Over decades, several hypotheses for sex allocation have been proposed and tested repeatedly. However, many studies have shown inconsistent results among species, populations, and study years. We investigated sex ratio patterns in a population of Japanese Tit Parus minor, which is closely related to the Great Tit Parus major. In this fouryear study, a total of 1500 offspring from 191 nests were sexed. We found that the proportion of male offspring in each brood was negatively related to laying date and clutch size only in the first clutches. This corresponds to our prediction that in sexually dimorphic Japanese Tits, females may control sex ratio by adjusting clutch size or the timing of laying to reduce costs of rearing. However, this relationship did not appear in later clutches, indicating that manipulation of sex ratios occurred only in part of the season. Parental quality was not related to sex ratio in either first or later clutches. Our results suggest that female Japanese Tits may control the sex ratio to balance reproductive effort and self-maintenance rather than to raise the offspring fitness benefits in relation to the quality of parents, and that they use a different sex allocation strategy between first and later clutches.
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Vol. 50 • No. 2