Extensive color polymorphism in the southeast Asian oriental dwarf kingfisher Ceyx erithaca: a result of gene flow during population divergence?

Abstract

Extensive plumage color polymorphism in the oriental dwarf kingfisher Ceyx erithaca has long intrigued ornithologists of the Indo–Malayan region. A large proportion of birds in Sumatra, Peninsular Malaysia and Borneo have plumages intermediate between the northerly black form and the southerly rufous form. We used molecular genetic techniques to investigate whether the pattern is likely caused by selection, contemporary hybridization or past introgression of genes. These data consist of DNA sequences of a mitochondrial gene (ND2) and five nuclear intronic loci. Phylogenetic analyses indicated that birds from continental southeast Asia (excluding the Thai–Malay Peninsula) are well differentiated from those from insular southeast Asia. This genetic distinction correlates well with a fixed difference in mantle coloration. Northern birds have black mantles and are consistently dark; southern birds, though highly variable in parts of the plumage, consistently possess a rusty brown mantle. Multivariate analyses of morphometric data also support clustering birds in the two groups defined by mantle coloration. Coalescent analyses suggest that gene flow occurred after initial population splitting. We hypothesized that the dynamic geography of the Indo–Malayan Archipelago, as driven by eustatic sea‐level changes related to glacial cycles, is responsible for extensive secondary contact, and gene flow, between the two color forms. Such periodic contacts, after initial separation of the populations, may be equally common in other geographically dynamic regions of the world.