FEMALE PREFERENCE FOR APPARENTLY SYMMETRICAL MALE SEXUAL ORNAMENTS INTHE BARN SWALLOW HIRUNDO-RUSTICA

The outermost tail feathers of barn swallows Hirundo rustica apparentl y reliably signal the quality of males, because individuals with the l ongest tails have the lowest degree of fluctuating asymmetry (random d eviations from symmetry in the otherwise symmetrical tail trait) despi te the size of their secondary sexual character. I experimentally test ed whether females preferred males with symmetrical tails without alte ring the aerodynamic properties of birds by painting the tips of the o utermost tail feathers with white or black correction fluid. Unmated m ales were randomly assigned to one of four treatments: (i) asymmetrica l tails, where the outermost 20 mm of one tail feather was painted whi te and the other black; (ii) symmetrical tails where the outermost 1 0 mm of both tail feathers was painted white (symmetric I); (iii) symme trical tails where the outermost 20 mm of both tail feathers was paint ed white (symmetric II); or (iv) controls where the outermost 20 mm of both tail feathers was painted black. The experimental treatment affe cted the duration of the premating period since it took longer for asy mmetrical males to acquire a mate than for either group of symmetrical males or control males. This gave rise to a delayed start of laying a mong males with apparently asymmetrical tails. The seasonal production of fledglings therefore decreased from control males through males wi th either symmetrical treatment to males with the asymmetrical treatme nt. Females therefore pay direct attention to the level of fluctuating asymmetry in secondary sexual characters even when the asymmetry does not affect the aerodynamic properties of males.