Offspring sex ratio is unrelated to male attractiveness in dark-eyed juncos (Junco hyemalis)

Sex allocation theory predicts that parents should bias investment toward t he offspring sex that confers higher relative fitness on the parents. When variance in reproductive success is higher in males than females, and some males are more attractive to females than others, thereby achieving higher reproductive success, female parents mated to attractive males are expected to bias reproductive allocation toward sons. Modification of the primary s ex ratio is one mechanism by which avian parents may bias allocation. In ma te choice trials, captive female dark-eyed juncos (Junco hyemalis) prefer t o associate with males whose circulating testosterone level has been elevat ed experimentally. We asked whether females socially mated to testosterone- treated males in nature might bias the sex ratio of progeny in favor of son s. We determined the primary sex ratio of broods using a sex-linked molecul ar marker, the CHD gene on the W chromosome. We found no relationship betwe en the hormonal treatment of males and the primary or secondary sex ratios of offspring produced by their social mates. Sex ratio was also unrelated t o breeding-season date, study year, male viability, or female age. While un likely, it is possible that female juncos are not able to manipulate the pr imary sex ratio of their broods. More likely, possible benefits to female j uncos of producing attractive sons may be small because (1) attractive male s do not necessarily have higher fitness, depending on the trade-offs invol ved for males, and (2) when such benefits exist, they may be outweighed for females by the costs of compensating for the reduced paternal care of attr active males.