The effect of tail streamer length on aerodynamic performance in the barn swallow

The elongated tail of the male barn swallow (Hirundo rustica) is regarded a s one of the classic examples of a male trait exaggerated by female choice. However, recently a hypothesis has been proposed suggesting that the strea mers, or elongated outer tail feathers, may aid aerodynamic performance thr ough the Norberg mechanism, providing lift at slow speeds and high angles o f attack when the tail is fully spread. The possibility exists that the tai l screamer has evolved under natural selection, sexual selection, or a comb ination of both selection pressures. We tested these three hypotheses by re ducing the streamer length of free-flying swallows and measuring their aero dynamic performance, using stereo-video. Measurements of flight performance were made from the digitized three-dimensional flight paths. Five flight v ariables best described the individual variation in night performance. Four of these five parameters-mean velocity mean curvature, maximum agility, an d mean rate change of curvature in the XY plane-had significant second-orde r polynomial relationships with tail streamer manipulation. The first and s econd principal components (from principal components analysis of the fligh t variables) also showed similar relationships with streamer manipulation T he combination of a curvilinear relationship between night performance and streamer length and an aerodynamic optimum between 0 and 20 mm reduction is only predicted if both natural and sexual selection have been acting on st reamer morphology. Our data therefore suggest that sexual selection has ext ended streamer length by around 10 mm beyond its aerodynamic optimum. We su ggest that both natural and sexual selection have been important in shaping rail morphology in the barn swallow and the relative importance of both se lection pressures is discussed.