THE INFLUENCE OF HATCHING DATE ON DIFFERENT LIFE-HISTORY STAGES OF GREAT SKUAS CATHARACTA-SKUA

We investigated the effect of hatching date on chick body condition, c hick survival, post-fledging survival and eventual breeding performanc e as an adult (laying date and clutch volume) of Great Skuas in Foula (Shetland). In 10 out of 16 years there was a seasonal decline in chic k body condition. In the remaining years, there was no detectable effe ct of hatching date on chick condition. Differences among years were s tatistically significant. Chicks in early broods survived better than late-hatched chicks in four years, and no trend could be detected in o ne year. Annual variation in the seasonal pattern of chick growth and survival, and of adult territorial attendance, suggests that environme ntal conditions play a role in determining the seasonal decline in per formance. Post-fledging survival in relation to hatching date was stud ied by analysis of ringing recoveries. Hatching date affected post-fle dging survival, but only in the first year of life. Breeding parameter s of birds of known hatching date were studied when they came back to breed in Foula. Hatching dale of an individual did not affect its layi ng date or volume of the clutches laid later in life. This suggests th at laying dale has a low heritability, or none at all. Birds that laid early in the season produced more future recruits than later birds. A fter an early breeding attempt, skuas had a higher overwinter survival probability than after a late season. These results suggest that layi ng date is an indicator of individual quality in skuas, in the sense t hat early birds apparently are more competent at gathering resources f or reproduction and survival.