across the species range. Using unsupervised and unbiased clustering algorithms that assigned group membership independent of a priori taxonomic designations, we found that currently recognized subspecies did not consistently sort in accordance with subspecies designation based on plumage color. However, song analyses grouped birds into two clusters; one that included 89% of all putative E. t. extimus, and another that included 100% of specimens designated as E. t. adastus, E. t. brewsteri, E. t. traillii and a small percentage of E. t. extimus (11%). Our results are consistent with previous hypotheses of conservative plumage evolution in tyrannids and species differentiation based on song, and support the subspecific status of E. t. extimus.