Penguin colonies are so large that their masses of excrement can be seen from space, offering scientists a useful means of studying the Antarctic birds. An analysis of 30 years of satellite images of penguin guano shows how the birds' diets are shifting in response to warming.
Penguin droppings not only amass in huge volumes, but also stand out clearly against the white snow blanketing most of Antarctica. In 2024, scientists discovered four new colonies of emperor penguins by scanning satellite imagery for the signature brown streak left by their excrement.
For the new study, scientists analyzed subtle shifts in the color of Adélie penguin guano to track changes in their diet. First, they collected samples of guano and analyzed their color and contents, which allowed them to infer what penguins were eating by studying satellite imagery. Diets rich in krill — tiny, shrimplike crustaceans — lent guano a pink color.
Scanning roughly 4,000 images gathered between 1984 and 2013, scientists found that when sea ice is abundant, penguins eat more fish, and when sea ice is scarce, they rely more on krill.
With warming, sea ice is declining, hitting its lowest mark ever in 2023. And that is a problem for penguin chicks, which are typically heavier when fed fish than when fed krill, and are more likely to survive until adulthood. The new study, published in Current Biology, finds that in places where penguins are eating fewer fish, their numbers are dropping.
"Adélie penguins are an iconic species breeding all around the continent of Antarctica," said coauthor Michael Polito, an ocean scientist at the University of California, Santa Cruz. "They act as a 'canary in the coal mine,' and our study illustrates how recent warming has disrupted the Antarctic marine food web they rely on to the detriment of many of their populations."
Warming is not the only challenge facing penguins. At the same time that the loss of sea ice is cutting off access to fish, industrial supertrawlers are vacuuming up thousands of tons of Antarctic krill each year to feed a global market for dietary supplements and animal feed.
Scientists say both the retreat of sea ice and the loss of krill are driving declines of emperor penguins, which were listed as endangered by the International Union for Conservation of Nature earlier this year. A recent study found that krill fishery managers are not accounting for the compounding effects that krill fishing and warming are having on penguins.
Supertrawlers Are Taking Antarctic Krill That Whales Depend On