The function and evolution of the tail streamer in hirundines

The morphology of a bird's tail may result from compromisesbetween aerodynamic efficiency, phylogenetic constraints andselection for non-aerodynamic characteristics, such as mateattraction. A good example of a trait shaped by trade-offsbetween aerodynamic efficiency and reproductive benefits mediatedthrough female preference is the tail streamer of the barnswallow. Here we use a standardized task to measure the impactof manipulated tail streamer lengths on maneuvering flightin the barn swallow and in the sand martin, a closely relatedspecies that lacks a streamer. Our results show that the tailstreamer of the barn swallow has a role in maneuvering flight.However, the outer tail feather is approximately 12 mm (9-20%)longer than the aerodynamic optimum for maneuvering flight.Furthermore, we show that the addition of artificial tail streamersto the sand martin, enhances maneuverability even at smallstreamer lengths, thereby implying that tail streamers mayhave evolved via natural selection for increased flight performance.Our results therefore suggest that initial tail streamer elongationin the barn swallow has a functional explanation in terms ofincreased aerodynamic performance. However, female choice hasbecome associated with this trait, promoting the developmentof a costly handicap.