The last time scientists visited the remote French island of Île aux Cochons in the Indian Ocean, they found more than 2 million king penguins, including 500,000 breeding pairs.
That was 1982, and it gave the island the distinction of being the largest such colony in the world. But in the first comprehensive new count since then, researchers report in Antarctic Science that only about 200,000 king penguins now inhabit the island.
Of those, about 60,000 were in pairs, reports the BBC. The new numbers are based on aerial photos taken in 2015 and 2017 of the island, which sits between Antarctica and the tip of Africa, per the Guardian.
For now, researchers have no clear answer on what explains the population plunge, though followup field studies could shed light. “It is completely unexpected, and particularly significant since this colony represented nearly one third of the king penguins in the world,” says lead author Henri Weimerskirch of the Centre for Biological Studies in Chize, France.
One theory revolves around the weather: A particularly strong El Niño in the late 1990s warmed regional waters and might have pushed penguin prey such as fish and squid out of foraging range long enough to have devastating consequences on such a dense population, according to a news release.
Those effects could have been amplified against the larger backdrop of climate change in general, notes the Guardian. Other theories include diseases such as avian cholera, or the arrival of invasive species such as rats or mice.
(On the flip side, researchers found a previously unknown colony of a different type of penguin.)
This article originally appeared on Newser: Biggest King Penguin Colony Sees a Catastrophic Drop