Population Genetic and Hybridization Studies of Three Bornean Bird Species: Mountain Black-eye (Chlorocharis emiliae), White-rumped Shama (Copsychus malabaricus), and Oriental Magpie-robin (Copsychus saularis)

Mountain Black-eye (Chlorocharis emiliae), Oriental Magpie-Robin (Copsychus saularis) and White-rumped Shama (C. malabaricus) are Bornean passerines with different population dynamics. To determine the effect of these differences on their population structures, I examined mitochondrial DNA sequences and morphological characters of populations within each species in Sarawak and Sabah, Malaysia. Chlorocharis is a white-eye (Zosteropidae), endemic to Borneo, with a unique “sky island” distribution. Molecular and previously obtained morphological data support its traditional division into at least three subspecies. An unexpected result of my comparisons was a marked genetic subdivision between its Sabah and Sarawak populations. C. malabaricus and C. saularis are widespread lowland thrushes (Turdidae). Both are divided into two subspecies on Borneo based on plumage, and these subspecies meet near the Sabah-Sarawak border. My comparisons indicated that C. malabaricus subspecies, for the most part, do not hybridize and can be considered full species, whereas C. saularis subspecies hybridize extensively and should be retained as subspecies. The distinct population characteristics of the three species may be attributed to their different life styles. Chlorocharis occurs mainly on Borneo’s highest peaks, and even during glacial periods when montane forest descended in elevation its populations apparently remained isolated from one another. C. malabaricus is a forest interior species that was divided into two populations in the early Pleistocene. Its populations have only recently come in contact with one another, presumably because of the species’ limited dispersal propensity. C. saularis on the other hand is an open country species. It too was subdivided in the early Pleistocene, but its populations have greater dispersal propensity and may have moved more into rapidly contact with one another than C. malabaricus populations. The most important discovery of this study is that despite different population dynamics and habitats, the three species exhibit the same population subdivision in the vicinity of the Sabah-Sarawak border. In Copsychus, the subdivision may be explained by the withdrawal of populations to rainforest refugia during the Pleistocene. However, no simple explanation exists for Chorocharis, which as a montane species should not be similarly subdivided. It should have expanded, not contracted, during glacial events