Song but not colour divergence constrains hybridization in birds.

Mating signals, such as songs or plumage ornamentation, are key prezygotic barriers that promote behavioral isolation by ensuring that individuals are able to mate with conspecifics and avoid the costs of hybridization. Previous family-level comparative studies in birds have revealed that song similarity is positively correlated with the incidence of hybridization, whereas genomic research in avian hybrid zones suggests that plumage divergence does not necessarily prevent gene flow. However, no study has jointly evaluated the relative contribution of both signalling modalities to hybridization incidence within a broad macroevolutionary framework. We combined a large-scale dataset on documented hybridization events across bird species with quantitative measures of acoustic and plumage divergence and tested their association using phylogenetically informed comparative models. We found that species-level acoustic divergence showed a strong negative association with the number of hybridization partners after accounting for sympatry. In contrast, male plumage divergence did not predict hybridization incidence, yet female plumage divergence showed a significant negative association. These results indicate that song represents a major reproductive barrier in birds, probably because it is a long-range and evolutionarily labile signal. Moreover, male plumage coloration may play a more limited role in reproductive isolation, potentially because male ornaments may introgress more readily across species boundaries.