Divergence in vocalizations indicates cryptic speciation in Camptostoma tyrannulets

The tyrant flycatcher genus Camptostoma currently comprises two visually similar species: the Northern Beardless-Tyrannulet (C. imberbe), ranging from the southern United States to Costa Rica, and the Southern Beardless-Tyrannulet (C. obsoletum), found from Costa Rica to Uruguay. Vocal and genetic differences suggest that the Southern Beardless-Tyrannulet may include multiple species, but no analysis of acoustic trait variation has been conducted within the genus. We analyzed a geographically comprehensive sample of sound recordings to measure geographic variation in vocalizations across the genus, testing the hypothesis that the Southern Beardless-Tyrannulet includes multiple species. We find that Camptostoma comprises eight vocally distinct populations, two within the Northern and six within the Southern Beardless-Tyrannulet. These populations exhibit diagnostic differences in multiple different vocalizations, including vocalization types that are typically species-specific in the Tyrannidae. Their range limits and contact zones largely corresponded with known dispersal barriers, ecotones, and suture zones, with little evidence of gene flow between adjacent populations. Our results strongly suggest that Camptostoma comprises as many as eight vocally distinct biological species, underscoring the underestimated diversity of species in birds with conservative plumage evolution.