Extensive historical and contemporary hybridization suggests premating isolation in Vermivora warblers is not strong: A reply to Confer et al.

We present comments on an article published by Confer et al. (Ecology and Evolution, 10, 2020). Confer et al. (2020) aggregate data from multiple studies of social pairing between Vermivora chrysoptera and V. cyanoptera, two wood warblers in the family Parulidae that hybridize extensively where they co-occur. From analysis of these data, they conclude there is near-complete reproductive isolation between these two species. In our reply, we show that this finding is not supported by other lines of evidence, and significant drawbacks of their study design preclude such strong conclusions. In our critique, we show that (a) coarse-scale plumage classifications cannot be used to accurately estimate hybrid ancestry in Vermivora; (b) extra-pair paternity is very high in Vermivora and is likely facilitating hybridization, yet was not considered by Confer et al. (2020), and we suggest this will have a substantial influence on the interpretation of reproductive isolation in the system; and (c) the central finding of strong total reproductive isolation is not compatible with the results of other long-term studies, which demonstrate low isolation and high gene flow. We conclude with a more comprehensive interpretation of hybridization and reproductive isolation in Vermivora warblers.