The interactive effects of age and food supply on the breeding ecology of great skuas.

Age-specific variation in the reproductive performance of great skuas in Shetland, U.K., was investigated during a period of fluctuating sandeel availability between 1988 and 1993. Increased sandeel abundance was associated with earlier laying dates, increased clutch volumes and improved fledging success. Sandeel abundance had no effects on clutch size and hatching success. Parental age improved haying date and clutch size in a nonlinear manner. Laying date became earlier in the youngest age classes, with the effect being negligible in older birds. Clutch size increased with age up to 18 and then declined in older birds. Clutch volume increased with age and the probability of nest predation declined with age. There were no significant interactive effects of age and year on laying date nor clutch volume, indicating that birds of all ages benefited equally from increases in sandeel abundance. This suggests that lack of breeding experience rather than deficient foraging skills are responsible for young birds laying later. There was a significant interactive effect of year and age on fledging success that was associated with changes in sandeel availability. Success was uniformly low for birds of all ages in 1988-90 when sandeel abundance was low. Success increased with age in 1991 and 1992 during which time food supply improved. During 1993 food supply was abundant and fledging success was high in all age classes. Age-specific improvements in laying date, clutch size and nest predation were best explained by learning of skills associated with experience of breeding. Improvements in fledging success with age were probably best explained by the learning of skills associated with foraging.