A tiny swift with glossy upperparts (many forms showing a greenish-blue tinge), a white belly, and a bat-like, fluttering flight action. Some forms show a white rump. Occurs over most habitats throughout range, often within flocks that regularly mix with other swiftlet species. Away from nesting colonies, always seen in flight. Smaller and shorter-winged than Uniform, Papuan, Bare-legged, Mayr’s, Mountain, and White-rumped Swiftlets. Further distinguished from those species and from Edible-nest, Sulawesi, Seram, and Halmahera Swiftlets by white belly and flight action.