The function and evolution of the tail streamer in hirundines

The morphology of a bird's tail may result from compromises between aerodyn amic 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 med iated through female preference is the tail streamer of the barn swallow. H ere we use a standardized task to measure the impact of manipulated tail st reamer lengths on maneuvering flight in the barn swallow and in the sand ma rtin, a closely related species that lacks a streamer. Our results show tha t the tail streamer of the barn swallow has a role in maneuvering flight. H owever, the outer tail feather is approximately 12 mm (9-20%) longer than t he aerodynamic optimum for maneuvering flight. Furthermore, we show that th e addition of artificial tail streamers to the sand martin, enhances maneuv erability even at small streamer lengths, thereby implying that tail stream ers may have evolved via natural selection for increased flight performance . Our results therefore suggest that initial tail streamer elongation in th e barn swallow has a functional explanation in terms of increased aerodynam ic performance. However, female choice has become associated with this trai t, promoting the development of a costly handicap.