The function of long tails in female barn swallows (Hirundo rustica): An experimental study

The outermost tail feathers in male barn swallows (Hirundo rustica) are the target of a strong directional female mate preference. The tail ornament is also expressed in females, since females have considerably longer tails than juveniles, either due to (1) a strong genetic correlation between the characters in the mio sexes, or (2) direct sexual selection on females. To discriminate between these two hypotheses, we manipulated the length of the outermost tail feathers in female barn swallows shortly after arrival by either shortening or elongating the outermost tail feathers, or maintaining their length among control individuals. Start of laying of the first clutch, reproductive performance, or provisioning of offspring did not show any significant differences among treatments. Original female tail length before manipulation was unrelated to reproductive performance, while male tail length explained some variation in the number of clutches and, to some extent, the total number of eggs laid per year. Females with longer tails arrived earlier at the breeding grounds. Manipulated female tail length was positively correlated to the tail length of their mates. Our results support the correlated response hypothesis but do not support the sexual selection explanation for the existence of exaggerated tail feathers in female barn swallows.