What can penguin poop tell us about how climate change is affecting the Antarctic ecosystem? A lot, according to scientists who used decades of NASA satellite images to analyze changes in the diets of Antarctic Adélie penguins.

The birds’ guano color changes depending on what they eat, and that color tells the researchers that penguins living in areas with more sea ice consume more fish, while those living in areas with less sea ice consume more krill.

The findings, published in Current Biology, analyze penguin colonies across nearly the species’ entire range between 1984 and 2013.

Penguin chicks that eat fish grow larger; populations that consume more krill experience long-term population declines, scientists find.

Researchers say the findings suggest that shrinking sea ice is altering the Antarctic food web by reducing the availability of fish and forcing penguins to depend on krill, which is also becoming less abundant in some areas. Because Adélie penguins are widespread across Antarctica, scientists say they provide an important indicator of broader ecosystem changes driven by climate warming.

“Antarctica has experienced rapid environmental change in recent decades, and this approach gives us a new and powerful tool to learn how it has affected penguins,” says co-author Michael Polito, a professor of ocean sciences at the University of California-Santa Cruz.

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