Albatrosses and giant petrel monitoring
Four species of albatross breed in large numbers on Bird Island: wandering albatross (Diomedea exulans), black-browed albatross (Thalassarche melanophris), grey-headed albatross (T. chrysostoma) and light-mantled sooty albatross (Phoebetria palpebrata). There is also a single male white-capped albatross (T. steadi) that has bred in a mixed pair with a female black-browed albatross since summer 2007/08. Although all species lay a single egg, are long-lived, usually mate for life, and have fairly wide ranging diets consisting of varying proportions of fish, squid and crustacea, many other aspects of their behaviour and life-styles are surprisingly different. The wandering albatross lays eggs in December, the chick hatches in March, and is raised during the long austral winter. In contrast, the other albatrosses are summer breeders, laying eggs in October and fledging chicks in April–June. The black-browed albatross is an annual breeder, whereas wandering, grey-headed and light-mantled sooty albatrosses are all biennial, fledging a chick at most once every two years; however, a pair that fails early will often try again the following season.
With the exception of the wandering albatross, overall foraging ranges of these species at sea overlap considerably (at least during chick-rearing), but the areas of greatest usage differ. There is consistent targeting of areas with specific oceanographic characteristics, such as fronts, shelf regions, eddies and upwelling: nearby neritic (shelf) waters are exploited by wandering and black-browed albatrosses; the Antarctic Polar Frontal Zone by black-browed and grey-headed albatrosses; the central Scotia Sea and distant shelf-slope and shelf of the southern Scotia Arc by black-browed, grey-headed and light-mantled sooty albatrosses; and the shelf slope and northern subantarctic and subtropical waters by wandering albatross. Migration patterns and nonbreeding distributions are even more variable. All species except the light-mantled sooty albatross are in long-term decline, partly or wholly because of incidental mortality of adults and juveniles in fisheries. Monitoring of different aspects of their life-histories and behaviour – population size, demography, diet and foraging ecology — can therefore tell us about changes in food availability and fishing pressures in different environments.
The two large, surface-nesting petrels, the northern and southern giant petrel (Macronectes halli and M. giganteus) are monitored intensively on Bird Island. Similar in many respects to albatrosses, these lay a single egg, are long-lived and usually mate for life. They are easy to tell apart in the field — northern giant petrels have a red, and southern giant petrels have a green tip to their bill. There are a few mixed pairs, always of a male southern and a female northern giant petrel, and hybrids are very rare (<0.1% of birds). The southern giant petrel also has a white colour form (morph), usually with a few black feathers, which is very rare at South Georgia (<1% of the population).
Southern giant petrels nest on average six weeks later than northern giant petrels. This has major implications for the ability of the large males to exploit the huge amount of carrion available from the recovering Antarctic fur seal population at South Georgia. Timing of breeding therefore seems to have an important effect on population trajectories; despite comparable life-histories and diet, numbers of northern giant petrel are increased rapidly whereas those of southern giant petrel are stable or increasing slowly. Both are subject to incidental mortality in longline fisheries, and are listed under the international Agreement on the Conservation of Albatrosses and Petrels (ACAP).
Population sizes and trends (albatrosses and giant petrels)
Methods used for assessing breeding population size on Bird Island vary with species. Each year, all pairs of wandering albatross, eight colonies of grey-headed albatross, five colonies of black-browed albatross, and well-demarcated study areas of light-mantled sooty albatross, and northern and southern giant petrels are counted. For all species except light-mantled sooty albatross, all breeding pairs on the island are also counted every 10 years. The albatrosses, with the exception of light-mantled sooty albatross, are in decline, whereas northern giant petrel is increasing rapidly probably because of the high availability of Antarctic fur seal carrion.
Timing of breeding and success (albatrosses and giant petrels)
The study colonies of albatrosses and giant petrels are visited daily or on consecutive days to record laying dates from direct observation. For most species, this shows remarkably little variation from year to year. Once pairs have laid, the nests are marked and the frequency of visits drops to at least weekly for the remainder of the season in order to record timing of failure and measure hatching success (chicks hatched/eggs laid) and fledging success (chicks fledged/chicks hatched), which can be combined into overall breeding success (chicks fledged/eggs laid).
Unsurprisingly, breeding success and timing of failure varies with species and year. When there is a lot of late snowfall, hatching success tends to be lower in species that nest relatively early in the season. In some species, including black-browed albatross, annual variability in breeding success is high. This is probably a consequence of changes in the availability of Antarctic krill, which is a key component in their diet. Breeding success appears to have increased in recent years, which could potentially be related to a slight improvement in conditions and/or reduced competition for prey now that the population has declined.
Chick measurements (albatrosses and giant petrels)
With the exception of light-mantled sooty albatrosses which may nest in sites that are inaccessible, a large sample of chicks of the other albatross and of the two giant petrel species are weighed at peak mass and/or close to fledging each year.