Off the coast of western Antarctica, a tiny archipelago called the Danger Islands has been home to more than 1.5 million Adélie penguins that were hiding in plain sight.

The islands were long known to have penguins, but how many remained a mystery until satellite images and an on-the-ground survey revealed the colonies’ immense sizes.

The new counts include the third and fourth largest colonies of Adélie penguins in the world; in all, they increase the region’s known penguin abundance by nearly 70 percent.

The findings, published in Scientific Reports on Friday, are helping inform plans for conserving Antarctica’s waters, some of which have seen alarming declines of Adélie penguins. They also highlight how researchers can combine in-person and satellite data as never before.

VIDEO: A penguin mega-colony has been discovered off the western coast of Antarctica.

“The combination of several different image datasets is what made this paper stand out for me,” says Luba Reshitnyk, a marine geographer at the Hakai Institute who wasn’t involved with the study. “Mapping penguins (or more accurately, their habitat) from space is very exciting!”

Changing Climate

Of the Antarctic Peninsula’s three types of penguins—Adélies, chinstraps, and gentoos—Adélies are the only solely Antarctic species, and they require Antarctic conditions.

But along the Antarctica Peninsula’s western side, seas have gotten hotter in the past 40 years, and winter air temperatures there have increased about nine degrees Fahrenheit. Sea ice-free seasons have lengthened by up to three months; nearly 600 of the region’s 674 glaciers are in retreat.

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The shifting conditions have altered the area’s food web, changing when and where Adélies can eat. The extra warmth also means extra rain, which floods or destroys penguin nests, drowns eggs, and causes chicks to literally freeze to death. The result: nearly every Adélie colony along the western peninsula is in decline.

On the eastern Antarctica Peninsula and the Danger Islands, however, Adélies thrive. Here, winds push ice up and around the tip of the peninsula, and a slowly churning vortex of seawater pins it against land. As a result, sea ice lasts far longer, making the area friendlier for Adélies.

The Danger Islands were known to have penguins, but getting to the islands through ice-choked waters—the islands’ namesake—made studying them difficult. Enter satellites, which ecologists have increasingly used to map Antarctic life.

“It’s like everybody’s been studying Antarctica under their own street lamps,” says study co-author Heather Lynch, a biostatistician at Stony Brook University. “Satellites are like turning on the lights.”

The number-one way to find penguins from the air? Look for their number-two. Mega-colonies of penguins are so large that they leave prodigious amounts of guano—feces—in their wake. These poop-stained landscapes look distinctive from above, so satellites in orbit, such as NASA’s LANDSAT fleet, can spot them.

In 2014, Lynch and her colleague Matthew Schwaller published an algorithm that could analyze satellite photos of Antarctica and flag areas as guano-smeared. The Danger Islands—a nine-island archipelago—appeared as a hotspot.

Algorithms could only do so much, however. In LANDSAT images, a single pixel is about a hundred feet wide, far too coarse to spot individual penguins. “It’s like saying I saw smoke there, but is there really a fire?” says study co-author Hanumant Singh, a Northeastern University roboticist.

To get a better look, Lynch’s team ventured to the islands in December 2015, counting penguin rookeries from the ground and with an off-the-shelf aerial drone. The team then made massive photomosaics of some of the islands and programmed a neural network to count their nesting sites. Checking the algorithm’s work also required some researchers to count by hand.

“Biologists are the most patient people I know of,” says Singh.

Informing Protections

Already, the findings are helping shape the strategy of CCAMLR (pronounced camel-arr), the international commission charged with conserving Antarctica’s marine life.

Antarctic Argentine Institute scientist Mercedes Santos, who helps CCAMLR design marine protected areas (MPAs), says that the discovery is strengthening the case for a proposed MPA encompassing the western Antarctic Peninsula and the southern Scotia Arc.

In a statement, Santos said that the proposed MPA includes an 18.6-mile (30-kilometer) buffer around penguin colonies, including the ones on the Danger Islands, to protect the birds’ feeding grounds as they reproduce.

“The Danger Islands are so small, they don’t even appear on the planning map that CCAMLR’s using,” Lynch adds. “It’s insane how many penguins there are in such a small area.”

Next, Lynch’s team will work out how these massive colonies eke out a living—an added benefit of putting boots on the ground. Tissue samples collected on the 2015 expedition should reveal whether the penguins eat krill or fish, and soil samples will reveal how long penguins have lived on the island.

“We can discover areas in satellite imagery … but at the end of the day, we’re going to get the best data on the ground,” says Lynch. “We can’t just hang up our boots and do it all in space.”