Genetic, bioacoustic and morphological analyses reveal cryptic speciation in the warbling vireo complex (Vireo gilvus: Vireonidae: Passeriformes)

Cryptic species are closely related taxa that are difficult to separate morphologically, but are reproductively isolated. Here we examine the warbling vireo complex (Vireo gilvus), a widespread songbird speculated to be comprised of more than one cryptic species. We included three taxa within the complex: two of the western (Vireo gilvus swainsonii and Vireo gilvus brewsteri) subspecies and the single eastern (Vireo gilvus gilvus) subspecies. We used mtDNA and microsatellite loci to assess the congruence of genetic data to the current subspecies boundaries. We then incorporated bioacoustic, morphometric and ecological niche modelling analyses to further examine differences. We found two genetic groups with mtDNA analysis, splitting eastern and western warbling vireos. Microsatellite analyses revealed four genetic groups: an eastern group, a Black Hills group and two western groups that do not agree with current western subspecies boundaries based on phenotypic data. Our results suggest that eastern and western warbling vireos have been reproductively isolated for a long period of time and therefore may be best treated as separate species. However, more research into areas of contact to examine the presence of hybridization is advised before making a taxonomic revision. Differences between the two western genetic groups appear less clear, requiring additional research.