Higher Predators – Bird Island – 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 albatross (Phoebetria palpebrata). Although all species lay a single egg, are long-lived, usually mate for life, and have 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 its egg in December, the chick hatches in March, and is raised during the long austral winter. In contrast, the other albatrosses are summer breeders, laying a single egg in October and fledging chicks in April-June. The black-browed albatross is an annual breeder, whereas wandering, grey-headed and light-mantled 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.
The foraging ranges of the three smaller albatross species show considerable overlap, at least during chick-rearing, but the areas of greatest usage differ. Adults consistently target productive 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 albatrosses; and the shelf slope and northern subantarctic and subtropical waters by wandering albatross.
Migration patterns and nonbreeding distributions are even more variable. Populations of all species at South Georgia except the light-mantled 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.
Petrels
The two large, surface-nesting petrels, the northern and southern giant petrel (Macronectes halli and M. giganteus) are also monitored 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 have increased whereas those of southern giant petrel are stable. 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
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 albatross, and northern and southern giant petrels are counted. For all species except light-mantled albatross, all breeding pairs on the island are also counted every 10 years. Numbers of wandering albatross Loading.., grey-headed albatross Loading.. and black-browed albatross Loading.. are all in decline, whereas northern giant petrel are increasing Loading.. probably because of the high availability of Antarctic fur seal carrion.
Breeding adults and chicks are ringed in the main study areas (for wandering albatross, this is all of Bird Island). Ringing has been carried out for many years, so a large proportion of birds at these sites are of known age. Annual resighting programmes for ringed birds allows the calculation of survival rate and breeding frequency of adults, and of return rates of fledglings, in order to determine the key processes underlying observed changes in population size. In addition, the close monitoring of breeding histories (timing, success etc.) of marked birds allows analyses of the importance of factors that include age, senescence, quality, mate change etc. on individual breeding performance.
Timing of breeding and success
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, timing of breeding 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 or reduced competition for prey now that the population has declined.
Chick measurements
With the exception of light-mantled albatrosses which may nest in sites that are inaccessible, a large sample of chicks of the other albatross species and of the two giant petrel species are weighed at peak mass or close to fledging each year.
All chicks are weighed at a particular age or on a particular date, hence the need to know exact timing of hatching for certain species. Bill dimensions are recorded for wandering albatross and giant petrel chicks close to fledging as this can be used to sex chicks (male bills are much larger than those of females) to examine differences in survival. Weight data from each species provides information on annual changes in food availability. For example, the changes in peak mass from year to year in grey-headed albatrosses should be indicative of the abundance of their two main prey, Antarctic krill and a squid Martialia hyadesi which they catch in the region of the Antarctic Polar Front.