WASHINGTON, D.C. — Want to know what an Adélie penguin is eating? The best way to find out is to make one regurgitate its meal. This is about as pleasant for a bird and researcher as you might think. It’s also invasive, time-consuming and costly. So scientists have been looking for other ways to determine what these birds eat. And now they have one. It relies on images taken by satellites.
Landsat cameras have been taking images of the Earth from space since the 1970s. The photos don’t reveal individual penguins, let alone what they dine on underwater. What the images do show, though, is poop. Lots of it left behind by Adélie penguins throughout their breeding season.
Scientists have found that poop to be useful. The penguins cluster together at a predictable rate. That has led researchers to figure out how to count penguin colonies just from their huge poop stains. Last year, for instance, scientists reported using feces to find a supercolony of 1.5 million Adélie penguins. They were on the Danger Islands off the coast of the Antarctic Peninsula. That finding came from a group led by Stony Brook University ecologist Heather Lynch.
Figuring out food preferences from those images is a bit harder. But it, too, starts with poop.
An Antarctic journey
Casey Youngflesh is an ecologist at the University of Connecticut in Storrs. Until a few months ago, he was a graduate student in Lynch’s lab. During that time, he made several trips to the Antarctic Peninsula. He and other researchers visited Adélie penguin colonies by boat. They started from either the tip of South America or the Falkland Islands. Then they crossed some of the roughest waters on the high seas by boat. “It can get a little bit hairy sometimes,” Youngflesh says, “especially on the smaller vessels.”
Timing was essential. The birds spend the dark winters following the sea ice. They return to land to raise chicks during the southern summer. Visit too early and the colonies wouldn’t have started to nest. Visit too late and the colonies would be a mess. Large chicks would be running amok and poop would be mixed with mud. “Everything’s a lot cleaner and neater earlier in the season,” Youngflesh explains.
He and the other researchers on these trips gathered lots of data from the penguin colonies they visited. They at times counted birds or checked on how densely the birds packed into a site. Sometimes they gathered poop in little smell-proof bags and brought them back to the ship.
To most people, the poop looks pink. (It also stinks, as you might expect.) This guano gets its color from the carotenoids (Kah-ROT-eh-noyds) in the carapace, or shell, of the krill that the penguins eat. Carotenoids are pigments. They make a carrot orange, a banana yellow and an Adélie penguin’s poop pink. But what else a penguin eats can alter that color. And so those subtle changes in color can indicate the range of what these birds consume.
Back on the ship, Youngflesh would take each sample and make a “poo patty.” Each was “kind of the size of a hamburger patty,” he says. (And, from the picture he supplied, looked a bit like one, too.) He’d run the patty through a spectrometer (Spek-TROM-eh-tur). This device measures a sample’s colors. It even detects wavelengths that the human eye can’t see, such as infrared and ultraviolet.
Then each patty went into a dehydrator so it could be shipped back to the lab. There, Youngflesh would measure its nitrogen-15 levels. (Nitrogen-15 is a form of the element nitrogen.) Those levels correlated with where in the food web the penguin had been eating — higher (fish) or lower (krill).
This poop came from a dozen or so colonies along the Antarctic Peninsula. Youngflesh used math to translate the detailed spectrometer data to the less-detailed data in the Landsat photos. Afterward, each pixel depicting poop on a Landsat image could be linked to the dominant item on the penguin menu: fish or krill.
Adélie penguins in West Antarctica tend to eat more krill. Those in East Antarctica eat more fish. Youngflesh shared his findings December 12, here, at the American Geophysical Union’s fall meeting in Washington, D.C.
Scientists have done diet studies of individual penguin populations. It’s not easy, though, to do that frequently. The new technique will let researchers get a snapshot of what Adélie penguins across Antarctica are eating, year after year, Youngflesh notes. Going back through archived Landsat photos didn’t reveal any big changes over time in what these penguins eat. However, now researchers will be able to monitor that as the region changes. That will let them provide data to Antarctic ecosystem managers.
Youngflesh says researchers might be able to apply this method to other seabirds, “if they’re nesting on the ground and pooping all over the place.” Someone would have to collect more samples, though, to calibrate the satellite data as he did for the Adélie penguins. And if anyone should want finer data about how a penguin’s diet differs from bird to bird or day to day, there aren’t many good substitutes approaching individual birds and getting them to give up their lunch.