Abstract
The Oriental Dwarf Kingfisher species-complex of South and Southeast Asia comprises two forms, the dark-backed Ceyx erithaca of India and Indochina and the rufous-backed Ceyx rufidorsa of Java and the Lesser Sunda Islands. Between these two extremes, the large area of Sundaland and the Philippines is occupied by individuals that have a rufous back, characteristic of C. rufidorsa, but exhibit a range of phenotypes that are intermediate between C. erithaca and C. rufidorsa. These potential intermediates have intrigued avian taxonomists for generations. To investigate the species dynamics of the two forms and understand the demographic history of the intermediates, we generated a genome-scale dataset (ddRAD) representing multiple individuals across the entire range of the complex. Our findings support the distinctiveness of the two forms based on back colour. Demographic analysis suggests the two populations were isolated c. 820 000 years ago followed by secondary contact c. 140 000 years ago, with asymmetrical dispersal of C. rufidorsa into C. erithaca. Although some limited introgression appears to have occurred more recently between the two taxa in the northern parts of their range, we were unable to find any association of recent hybridization with the intermediate plumages of C. rufidorsa. We also found no support for the commonly recognized Borneo subspecies motleyi.