In her study, Mia Momberg, a postdoctoral research fellow at the University of Pretoria, highlights the effect of climate change on wandering albatrosses, the world’s largest flying bird. With its massive wingspan of 3.5 metres, these pelagic giants of the sky are oceanic nomads spending most of their 60 life years at sea, only reverting to land for breeding; every two years after they have reached sexual maturity.
They are found in the Southern Ocean and scattered islands within this ocean such as Marion Island, one of the islands of Prince Edward Islands, sub-Antarctic islands situated just over 2000km south of South Africa.
These two islands support about 50% of the entire world’s wandering albatross breeding population, estimated to be around 20,000 mature individuals. Every year scientists from South African universities survey Marion Island to find and record each wandering albatross nest.
The species, listed as vulnerable by the International Union for Conservation of Nature, faces huge risks while in the open ocean, in particular due to bycatch from longline fishing trawlers.
The Study to Understand Environmental Variables
Momberg was part of a 2021 study group that investigated the environmental variables affecting the birds’ choice of nest site on Marion Island. The birds make their nests, a mound of soil and vegetation, on the ground. The study included wind characteristics, vegetation and geological characteristics at nest locations from three breeding seasons.
Elevation turned out to be the most important variable – the albatrosses preferred a low (warmer) site and coastal vegetation. However, these preferences also point to dangers for these birds from climate change. The greatest risk to the availability of nesting sites in future could be devastating to the population.
Variables Influencing Nest Site Selection
Marion Island is of volcanic origin and has a rough terrain. Some areas are covered in sharp rock and others are boggy with extremely wet vegetation, with rain and strong wind on most days. The island is ideal for studying climate change as the Southern Ocean is experiencing some of the largest global changes in climate.
Over the past 50 years, the west coast of the Antarctic Peninsula has been one of the most rapidly warming parts of the planet. This warming is not only restricted to the land but can also be noted in the Southern Ocean. Upper ocean temperatures to the west of the Antarctic Peninsula have increased over 1°C since 1955. Another reason why Marion Island is ideal for scientific studies is that it is relatively undisturbed by humans.
Factors Affecting Breeding Sites
Using GPS coordinated nest data from the entire breeding population on Marion Island, Momberg and her colleagues aimed to determine which factors affected where these birds breed. With more than 1,900 nests and 10,000 randomly generated points where nests are not present, it was found that the following are of high importance:
- Elevation (which on this island is also a proxy for temperature);
- Terrain ruggedness and slope;
- Distance to the coast and underlying geology;
- Vegetation type and
- Wind speed and turbulence.
These variables were ranked according to their influence on the statistical model predicting the likelihood of a nest being present under the conditions found at a certain point.
The most important variable was elevation. The majority of the nests were found close to the coast, where the elevation is lower. These areas are warmer, meaning chicks would be less exposed to very cold temperatures on their open nests. The probability of nests being present also declined with distance from the coast, probably because there are more suitable habitats closer to the coast.
Vegetation type was strongly determined by elevation and distance from the coast. This was an important factor as the birds use vegetation to build their nests. In addition, dead vegetation contributes to soil formation on the island, which is also used in nest construction.
The probability of encountering nests is lower as the terrain ruggedness increases as these birds need a runway of flat space to use for take-off and landing. During incubation, the adults take turns to remain on the nest. Later they will leave the chick for up to 10 days at a time, but continue to feed it for up to 300 days.
Areas with intermediate wind speeds were those most likely to have a nest. At least some wind is needed for flight, but too much wind may cause chicks to blow off the nests or become too cold.
Changing Climate – A Delicate Balance
Changing climates may upset this delicate balance. Human-driven changes will have impacts on temperature, rainfall and wind speeds, which in turn will affect vegetation and the distribution patterns of other species as well and not only that of wandering albatrosses.
By 2003, Marion Island’s temperature had increased by 1.2°C compared to 50 years before. Precipitation had decreased by 25% and cloud cover also decreased, leading to an increase in sunshine hours. The permanent snowline which was present in the 1950’s no longer exists. These changes have continued during the 20 years since their initial documentation and are likely to continue.
Strong vegetation shifts were already documented in the sub-Antarctic years ago. Over 40 years, many species have shifted their ranges to higher elevations where the temperatures remain cooler. Wind speeds have also already increased in the Southern Ocean and are predicted to continue doing so, which may affect the size of areas suitable for nesting.
If nesting sites move to higher elevations on Marion Island, as temperatures warm and some areas become unsuitable due to changes in vegetation or wind speeds, it is likely that the suitable nesting area on the island will shrink considerably.
The study added to what is known about elements affecting nest-site selection in birds, (such as wind, an under explored element), influencing nest-site selection in large oceanic birds. The results could also provide insights that apply to other surface-nesting seabirds.
