SEXUAL SELECTION AND TAIL STREAMERS IN THE BARN SWALLOW

The functional significance of elongated, narrow tips of the tail feat hers of certain birds, so-called tail streamers, has recently been dis cussed from an aerodynamic point of view, and the effects of sexual se lection on such traits have been questioned. We review our long-term f ield studies using observational and experimental approaches to invest igate natural and sexual selection in the barn swallow, Hirundo rustic a, which has sexually size-dimorphic outermost tail feathers. Experime ntal manipulation of the length of the outermost tail feathers has dem onstrated sexual selection advantages of tail elongation and disadvant ages of tail shortening, with opposite effects for natural selection i n terms of foraging efficiency, haematocrit and survival. These findin gs are contrary to the prediction of a general deterioration from both shortening and elongation, if the tail trait was determined solely by its effects on aerodynamic efficiency and flight manoeuvrability. Pat terns of sexual selection in manipulated birds conform with patterns i n unmanipulated birds, and selection differentials for different compo nents of sexual selection in manipulated birds are strongly positively correlated with differentials in unmanipulated birds. Age and sex dif ferences in tail length, and geographical patterns of sexual size dimo rphism, are also consistent with sexual selection theory, but inconsis tent with a purely natural selection advantage of long outermost tail feathers in male barn swallows.