Sex-specific nestling body mass in relation to brood sex composition in the Eurasian Sparrowhawk Accipiter nisus

The trade-off between brood size and offspring quality, as predicted by life history theory, has been extensively studied in birds. However, in sexually size-dimorphic birds, where the larger sex requires more investment from the parents, the potential additional trade-off between brood sex ratio and offspring quality has received less attention. Effects of brood sex composition on nestling fitness can have implications for optimal sex allocation strategies. A harmful effect of a higher proportion of the larger sex on either one of the sexes predicts selection towards parents producing the less harmful, smaller sex. In addition to Fisherian equal allocation this should lead to considerable sex ratio biases in extremely sexually size-dimorphic birds. We tested this in the Eurasian Sparrowhawk Accipiter nisus, where female nestlings require about 40% more parental investment than male nestlings. Sex-specific nestling body mass, calculated as the relative difference with the average body mass, corrected for age, for both sexes, was used as an indicator of nestling quality. With a sample size of 120 broods, containing 483 nestlings, we found that individual nestlings were of lower body mass when they hatched in larger and female dominated broods. This effect was particularly pronounced in female nestlings. Under these conditions, sex ratio theory predicts a male bias in the population. However, this is not supported by empirical evidence from literature.