Massive bioacoustic analysis suggests introgression across Pleistocene land bridges in Mixornis tit-babblers

Because of complex patterns of Pleistocene land connections across its land masses, the Sundaic region is particularly interesting for studying the mechanisms of biological differentiation. Using an unusually large bioacoustic dataset, we studied patterns of vocal differentiation among populations of the widespread Striped Tit-Babbler Mixornis gularis complex. We demonstrate diagnostic differences in song characteristics between distant populations found on separate landmasses, but we show that these differences emerge as clinal once populations on small, rarely-sampled and geographically intermediate islands are included in the analysis. This pattern suggests bioacoustic clinality among different island members of the complex in contrast to results from previous morphological research. Our vocal data additionally uncover cases of potential erroneous taxon attribution and confirm cases of suggested synonymizations in this complicated species complex. Our study supports calls for the integration of bioacoustic data in evolutionary research and underscores the importance of extensive taxon sampling, especially in archipalegic settings such as Sundaland where numerous geographically intermediate island populations may serve as conduits for gene flow during Pleistocene drops in sea level.