Among white-bellied glossy swiftlets of the Collocalia group, A. R. Wallace was first to recognise the Makassar Strait, separating Borneo and Sulawesi, as a geographical barrier between different phenotypes: plain-tailed to the west and spot-tailed to the east. Other morphological characters used to define species within the group have been blue or green gloss to the dorsal plumage, and the presence or absence of a single minute tufted feather on the hallux. The value of these characters as taxonomic markers is now known to be unreliable due to the discovery of phenotypically mixed populations east of the Makassar Strait, from North Maluku province, I ndonesia, through Papua New Guinea to New Ireland. We combine field observations of plumage characters with genetic evidence to establish taxonomy of Collocalia group swiftlets. Sequencing specific mitochondrial genes (Cytb and ND2), the nuclear-encoded Fib gene, and a subset of mitochondrial genomes provided data for phylogenetic analysis. Genetic divergence of c.4.7% is observed between two Collocalia clades either side of the Makassar Strait: the plain-tailed C. affinis cyanoptila sampled at Fraser's Hill, Peninsular Malaysia, and a phenotypically mixed population of C. esculenta spilura from North Maluku, Indonesia. Each population formed high-affinity genetic clades, within which divergence was <0.5%. These findings are consistent with geographic but not phenotypic separation between populations. We therefore conclude taxonomy based on these plumage features in glossy swiftlets of the Collocaliini is unreliable.