EFFECT OF PREDATION BY SKUAS ON BREEDING SUCCESS OF THE CAPE PETREL DAPTION-CAPENSE AT NELSON-ISLAND, ANTARCTICA

Breeding success of Cape petrels at Nelson Island (South Shetland Isla nds) in 1991/1992 averaged 29%. Predation by skuas accounted for 76% o f all nesting failures. Direct attacks prevailed over opportunistic pr edation. Intensity of observer disturbance alone could not account for the inter-colony variation in predation rate, which tended to increas e with colony size, presence of nearby breeding skuas and local availa bility of other prey species (penguins). Predation was significantly l ower on nests that could be attacked only, or more easily, from the ai r and nests with close neighbours. Selective predation on nests in whi ch relatively narrower eggs were laid was independent of nest-site cha racteristics and laying date, suggesting an effect of parental age/exp erience on breeding success through antipredator behaviour. Developmen t of oil-spitting behaviour in growing chicks balanced the parental at tendance and effort in nest defence, which declined as chick age incre ased. Increased predation on lighter but not smaller chicks may reflec t the lower ability of such chicks to defend themselves by spitting st omach oil. Seasonal variations of predation rate on Cape petrel nests did not result from predator swamping by numbers of simultaneously act ive nests, but rather reflected temporal availability of alternative p rey. Success of all phases of nesting declined with later laying date.