The function and evolution of the tail streamer in hirundines

The morphology of a bird's tail may result from compromises between aerodynamic efficiency, phylogenetic constraints and selection for non-aerodynamic characteristics, such as mate attraction. A good example of a trait shaped by trade-offs between aerodynamic efficiency and reproductive benefits mediated through female preference is the tail streamer of the barn swallow. Here we use a standardized task to measure the impact of manipulated tail streamer lengths on maneuvering flight in the barn swallow and in the sand martin, a closely related species that lacks a streamer. Our results show that the tail streamer 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 streamers to the sand martin, enhances maneuverability even at small streamer lengths, thereby implying that tail streamers may have evolved via natural selection for increased flight performance. Our results therefore suggest that initial tail streamer elongation in the barn swallow has a functional explanation in terms of increased aerodynamic performance. However, female choice has become associated with this trait, promoting the development of a costly handicap.