Geographic variation, population genetic structure, and taxonomy of the Variable Antshrike (Thamnophilus caerulescens)

The Variable Antshrike (Thamnophilus caerulescens) includes eight recognized subspecies across a broad range from Peru to eastern Brazil. Previous genetic studies suggested deep divergence among some populations, but limited geographic sampling hindered taxonomic resolution. We analyze plumage, morphology, vocalizations, and genome-wide genetic markers across the full range of T. caerulescens to assess geographic variation and population structure. We also conducted field playback experiments to test vocal differences as potential premating reproductive barriers. Our data reveal nine phenotypically distinct populations, including seven of the eight currently recognized taxa and two new taxa. Increased geographic sampling uncovered several hybrid zones, some spanning hundreds of kilometers. The geographically isolated T. c. cearensis showed substantial genetic and vocal differences from the remaining taxa, overlapping in acoustic trait space only with the distant T. c. melanchrous. In playback experiments, T. c. cearensis responded to its own and T. c. melanchrous songs but ignored others. These results suggest strong behavioral isolation and likely intrinsic postzygotic barriers between T. c. cearensis and other taxa, supporting its recognition as a separate species. In contrast, most other taxa exhibit only modest differentiation in traits linked to reproductive isolation and show weak barriers in contact zones.