Breeding biology and population dynamics of giant petrels Macronectes at South Georgia (Aves: Procellariiformes)

The breeding biology and population dynamics of the two species of giant petrels, Macronectes halli and M. giganteus, were studied at Bird Island, South Georgia during 1978–81. The mean laying date of M. halli was in early October, five to six weeks ahead of M. giganteus. In both species males take the larger share (c. 54%) of incubation. The eggs of the two species are of the same size but in M. halli the egg formed a significantly higher proportion of the female body weight. On hatching M. giganteus chicks are 5% heavier than M. halli chicks but there are no sexual differences in weight. Brooding and guarding in M. halli is 11 days longer than in M giganteus and males undertake 60–68% of the duties during this time. Males of both species take five to six days longer to fledge than females and M. halli chicks have a seven to nine day shorter fledging period. The sex ratio amongst chicks is 1: 1. Chick growth (weight, culmen length and wing length) is summarized. Males reach higher peak and fledging weights than females in both species and have significantly larger bills through most of the fledging period. The increase in wing length was similar in both sexes until near the end of the fledging period. Adult M. halli tend to be shorter-winged, larger-billed and lighter in weight than M. giganteus at Bird Island. Chick mortality was on average c. 10% higher in M. giganteus than in M. halli leading to a lower overall breeding success in the former. Whilst the breeding population of M. giganteus seems to have stayed relatively stable M. halli has increased in numbers over the last 20 years. However 26-9-57–5% of birds colour-ringed in 1978 did not breed in the subsequent two breeding seasons. Mean annual survival of breeding adults was c. 0–90. Breeding commenced between age 6 and 9 years with males possibly starting earlier than females. The increased population of M. halli and the better survival of their chicks is attributed to the considerable increase in breeding Antarctic fur seals Arctocephalus gazella which provide an abundance of carrion for M. halli nestlings during the first 30 days after hatching.