POPULATION-DYNAMICS OF BLACK-BROWED AND GRAY-HEADED ALBATROSSES DIOMEDEA-MELANOPHRIS AND D-CHRYSOSTOMA AT BIRD ISLAND, SOUTH GEORGIA

Population dynamics of Black-browed and Grey-headed Albatrosses Diomedea melanophris and D. chrysostoma were studied at Bird Island, South Georgia, for 17 consecutive years (1975-1991). Over this period, almost all the Grey-headed Albatross colonies decreased at an average rate of 1.8% per annum. Although the total Black-browed Albatross population increased (at 0.8% p.a.), 14 of the 23 colonies (including both study colonies) decreased. Black-browed Albatrosses follow an annual breeding cycle: over 80% of birds successful in rearing a chick and 75% of those failing to do so returned to breed the next year, 5-10% of both categories delayed one further year (even when still paired). Grey-headed Albatrosses are essentially biennial: <1% of successful birds bred the next year, 68% returned to breed 2 years later, 11% the next and 5% not until the fourth year. In contrast, over 50% of birds that failed to rear a chick bred the next year, 23% delayed for one further year. Birds which failed after March did not return the next year, whereas 80% of birds which failed during incubation did breed the following year. Grey-headed Albatrosses showed higher (39%) and more consistent breeding success than Black-browed Albatrosses (29%, including 4 years of almost complete breeding failure); hatching success was similar in both species, and fledging success was the main source of variation in Black-browed Albatross productivity. We link this to the dependence of this species on Antarctic krill, a variable resource largely absent in 3 of the 4 years of widespread failure. Modal age of first breeding was 10 years for Black-browed and 12 years for Grey-headed Albatrosses. Overall, survival rate of juvenile Black-browed Albatrosses to recruitment as breeding birds averaged 28% and 14% for 1960s and 1970s cohorts, respectively; for Grey-headed Albatrosses comparable values were 38% and 6%. Studies of immigration and emigration between colonies showed that no breeding birds moved, that philopatry of fledglings was generally high but that recruitment rates need to be increased by 1-3% to allow for juveniles returning to non-natal colonies. Annual survival of adults averaged 93% (s.e. +/- 0.6) and 95% (s.e. +/- 0.8) for Black-browed and Grey-headed Albatrosses, respectively; for sexed birds, rates were similar for Grey-headed Albatrosses but 2% higher for female Black-browed Albatrosses. With our data, lifetime productivity was 30% higher for Black-browed Albatrosses, but this could be offset by a change in adult survival rate of less than 1%. The proximate reason for the population decline in the studied colonies was high juvenile mortality, which has increased since the 1960s. It is likely that this reflects incidental mortality associated with fisheries (particularly entanglement in long-lines for tuna and collision with net-monitor cables of trawlers), but data are few and mainly circumstantial. Many differences between Grey-headed and Black-browed Albatrosses (e.g. the former showing later sexual maturity, less frequent breeding, higher and more consistent survival) probably relate to basic differences between annual and biennial breeding. Some differences (e.g. breeding success) probably relate to differences in diet; others (e.g. in juvenile survival) may reflect different at-sea distributions. Comparisons with other species revealed species and possibly site-specific differences rather than consistent patterns for annual and biennial breeders.