Medium-sized dark swift with small white throat patch and rump patch. Commonly nests in building and cave crevices and often seen in small flocks over valleys, cities, and forests. Plumage pattern is similar to Pacific Swift, which has darker throat and very different shape, with longer, more pointed wings and long, sharply forked tail. White plumage patches distinguish it from Aerodramus swiftlets. Gives a high-pitched, twittering trill.