The iridescent blue back is the best way to distinguish this species from other swiftlets within its range, but this feature is typically only visible when the bird is flying low (often over water) or on the nest. Relatively well-demarcated white belly and dark rump also serve to distinguish this species from Aerodramus swiftlets. Common and well-adapted to human habitation, nesting socially in both caves and manmade structures. Flies in flocks over forests, fields, and water bodies. Gives sharp grating twitters at the nest, but generally quiet.