Ecological and evolutionary drivers of geographic variation in songs of a Neotropical suboscine bird: The Drab-breasted Bamboo Tyrant (Hemitriccus diops, Rhynchocyclidae)

Understanding the evolutionary and ecological mechanisms that shape the spatial divergence of signals involved in reproductive isolation is a central goal in studies of speciation. For birds with innate songs, such as the suboscine passerine birds, the integration and comparison of both genetic and ecological factors in explaining song variation at the microevolutionary scale are rare. Here, we evaluated the evolutionary and ecological processes underlying the variation in the songs of the Atlantic Forest endemic Drab-breasted Bamboo Tyrant (Hemitriccus diops), testing the effects of both stochastic and adaptive processes, namely the stochastic and acoustic adaptation hypotheses, respectively. We combined vocal, genetic, and ecological (climate and forest cover) data across the species' range. To this end, we analyzed 89 samples of long and short songs. We performed analyses on raw and synthetic data song variables with linear mixed models and multivariate statistics. Our results show that both song types differ in spectral features between the 2 extant phylogeographic lineages of this species, but such vocal divergence is weak and subtle in both song types. Overall, there is a positive relationship of acoustic distances with the amount of forest cover in long songs. Our results suggest that there is cryptic geographical variation in both song types and that this variation is associated with low levels of genetic divergence in both songs and with ecological factors in long songs. LAY SUMMARY Birds with innate songs constitute good models to evaluate the role of ecological and evolutionary forces in shaping vocal geographic variation. Here, we tested if selective and stochastic–neutral factors underlay the spatial variation in two types of songs in a Neotropical suboscine bird. To this end, we contrasted the variation in songs with genetic and environmental variation across the entire species range. We found that there is cryptic geographical variation in both song types and that this variation is associated with low levels of genetic divergence in both songs and with ecological factors in long songs.