Comparative bioacoustics of multiple eastern versus western songbird pairs in North America reveals a gradient of song divergence: Raw acoutic measurements

Vocalizations are one of the key premating reproductive barriers that could affect species formation. In song-learning birds, vocal traits are sometimes overlooked in species delimitation, as compared to morphological or plumage-based differences. In this study, we assessed geographic variation in songs of eight pairs of oscines on two scales: (1) comparing primary songs of species/subspecies pairs whose breeding grounds are eastern and western counterparts of each other in the continental North America, and (2) for each counterpart, identifying and comparing possible variation among their populations. We found that there were strong differences in the songs between eastern and western taxa, though the magnitude of that difference was not correlated to a mitochondrial DNA-based estimates of divergence. Additionally, we found that within-taxa geographic variation was not common in our focal taxa, beyond a single species (Townsend’s warbler, Setophaga townsendi). The result of this study provides a standardized, quantitative comparison of eastern and western songbirds, and implies the possible effectiveness of vocalizations as a reproductive barrier as this geographic scale may not be as strong as has been previously suggested.

Methods
The bioacoustic data was collected using the bioacoustic software RavenPro (https://www.ravensoundsoftware.com/). All measurements are raw measurements of sound frequency and duration exported from RavenPro. The details of collection method are described in the primary article.