Cryptic species-level diversity in Dark-throated Oriole Oriolus xanthonotus

Abstract

Borneo is part of the Sundaland biodiversity hotspot, yet this large tropical island still harbours much unrecognised biodiversity. In this integrative study, we combine the results from phylogenomic, bioacoustic, biometric and morphological analyses, and show that the Sundaic species Dark-throated Oriole Oriolus xanthonotus comprises two species-level taxa, both of which occur on Borneo. The eastern species, here re-named Ventriloquial Oriole O. consobrinus, is characterised by plumage distinctions, most notably a more male-like coloration in females, and a characteristic ventriloquial song unique among the region's orioles. The precise contact zone of the two species on Borneo is incompletely mapped. Their populations on Borneo show pronounced character displacement whereby individuals of O. xanthonotus on other landmasses are significantly larger than Bornean individuals, presumably to prevent non-adaptive hybridisation with the larger Ventriloquial Oriole along their Bornean contact zone.