Abstract
Sex allocation theory proposes that parents should bias the sex ratio of their offspring if the reproductive value of one sex is greater than that of the other. In the monogamous blue tit (Parus caeruleus), males have a greater variance in reproductive success than females, and high-quality males have higher reproductive success than high-quality females due to extrapair paternity. Consequently, females mating with attractive males are expected to produce broods biased toward sons, as sons benefit more than daughters from inheriting their father's characteristics. Song and plumage color in birds are secondary sexual characters indicating male quality and involved in female choice. We used these male sexual traits in blue tits to investigate adaptive sex ratio manipulation by females. We did not find any relationship between male color ornamentation and brood sex ratio, contrary to previous studies. On the other hand, the length of the strophe bout (i.e., the mean number of strophes per strophe bout) of fathers was positively related with the proportion of sons in their broods. The length of the strophe bout is supposed to reflect male quality in terms of neuromuscular performance. We further showed that sons produced in experimentally enlarged broods had shorter strophe bouts than sons raised in reduced broods. These results are consistent with the hypothesis that females adjust the sex ratio of their broods in response to the phenotype of their mate.