How do you spot a penguin from space? Look for its poop. Scientists have discovered 10 emperor penguin colonies along the Antarctic coast by scanning satellite photos for the reddish-brown stains of their guano. The approach--apparently the first time anyone has used satellite images to locate the breeding populations of an animal--could prove to be a valuable new way to monitor how penguins are responding to climate change.
Emperor penguins grow up to 1.2 meters tall, but that doesn't make them easy to find. They breed on sea ice during the dark, unbearably cold Antarctic winter. By the time scientists start showing up for the summer field season in December, the adults are mostly gone, the chicks are fledging, and the sea ice on which they breed is breaking up. Most of the known colonies have been found when people--usually geologists doing something unrelated, such as conducting aerial surveys--stumble across them.
Penguin biologist Phil Trathan and cartographer Peter Fretwell, both of the British Antarctic Survey, wondered if it was possible to do better by tracking the penguins from space. The birds themselves don't show up in satellite pictures; their black-and-white bodies are too similar to the white ice with black shadows. Not so with guano. "The poo just sort of stands out at you," says Trathan. Emperors are the only penguins that breed on the sea ice, so he knows who's doing the pooping.
Fretwell scanned the coastal sections of the Landsat Image Mosaic of Antarctica, a conglomeration of hundreds of satellite images of the continent taken between 1999 and 2004. "It sends your eyes a bit strange after a couple of days," says Fretwell. He also performed spectral analysis, using a computer to find the areas that are more reddish, like guano, than bluish, like ice. Then the eye can tell if the reddish area is a smear of guano or, say, an island.
Of the 34 previously known colonies, six had disappeared; the ice in those places was squeaky clean. The researchers also found 10 unknown colonies, represented by washes of guano, they report online this week in Global Ecology and Biogeography. "The big question is, why are some of the colonies missing?" says Trathan. He suspects climate change; the lost colonies were generally farther north, in areas with warmer temperatures. That jibes with previous research (ScienceNOW, 26 January), which has suggested that climate change could be a serious problem for emperors: The birds have to stick to the coast, so they can only move so far south as the temperature increases.
The satellite pictures have 15-meter resolution, which is good enough to find the poop stains but not good enough to count beaks. The researchers' next step is to assemble higher-resolution images and aerial photographs that will let them come up with better population estimates for the birds.
"I think it's a really clever idea," says Gerald Kooyman, a marine biologist at the Scripps Institution of Oceanography in San Diego, California, who has been trying to get better counts of emperor penguins for years. He says he's surprised, however, that guano shows up so much in the satellite photos. Compared with other penguins, says Kooyman, emperor penguins are "pretty tidy birds."