ASPECTS OF McCORMICK SKUA BREEDING BIOLOGY

The breeding behaviour of the McCormick Skua was studied at Cape Royds, Antarctica, for three summers. These observations followed the work of Young (1963a, b). Records of banded birds suggest that at the age of five to six years birds return to breed in the area where they were reared. Breeding pairs retain the same territory and mate over consecutive summers. The male will retain a territory and remate if the female does not return, while there is some evidence to suggest that if the male is absent the female will leave the territory and remate elsewhere. Unfavourable weather frequently forced incubating birds to desert their eggs and as a result some pairs laid three clutches (at the most two clutches of two eggs and a clutch of one). The death of sibling chicks from fighting was examined by comparing the early growth of 11 successful pairs of chicks with that of 12 pairs in which one of the siblings died as a result of fighting. From a comparison of weights at hatching and the interval between the hatching of members of chick pairs, it appears that the difference in weight between twins and the gain in weight by one chick in the first few days is critical for the survival of the twins. The breeding success, calculated from the total eggs laid and the total number of chicks fledging, was 40%, 42% and 17% for the three respective summers. Storms (periods of wind with snow) were the main factor influencing breeding success.