Can song discriminate between Macgillivray's and Mourning Warblers in a narrow hybrid zone?

Areas where divergent taxa come into secondary contact are important in identifying the reproductive barriers that may have evolved between them. Here we investigate whether song variation plays a role in maintaining a recently described hybrid zone between MacGillivray's (Oporornis tolmiei) and Mourning Warblers (O. philadelphia) in northeastern British Columbia, Canada. We analyzed song recordings and genetic data from across the ranges of both species as well as where they occur in sympatry. We predicted that if song differences are an important pre-mating reproductive barrier between these taxa, song and genotype in the hybrid zone should covary. Using a discriminant function analysis, we show that the two species' songs are differentiated in allopatry but that songs converge in sympatry, with a weak and nonsignificant association between songs and diagnostic mitochondrial and nuclear genetic markers in the hybrid zone. These results suggest that song divergence is unlikely to be an important reproductive barrier between these taxa and that learning of the other species' song may be promoting continued hybridization. This result contrasts with studies of other species pairs that maintain song differences in sympatry and come into contact in this same geographic region.