In species with extrapair mating, females may choose a social mate who will contribute to the successful raising of their brood and a sire who can enhance the genetic quality of offspring. Female choice of social mate and genetic sire may thus be independent events, directed toward different types of reproductive benefits. Furthermore, if reproductive benefits derived through mating preferences differ for sons and daughters, a coupling between sex ratio adjustment and mate choice would be favored by selection. In this paper, we examined whether females adjust the primary sex ratio of offspring to the quality of the social male and/or the extrapair sire in a socially monogamous species with frequent extrapair mating, the tree swallow Tachycineta bicolor. Recent evidence suggests that females obtain compatible genes’ benefits through extrapair mating in this species. If genetic quality is more important for male than female fitness and contributes to higher variance in reproductive success among males than females, sex allocation theory would predict a male-biased sex ratio among extrapair offspring. However, we found no indication of sex ratio bias with paternity in mixed paternity broods. Instead, females skewed the sex ratio toward males in broods without extrapair paternity, which probably reflects a higher phenotypic quality of these males. Furthermore, we confirmed earlier findings that females in good condition produce male-biased broods. Thus, our results indicate that female tree swallows adjust the primary sex ratio to the phenotypic quality of their social mate and themselves and not to the genetic sire of their offspring.
Nest box checks and blood samples