A large dark swift with origami-like angularity in flight profile. Black overall with a slightly paler throat and a bright white rump patch. Long forked tail is prominent in flight and is often opened while the bird is banking. Migrates in enormous flocks, which give sharp trilling calls in flight. The only large “fork-tailed swift” in its range, but compare with Cook's, Blyth's, and Salim Ali's Swifts where ranges overlap.