TESTING THE FUNCTIONAL-SIGNIFICANCE OF TAIL STREAMERS

Studies of the evolution of elaborate ornaments have concentrated on t heir role in increasing attractiveness to mates. The classic examples of such sexually selected structures are the elongated tails of some b ird species. Elongated tails can be divided into three categories - gr aduated tails, pin tails and streamers, There seems to be little debat e about whether graduated and pin tails are ornaments; i.e. costly sig nals used in mate choice. However, in the case of streamers there is c onsiderable discussion about their function. It has been suggested tha t tail streamers could be (i) entirely naturally selected, () entirely sexually selected, (iii) partly naturally and partly sexually selecte d. The prime example of a species with tail streamers is the swallow ( Hirundo rustica) in which both sexes have tail streamers. In this pape r we discuss the aerodynamic consequences of different types of manipu lation of the streamer and/or outer tail feather. We make qualitative predictions about the aerodynamic performance of swallows with manipul ated tail streamers; these predictions differ depending on whether str eamers have a naturally or sexually selected function. We demonstrate that these hypotheses can only be separated if tail streamers are shor tened and changes in aerodynamic performance measured during turning f light.