 Hummingbirds cannot make the courtship dive sound when missing their outer tail feathers, and those same feathers could produce the dive-sound in a wind tunnel.The bird can sing at the same frequency as the tail feather chirp, but its small syrinx is not capable of the same volume.The sound is caused by the aerodynamics of rapid air flow past tail feathers, causing them to flutter in a vibration which produces the high-pitched sound of a courtship dive.