Diversification of a 'great speciator' in the Wallacea region: differing responses of closely related resident and migratory kingfisher species (Aves: Alcedinidae: Todiramphus)

The Collared Kingfisher species complex is the most widespread of the 'great speciator' lineages of the Indo-Pacific. They have shown a remarkable ability to spread and diversify. As a result of this rapid diversification, Todiramphus species are often found in secondary sympatry. In Southeast Sulawesi, Indonesia, two Todiramphus species are present, the breeding resident Collared Kingfisher Todiramphus chloris and the overwintering migratory Sacred Kingfisher Todiramphus sanctus. We investigated the effect of isolation on these closely related species by comparing their populations on mainland Sulawesi and its larger continental islands, with populations on the small, oceanic Wakatobi Islands. Within our wider analysis we provide further support for the distinctiveness of the Sulawesi Collared Kingfisher population, perhaps isolated by the deep water barrier of Wallace's line. Within Sulawesi we found that populations of Collared Kingfisher on the Wakatobi Islands had diverged from those on mainland Sulawesi, differing both in morphology and in mitochondrial DNA. In contrast, there was no divergence between Sacred Kingfisher populations in either morphology or mitochondrial DNA. We propose that a difference in habitat occupied by Collared Kingfisher populations between the mainland and continental islands vs. oceanic islands has caused this divergence. Mainland Collared Kingfishers are predominately found inland, whereas Wakatobi Collared Kingfishers are also found in coastal habitats. The larger body size of Wakatobi Collared Kingfisher populations may be a result of increased competition with predominantly coastal Sacred Kingfisher populations. The uniform nature of Sacred Kingfisher populations in this region probably reflects their consistent habitat choice (coastal mangrove) and their migratory nature. The demands of their breeding range are likely to have an even stronger selective influence than their Sulawesi wintering range, limiting their scope for divergence. These results provide insight into the adaptability of the widespread Todiramphus lineage and are evidence of the need for further taxonomic revision of Collared Kingfisher populations.