Reproductive Effort and Sex-Specific Offspring Performance in the Great Skua (Catharacta skua)

Life history theory predicts the optimisation of traits connected to reproduction in order to maximise lifetime reproductive output. Physiological and ecological trade-offs can constrain the simultaneous enhancement of two or more traits. Parents have to decide how to allocate their resources within and between reproductive events in order to maximise fitness. In a variety of circumstances differently sexed offspring can have different effects on the fitness of parents. Such a situation is likely to occur in species with sexual size dimorphism. The size difference can affect offspring performance, as has been observed in bird and mammal species with larger males. In this thesis I investigate sex-specific offspring cost and performance, reproductive costs of increased egg laying effort and parental response to these two factors in the great skua Catharacta skua. The great skua is a seabird with reversed sexual size dimorphism, that is the females are larger than the males. This important characteristic allows me to investigate different hypotheses that have been proposed in order to explain the vulnerability commonly exhibited by larger males, as I can separate the effect of size per se versus other male-specific traits. A combination of ecological field experiments and physiological measures is used to address these questions. Female great skua chicks were found to grow to a larger final size than males and needed more total energy to reach that size, as established with the doubly labeled water method. Daughters exhibited higher mortality than sons, which was exacerbated when hatching at very low mass. Poorer hatching condition slowed down female development more than male development in chicks which fledged successfully. Hatching sex ratio of control broods was not statistically different from a 50:50 ratio. Egg removal was used to experimentally increase egg production effort. Producing additional eggs negatively affected female condition, as measured by body mass index, pectoral muscle size and haematological parameters. Egg size declined in extended sequences and mothers produced a male-biased sex ratio in later eggs. Some signs of poor female body condition could still be detected one year after the experiment. In the post-experimental year breeding was significantly delayed, but this delay was recovered two years later. Provision of supplementary food slightly affected egg composition in extended sequences, and supplemented females did not bias the sex ratio. My results indicate that larger size is the main reason for sex-specific offspring vulnerability in size-dimorphic species, even though the disadvantage might be conferred at a time before size differences become apparent. Further, my results give strong evidence for facultative sex ratio adjustment, and the observed sex ratio is in the direction predicted for adaptive adjustment, based on higher vulnerability of daughters