The role of learned song in the evolution and speciation of Eastern and Spotted towhees

Abstract

ABSTRACTOscine songbirds can learn complex vocalizations that function in mate attraction and territory defense. Sexual selection pressures on such learned behavioral traits have been proposed to accelerate speciation. The Eastern and Spotted towhees are sister species that diverged relatively recently (0.28 Ma) but now have a partially overlapping range; widespread community-science recordings of these species, including songs from within their zone of overlap, enable us to investigate whether song differentiation could have facilitated their reproductive isolation. Here, we quantify 16 song features to analyze the geographic variation in Spotted and Eastern towhee songs and test for species-level differences. We then use a random-forest model to measure how accurately the two species’ songs can be distinguished from one another, as well as whether songs within their region of overlap are more easily distinguishable than those from non-overlapping regions. While no single song feature can reliably distinguish the two species, their songs can be distinguished with 89.5% accuracy using a random-forest model trained on 16 features. For each song feature, there is variation in the strength of the correlation across the longitudinal gradient and considerable differences within and between species. Finally, we analyze the limited publicly available genetic data from each species and find support for the hypothesis that they are reproductively isolated. Together, our results suggest that, in combination, small variations in song features may contribute to these sister species’ ability to recognize their species-specific songs.