SEX-RATIO ADJUSTMENT IN RELATION TO PATERNAL ATTRACTIVENESS IN A WILDBIRD POPULATION

When the relative fitness of sons and daughters differs, sex-allocatio n theory predicts that it would be adaptive for individuals to adjust their investment in different sexes of offspring. Sex ratio adjustment by females in response to the sexual attractiveness of their mate wou ld be an example of this. In vertebrates the existence of this form of sex ratio adjustment is controversial and may be confounded with sex- biased mortality, particularly in sexually size-dimorphic species. Her e we use PCR amplification of a conserved W-chromosome-linked gene to show that the sex ratio within broods of a natural population of sexua lly size-monomorphic collared flycatchers Ficedula albicollis is relat ed to the size of their father's forehead patch, a heritable secondary sexual character implicated in female choice. Experimental manipulati ons of paternal investment, which influence the size of this character in future breeding attempts, result in corresponding changes in the s ex ratio of offspring born to males in those breeding attempts. In con trast, manipulations of maternal investment have no effect on future s ex ratios, and there is no relationship between variables predicting t he reproductive value of the brood and nestling sex ratio. Analysis of recruitment of offspring reveals similar patterns of sex ratio bias. The results suggest that female collared flycatchers are able to adjus t the sex ratio of eggs ovulated in response to the phenotype of their mate. This finding is most consistent with ''genetic quality'' models of sexual selection.