Plumage assisted divergence in a vocally complex island endemic: The Dicrurus paradiseus species complex in Sri Lanka

Models of allopatric speciation within an island biogeographic framework suggest that the division of ancestral mainland populations leads to one or more allopatric island species predominantly through natural and sexual selection or genetic drift. Here we studied phenotypic divergence in a phylogenetic framework in the Dicrurus paradiseus allospecies complex in Sri Lanka, a continental island located in the Indian plate, to understand the complexity of phenotypic divergence on an island. Members of the genus Dicrurus are known as drongos and are conserved in morphology and plumage, but highly variable in vocalization due to vocal learning and mimicry. Two closely related drongos are found in Sri Lanka: the endemic D. lophorinus (or D. paradiseus lophorinus to many authors) found in the wet zone of the island and the widespread continental species D. paradiseus, which inhabits the dry zone. Sampling from all major populations, and voucher specimens from museums across their range in Sri Lanka, we examined phenotypic and genetic variation in this group. The phenotype showed two clusters: birds with a fish-like tail and erect crest (D. lophorinus), and birds with elongated tail streamers with backwardly curved crest (D. paradiseus). There was no significant difference in the vocal traits compared. The genetic variation was examined using two nuclear (Myo 2 and c-mos) and two mitochondrial (ND2 and Cytb) loci and the phylogenetic relationship was analyzed using the Bayesian inference coalescent-based species tree estimation method. The quantitative criteria for species delimitation provided a score sufficient to consider these two taxa as distinct species by considering measurements of body and plumage, acoustics, behaviour and distribution. The phylogeny supports distinct species status for the Sri Lanka Drongo (Dicrurus lophorinus) and that the D. lophorinus and D. paradiseus sister pair diverged since 1.35 mya. The variation in the crest and the tail plumage (components of phenotype) were the main contributors of the divergence, despite the similarity in general appearance and vocalization of the allopatric species.