Radar satellites can now watch emperor penguins through the Antarctic night

01The question

Emperor penguins are the only penguins that raise their young in the middle of the Antarctic winter. They gather on fast ice, sea ice that is frozen to the coast, where the fathers keep the eggs warm on their feet for more than two months.

Scientists already track penguin colonies from space by spotting brown stains of penguin droppings (guano) on the white ice in satellite photos. But photos need sunlight, and from mid-April to late August Antarctica is dark. The most important part of the breeding season was therefore invisible from orbit. The team asked whether radar, which doesn't need light, could see the penguins instead.

02What they did

The researchers used Sentinel-1, a free European radar satellite. They examined more than 1,100 radar images of three colonies (Atka Bay, Coulman Island and Cape Washington) covering the winters of 2017 to 2024. When the sun returned each spring, they switched to regular satellite photos.

By marking where each colony was in every image, often several times a week, they built the first year-round movement records for emperor penguin colonies. Coulman Island is the largest colony known, with about 19,800 adults at the last count.

03What they found

Huddled penguins show up in radar as a bright patch on darker, smooth sea ice. The team suspects this is because thousands of birds standing about a meter tall make the surface look rough to the radar. They could tell it was the birds, not just old stains, because the bright patch moved from image to image while the ice around it stayed still.

In winter, colonies barely moved, usually staying within about half a kilometer of where they started. From spring onward they split into many smaller groups (up to 19 at Coulman Island) and roamed, and some groups ended up more than 5.5 km away.

The surprise was where they went. Groups often walked away from the ice edge and sometimes changed direction all at once, even when the ice looked perfectly stable. At Atka Bay, the colony climbed up onto the neighboring ice shelf in seven of the eight years. The authors think changes in wind, rather than the ice, may be steering these moves.

04Why it matters

Emperor penguins depend on sea ice, and Antarctic sea ice has dropped sharply since 2016. Earlier modeling cited in this paper projects that most colonies could become quasi-extinct by 2100 if warming continues unchecked.

Being able to watch colonies all year round, using free data, lets scientists see how penguins respond to winter storms, early ice break-ups or giant icebergs as they happen. That knowledge could help decide where protected zones around colonies should be drawn.