In some birds, sexual selection, owing to female preference for long tails in mate choice, has led to tails large enough to constitute serious handicaps in flight, incurring a cost in terms of natural selection. Elongated tails also occur among species that depend heavily on flight for foraging, but then usually only the outer tail feathers are long, often being drawn out into narrow streamers. Apart from their role in mate choice, no functional explanation has ever been given of these tail streamers. Based on high-speed motion-picture films of swallows, Hirundo rustica, in free foraging flight and wind-tunnel experiments, I here report on a mechanical and aerodynamic function of the tail streamers in the swallow. Owing to the aeroelastic properties of the outer tail feather, the streamer causes an automatic and instantaneous downward deflection of the tail's leading edge when the swallow spreads and lowers the tail. Thus flow separation on top of the tail may be delayed to higher angles of attack, enabling much higher aerodynamic lift to be elicited from the tail before stall sets in. This improves manoeuvrability by allowing tighter turns, and the development of tail streamers is therefore governed largely by natural selection.