Scientists were collecting peat cores on South Georgia’s Bird Island when they made a surprising discovery – one which offers new insight into how climate change will affect seabird populations. 

Researchers were analysing the geochemistry of the cores, to get an insight into historic wind speeds on the island, when they found layers of ancient bird poo preserved in the peatland.

This bird poo, or guano, now gives them a window into 8,000 years of seabird history, in one of the most important seabird breeding sites on Earth: Bird Island is home to vast colonies of wandering albatrosses, petrels and penguins. 

The island is subject to the Southern Hemisphere westerly winds: powerful bands of wind that circle Antarctica and drive much of the Southern Ocean’s behaviour. Studying their past activity helps to improve predictions of how this might change as our climate warms.

When seabirds nest on the island’s slopes, their guano washes down into the valley peatlands below. Seabirds are top predators, so mercury from the fish they eat accumulates in their bodies and passes into their poo. That mercury becomes trapped in successive layers of peat, creating a chemical record of how many birds were nesting above.

Researchers are then able to analyse the amount and different isotopes of mercury at different depths of peat cores to reconstruct seabird populations across thousands of years. 

Since the first seabird colonies formed, the researchers identified five major phases of population expansion –  all five coincided with periods when the Southern Hemisphere westerly winds were less intense.

This connection between wind strength and seabird abundance has implications for the future. These Southern Hemisphere westerly winds have been intensifying in recent decades, driven by climate change, meaning we could see dramatic declines in seabird numbers on the island in years to come.

Stronger winds result in higher energy use for seabirds: it can reduce the length of feeding trips, the amount of food they provide their chicks and, ultimately, breeding success. Stronger winds also bring with them a greater risk of crash landings, flooded nests and thermal exposure. For species that nest on open ground, powerful gusts can blow chicks, and even adults, off their nests.

Seabirds play an integral role in the ocean’s food web system: penguins, albatrosses and petrels breed in huge numbers across the Southern Ocean; their guano recycles vital nutrients and some estimates suggest these nutrients fuel up to 75% of productivity in ecosystems across the wider global ocean.

Their decline would therefore have a huge impact on fisheries, ocean carbon cycling and the health of ecosystems billions of people depend on worldwide. 

Dr Stephen Roberts, a geologist at British Antarctic Survey, and co-author on the paper, said: “Discovering these mercury signatures in the peat cores was a really unexpected, and pleasant surprise.”

“It was incredible to see this connection between historic winds and seabird populations. As well as telling us about how past changes in the wind influence seabird populations, it really helps our understanding of how populations might change in the future where winds are expected to increase across the Southern Ocean,” he added.

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