Sex-biased environmental sensitivity: natural and experimental evidence from a bird species with larger females

The larger sex is often more vulnerable, in terms of development and survival, to poor conditions during early life. Differential vulnerability has implications for parental investment strategies such as sex ratio theory. When males are larger, it is not possible to separate the effects of larger size per se and other aspects of the male phenotype on vulnerability. Furthermore, offspring competition might favor the larger sex and thereby mask intrinsic, size-related effects. We studied sex-specific mortality in a bird species with reversed size dimorphism, the great skua Stercorarius skua, under natural and experimentally created poor conditions. Small eggs from extended laying sequences were used to create poor early conditions for the offspring, which were raised as singletons. Daughters had a lower survival in all treatment groups. Survival in natural broods was additionally affected by hatch date and position. Hatch weight was not different for sons and daughters but was lower in experimental than in natural nests. In natural nests, daughters fledged 10% heavier than sons, but in experimental nests, they did not reach a higher mass. The average survival difference between sons and daughters was not increased in experimental broods. However, hatch weight had a strong sex-specific effect. Very light females never survived, and survival probability of daughters increased with increasing hatch weight. By contrast, survival of sons over the same range of hatch weights was not related to weight. These findings support the hypothesis that larger (final) size per se is related to sex-specific offspring vulnerability during early life.