AGE-SPECIFIC BREEDING PERFORMANCE AND REPRODUCTIVE EFFORT IN GREAT SKUAS CATHARACTA-SKUA

(1) During a period of reduced food supply, egg-laying dates of great skuas Catharacta skua Brunnich on Foula, Shetland advanced and hatching success increased with age, although breeding success increased with age in only one of the two years studied. Clutch volume increased with age up to 14 years and decreased with age above this, and could be described by a quadratic equation. (2) There was no relationship between age and territorial attendance during incubation, but time spent foraging to provide food for chicks increased with age in 1989, while aggression increased with age in both years studied. These are both evidence of an increase in reproductive effort with age. (3) Older birds may have expended greater reproductive effort simply because they were better able to do so without incurring higher mortality or lower future breeding success. However, return rate between years declined with age for males, which could be a consequence of increased reproductive investment by older birds.