Using a radar satellite to track emperor penguins in winter is changing our understanding of the species

Freely available satellite radar imaging is being used for the first time to help track colonies of endangered emperor penguins in Antarctica throughout the winter months. Typically satellite images need sunlight to be useful, and Antarctica is dark in the winter. This has meant that colonies have been hidden at a critical time in the emperor penguin breeding cycle, when eggs are laid, chicks hatch and adults take part in those iconic huddles. Almost 70 colonies of emperor penguins, many of which have never been visited by humans, occupy the sea ice that is fastened to the coastline surrounding Antarctica. Now using synthetic aperture radar (SAR), an instrument operating on satellites that sends microwave pulses down to Earth, and measures how those pulses return, our research team has been able to track what the penguins do during these months. Unlike other forms of satellite imagery, SAR is not reliant on sunlight, or affected by cloud, and therefore provides radar imagery of Antarctica through the dark polar winter. SAR’s signal is sensitive to the properties of what it interacts with, and has been used for years in the poles to research things like the properties of sea ice and melt on glaciers. When looking at SAR images from the free European Space Agency satellite, Sentinel-1, showing the location of an emperor penguin colony during winter, my team started to notice a bright white splodge of pixels moving around against a backdrop of dark, smooth sea ice. Features like icebergs or rough sea ice can look bright in SAR imagery, but it didn’t make sense for those to be moving around at that time of year, when the sea ice was fixed and stable. Could it be the emperor penguin colony? We soon found the same thing at the locations of other colonies – clusters of bright moving pixels on stable ice. We were then able to obtain optical images from September when the sunlight first starts returning, and compare them with the SAR, and confirmed it was indeed the colonies we were observing. A group of adult and chick emperor penguins at Snow Hill Island, Antarctica. Peter Fretwell, British Antarctic Survey., CC BY Tracking through the whole year This finding opened up an exciting opportunity to track colonies throughout the whole breeding cycle, by combining SAR images in winter with optical images in spring/summer, and all using freely available data that has been routinely collected for years. We went back through eight years, from 2017 to 2024, and followed three of the largest colonies year-round. We were able to follow their movement, sometimes even daily from early in the winter until chicks grow their first feathers in December and January. Emperor penguin colonies in Sentinel-1 SAR imagery during winter. This image shows the Cape Washington colony. European Space Agency., CC BY We observed that colonies were typically less mobile in the winter, before spreading out and moving greater distances in the spring and summer. Interestingly, we found that the Atka Bay colony moved up from the sea ice onto the glacier ice that flows from the continent in seven of the eight years in the early spring. The colony did this despite the sea ice – the typically preferred habitat for the species – being stable. We need more research to understand why this is. Satellite remote sensing has been a crucial tool for observing the emperor penguins in the summer months since 2009. It was discovered then that colonies could be found in satellite images because of the dark guano stains they leave on the snow and ice. Overall, these observations have revolutionised our understanding of the species and have led to the discovery of many more colonies. The Atka Bay colony of emperor penguins in satellite radar imagery during winter. European Space Agency., CC BY After a period of stability, Antarctic sea ice has declined markedly since 2016 and likely entered a new state, characterised by lower extent and increased instability. Earlier this year, emperor penguins were added to the International Union for Conservation of Nature’s red list and classified as endangered, predominantly due to projected declines in sea ice. This makes better understanding and monitoring of emperor penguins more important than ever. The ability to observe colonies throughout the breeding cycle using freely available, routinely collected satellite data creates opportunities to better understand their response to changing environmental conditions.