Integrative taxonomy of an East Asian songbird indicates rapid dwarfism after island colonization

Animals that colonize islands often undergo significant evolutionary changes in comparison with their continental counterparts as a response to specific island conditions. The pace of such changes can be relatively fast, which poses challenges in the evaluation of the taxonomic status of island taxa. The Japanese and Manchurian Bush Warbler species complex (Horornis diphone–canturians), which breeds in East Asia and the Japanese Archipelago, is such an avian example. This species complex exhibits significant morphological differentiation between different taxa, and the taxonomic status and phylogenetic relationships within the complex are debated. Here, we updated the taxonomy of this species complex and shed insight on its evolutionary history using multi‐locus phylogeographic and acoustic analyses. Our results support the conventional treatment of the two continental taxa borealis and canturians as subspecies of H. canturians, contrary to some recent proposals that they are affiliated to H. diphone. We also document a reduction in body size, that is dwarfism, and vocal divergence in the nominate subspecies H. d. diphone, which is endemic to the remote and isolated Ogasawara Islands. These changes may have happened following colonization of these islands, which was estimated to have taken place approximately 0.2 million years ago. Although H. d. diphone is clearly distinctive and deserves recognition as an evolutionarily significant unit, H. d. diphone and other H. diphone samples were not reciprocally monophyletic. Because of this lack of reciprocal monophyly and a relatively recent divergence time, we advocate maintaining its current subspecies status. We also detected reduced genetic diversity, measured as heterozygosity, in H. d. diphone. We suggest that conservation efforts in the Ogasawara Islands should prioritize the protection of this endemic subspecies. Collectively, our findings suggest that the separation between the populations on the East Asian continent and the Japanese Archipelago, followed by colonization of remote oceanic islands through long‐distance dispersal, underlie rapid phenotypic and genetic diversification of the Horornis diphone–canturians species complex.