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The guano, or poo, of nesting birds has given researchers clues to the history of these sentinel seabirds.
Photo Credit: Angela Gallego-Sala
Scientific Frontline: Extended "At a Glance" Summary: Reconstructing Seabird Populations via Guano-Derived Mercury
The Core Concept: The analysis of mercury isotopes deposited from seabird guano into peatlands serves as a continuous geochemical proxy to reconstruct ancient seabird population dynamics and correlate them with historical climatic shifts over millennia.
Key Distinction/Mechanism: Rather than relying on scarce fossil records or observational data, researchers analyze mercury concentrations trapped in successive layers of peat. Because seabirds are apex marine predators, dietary mercury biomagnifies in their bodies and is excreted in guano, creating a highly accurate, depth-stratified chemical archive of colony density over an 8,000-year timeline.
Origin/History: This proxy method was discovered accidentally by researchers from the Swedish University of Agricultural Sciences, the University of Bern, and the British Antarctic Survey. While collecting peat cores on Bird Island, South Georgia, to analyze historic Southern Hemisphere westerly wind speeds, they identified a continuous 8,000-year mercury record. The data revealed that the first seabird colonies on the island established themselves between 6,800 and 6,100 years ago.
Major Frameworks/Components:
- Geochemical Isotope Profiling: The extraction and measurement of mercury concentrations and specific isotopes at varying depths within continuous peat cores.
- Paleoclimatic Correlation: The alignment of biological data (seabird colony expansion) with climatic data (historic wind speeds), demonstrating that the five major phases of population expansion coincided with periods of weakened Southern Hemisphere westerly winds.
- Trophic Biomagnification Modeling: The utilization of heavy metal accumulation in top-tier marine predators as a reliable indicator of historical biomass.
- Marine Ecosystem Nutrient Cycling: The ecological framework recognizing guano as a primary mechanism for recycling vital marine nutrients that fuel wider ocean productivity.
Branch of Science: Geochemistry, Paleoclimatology, Marine Ecology, and Ornithology.
Future Application: This methodology enables the creation of highly accurate predictive models assessing how modern, climate-driven intensification of atmospheric winds will impact seabird breeding success, foraging energy expenditure, and nest viability. It provides a baseline for forecasting long-term disruptions to global marine food webs and ocean carbon cycling.
Why It Matters: Seabirds function as critical barometers of ocean health and primary drivers of global marine nutrient recycling. By demonstrating that historic seabird populations thrived during periods of low wind intensity, this research highlights the severe threat posed by currently intensifying, climate-driven westerlies. A continued decline in these sentinel populations threatens to destabilize global fisheries, marine ecosystems, and ocean productivity.
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| Collecting peat cores on Bird Island. Photo Credit: Angela Gallego-Sala |
Layers of ancient bird poo preserved in the peatlands of the sub-Antarctic island of Bird Island have given scientists a window into 8,000 years of seabird history. They found that bird numbers rose and fell with changes in climate, offering new clues about how future climate change could impact seabird populations.
A new study published this week in The Proceedings of the National Academy of Sciences, led by the Swedish University of Agricultural Sciences and the University of Bern in collaboration with the British Antarctic Survey, has reconstructed how seabird populations on Bird Island, South Georgia have risen and fallen over millennia in response to natural shifts in climate.
An accidental discovery
Bird Island is one of the most important seabird breeding sites on Earth, home to vast colonies of wandering albatrosses, petrels and penguins. But it was the island’s peatlands – boggy, organic deposits that have been building up for thousands of years – that turned out to hold a remarkable archive of seabird history.
Scientists were collecting peat cores to reconstruct the past wind speeds of the Southern Hemisphere westerly winds – the powerful band of wind that circles Antarctica and drives much of the Southern Ocean’s behavior – to improve predictions of how this might change in the future as our climate continues to warm. While analyzing the geochemistry of the cores, which can be used as a proxy for historic wind speeds, they discovered an unexpected archive of seabird history.
When seabirds nest on the island’s slopes, their guano (or poo) washes down into the valley’s peatlands below. Because seabirds are top predators, 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.
By analyzing the amount and different isotopes of mercury at different depths of peat cores, the researchers were able to reconstruct seabird population changes continuously across thousands of years.
One of the study’s most striking findings emerged from the deeper layers of the peat cores. The record revealed that the first seabird colonies established themselves on Bird Island between 6,800 and 6,100 years ago – more than 1,000 years before other sub-Antarctic islands.
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 an unexpected, and pleasant surprise. And then, 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.”
The wind connection
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. The ‘Westerlies’ have been intensifying in recent decades, driven by climate change. If weaker winds historically allowed seabird populations to thrive, the strengthening of those winds today could contribute to the dramatic declines researchers are seeing in seabird numbers.
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 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.
Breeding success improves when winds are lower. Calmer conditions allow sea-ice to extend further across the Southern Ocean, increasing the amount of prey at the sea-ice edge for seabirds.
A warning for our oceans
Seabird populations have declined by up to 70% since the 1950s, driven by human activity (such as fishing) and climate change. However, the consequences of this extend beyond the fate of individual bird colonies. Seabirds are a barometer of ocean health but also play a key role in sustaining it. 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.
A sustained decline in seabird populations would likely ripple through the entire marine food web with consequences for fisheries, ocean carbon cycling and the health of ecosystems billions of people depend on.
Published in journal: Proceedings of the National Academy of Sciences
Authors: Chuxian Li, Stephen J. Roberts, Martin Grosjean, Adrien Mestrot, Martin Wille, Richard A. Phillips, Maxime Enrico, Kevin Bishop, Ulf Skyllberg, Dmitri Mauquoy, Clemens von Scheffer, Thomas Theurer, David Muirhead, Alex Whittle, Angela Gallego-Sala, Jeroen E. Sonke, François De Vleeschouwer, Nathalie Van der Putten, Pascale Braconnot, Olivier Marti, Stefan Osterwalder, Nina Buchmann, Thomas Frölicher, Eva Anthamatten, Aurea C. Chiaia-Hernández, Petra Zahajská, Catherine Jeandel, Krystyna M. Saunders, Sae Yun Kwon, Dingyong Wang, Richard Bindler, Louise Sime, and Dominic A. Hodgson
Source/Credit: British Antarctic Survey
Reference Number: es041826_01
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