Casey Youngflesh
The colour of penguin poop visible from space is offering a new window into how Antarctica's ecology and environment are changing over time. Adélie penguins are among the most abundant predators in the Southern Ocean. While they rely entirely on marine food sources, they nest on rocky outcrops surrounding the Antarctic continent.
Because penguin colonies often contain hundreds of thousands of nesting birds, they generate vast quantities of guano. These massive deposits are large enough to be easily identified in satellite imagery captured by NASA Landsat satellites.
A multidisciplinary team of scholars analysed thousands of satellite images dating back to the 1980s to examine these guano patches. The objective was to determine what these birds are eating and how environmental shifts are affecting their diets and long-term survival.
The study revealed a direct link between sea ice availability and dietary habits. When sea ice is abundant near a colony, Adélie penguins tend to rely more on small fish. Conversely, when sea ice diminishes, they depend heavily on krill, which are small shrimp-like crustaceans.
Krill possess a pink exoskeleton that penguins cannot fully digest, imparting a distinct rosy shade to their guano. Researchers used this colour variation to estimate dietary composition through a quantitative approach that links guano shade directly to the proportion of krill consumed.
To establish this link, physical guano samples were collected from several locations. Using a spectrometer inside a makeshift shipboard laboratory, researchers measured the exact colour of each sample as a satellite sensor would register it. Environmental chemistry tools were then used to analyse the contents, while statistical modelling mapped specific guano shades to corresponding diets.
This approach provides a rare glimpse into the broader Antarctic marine food web, which is typically hidden beneath the ocean surface. It marks the first direct effort to monitor animal diets from space.
By examining around 4,000 satellite images, the team tracked how penguin diets varied across space and time. What penguins eat differs across the continent and fluctuates both within a single year and from one year to the next.
The findings highlight how climate change is actively reshaping polar food chains. Since the 2010s, Antarctic sea ice has declined precipitously, with several years setting historical minimums. Less sea ice means fewer fish, which directly threatens penguin survival. Adélie chicks fed on fish are typically heavier and more likely to survive into adulthood than those raised on krill.
In areas where penguins consume fewer fish, colony populations have dropped significantly over the past few decades. Conversely, colonies with fish-rich diets have remained stable or increased. With further sea ice loss projected, Adélie penguins now face growing pressure alongside rising competition for fish and krill from humans and other marine life.