Different Maternal Quality Aspects Covary With the Offspring Sex Ratio and Condition in Bearded Reedlings ( <i>Panurus biarmicus</i> )

In addition to the intrinsic genetic quality of the parents, the differential allocation of maternal investment in offspring is recognized as a crucial determinant of offspring fitness. Sex allocation theory posits that females should adjust their brood sex ratio in response to the varying benefits associated with increased maternal investment in sons versus daughters. This differential allocation may occur in direct relation to the mother's condition or in relation to the attractiveness or quality of their social mate or extra-pair mates. In this study, we investigate whether the intrinsic quality of the mother, her social or extra-pair mate, influences the offspring sex ratio and offspring residual body mass of the socially monogamous bearded reedling (<i>Panurus biarmicus</i>). Our findings indicate that female intrinsic quality, in terms of tarsus length, is positively correlated with the proportion of sons produced per brood. Additionally, maternal body mass and blood sedimentation rate (a health indicator) were associated with nestling conditions, specifically residual body weight. However, the quality or attractiveness of the social mate did not affect nestling sex ratio or condition in terms of residual body mass. Furthermore, the sex ratio of extra-pair offspring did not deviate from an equal distribution; however, nestlings fathered by extra-pair males were in better condition than their half-siblings fathered by the female's social mate. Our results suggest that in bearded reedlings, offspring sex is related to maternal body size, which reflects intrinsic quality, but not to maternal health or body mass.