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, chick survival, post-fledging survival and eventual breeding performance 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 chick body condition. In the remaining years, there was no detectable effect of hatching date on chick condition. Differences among years were statistically significant. Chicks in early broods survived better than late-hatched chicks in four years, and no trend could be detected in one year. Annual variation in the seasonal pattern of chick growth and survival, and of adult territorial attendance, suggests that environmental conditions play a role in determining the seasonal decline in performance. Post-fledging survival in relation to hatching date was studied by analysis of ringing recoveries. Hatching date affected post-fledging survival, but only in the first year of life. Breeding parameters 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 laying date or volume of the clutches laid later in life. This suggests that laying dale has a low heritability, or none at all. Birds that laid early in the season produced more future recruits than later birds. After an early breeding attempt, skuas had a higher overwinter survival probability than after a late season. These results suggest that laying date is an indicator of individual quality in skuas, in the sense that early birds apparently are more competent at gathering resources for reproduction and survival.