Deviation from an equal sex ratio has been observed in several avian species and there is now descriptive as well as experimental evidence that females are able to modify the sex of their offspring within a brood. Less attention has been paid to consequences of sex ratio manipulation and sex specific differences e.g. in competitive disparities of male and female siblings and parent tactics to compensate for such competitive differences. In this study we examine differences in chick development in relation to sex and hatching order in the socially monogamous bearded tit Panurus biarmicus. Our results revealed that significantly more female than male nestlings were the biggest in the brood. Allometric measurements seem to be a good predictor of age (hatching order). This consequently means that female nestlings frequently hatch earlier and have a higher initial body mass. In contrast male nestlings seem to develop much faster than female nestlings. By modifying the hatching order in favour of female nestlings, mothers seem to promote daughters to compensate for their slower growth.