Large dark swift with long sickle-shaped wings and a forked tail, which is often kept closed in a needle-tipped point while in flight. Black overall with an ashy gray throat and a thin white rump. Distinguished from needletails by proportionally longer wings, longer and more slender body, and much longer tail. Closely related Pacific Swift has a broader and brighter rump patch and a whiter throat. Faint greenish gloss on the back is diagnostic but difficult to see. Like other "fork-tailed swifts", usually seen in flight over hills, forests, and open areas. Gives high thin notes as well as a grating trills. Breeds in limestone caves.