Female swallow preference for symmetrical male sexual ornaments

MANY secondary sexual characters are supposed to have evolved as a response to female choice of the most extravagantly ornamented males1, a hypothesis supported by studies demonstrating female preferences for the most ornamented males2–5. Comparative studies of elaborate feather ornaments in birds have shown that (1) ornaments have larger degrees of fluctuating asymmetry6 (small, random deviations from bilateral symmetry caused by an inability of individuals to cope with environmental and genetic stress during development of a character7) than other morphological traits, and (2) the degree of fluctuating asymmetry is often negatively related to the size of the ornament6. The negative relationship between ornament asymmetry and size suggests that ornament size reliably reflects male quality because the largest secondary sex traits demonstrate the least degree of fluctuating asymmetry. I manipulated tail length and tail asymmetry independently in male swallows (Hirundo rustica) to determine whether ornament size or asymmetry were used as cues in mate choice. Male swallows with elongated, symmetric tails mated earlier, and enjoyed larger annual reproductive success than did males with shortened tails and increased asymmetry. Females therefore prefer large as well as symmetric ornaments, which suggests that females in their mate choice use ornament asymmetry and size as reliable indicators of male quality.