Nest building is a sexually selected behaviour in the barn swallow

Females may use male nest building to assess male parental quality, and nes t size would then be a sexually selected trait. In the barn swallow, Hirund o rustica, females select their partner by his tail length, a character bel ieved to signal good genes. Both sexes participate in nest building, althou gh male participation is negatively related to his attractiveness as reflec ted by tail length. We tested the hypothesis that nest building is a sexual ly selected trait: females paired with males of high parental quality (as s hown by the male during nest building) may obtain a mate providing large am ounts of parental investment, while, as has been shown previously, females mated to attractive (long-tailed) males will acquire mates with good geneti c quality. Therefore, since nest building in barn swallows occurs after mat ing, we predicted a postmating sexual selection process by which the female invests differentially in reproduction depending on the male's nest-buildi ng effort (reflecting his willingness to invest in reproduction). The volum e of material in a nest was related to the male's contribution to nest buil ding and, in agreement with our hypothesis, in a multiple regression analys is, male tail length and nest material volume were negatively related to la ying date and positively to female investment in reproduction (total number of eggs laid during the breeding season). Moreover, females paired with lo ng-tailed males (which contribute very little to nest building), but using the same amount of nest material as females paired with short-tailed males, reduced the thickness of the nest and hence increased its capacity. Theref ore, in the barn swallow two different traits appear to be sexually selecte d: tail length of males owing to the good genes process and nest-building a bility owing to the good parent process. (C) 1998 The Association for the S tudy of Animal Behaviour.