Females may use male nest building to assess male parental quality, and nest size would then be a sexually selected trait. In the barn swallow, Hirundo rustica, females select their partner by his tail length, a character believed to signal good genes. Both sexes participate in nest building, although male participation is negatively related to his attractiveness as reflected by tail length. We tested the hypothesis that nest building is a sexually selected trait: females paired with males of high parental quality (as shown by the male during nest building) may obtain a mate providing large amounts of parental investment, while, as has been shown previously, females mated to attractive (long-tailed) males will acquire mates with good genetic quality. Therefore, since nest building in barn swallows occurs after mating, we predicted a postmating sexual selection process by which the female invests differentially in reproduction depending on the male's nest-building effort (reflecting his willingness to invest in reproduction). The volume of material in a nest was related to the male's contribution to nest building and, in agreement with our hypothesis, in a multiple regression analysis, male tail length and nest material volume were negatively related to laying date and positively to female investment in reproduction (total number of eggs laid during the breeding season). Moreover, females paired with long-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. Therefore, in the barn swallow two different traits appear to be sexually selected: tail length of males owing to the good genes process and nest-building ability owing to the good parent process. (c) 1998 The Association for the Study of Animal Behaviour.