Mystery of ‘massive decline’ in colony that once covered the hills of an island off Antarctica.

The world’s second-largest penguin colony has collapsed in just a few decades, falling from half a million breeding pairs in the 1980s to just tens of thousands in 2017.

Breeding colonies of king penguins (Aptenodytes patagonicus) occupy unvegetated ground on islands in the Southern Ocean, including the remote Ile aux Cochons. Helicopter and satellite images taken in 1982 and 1988 showed that the island hosted some 500,000 breeding pairs — a population that﻿ was second in size only to a colony of chinstrap penguins (Pygoscelis antarctica) in the South Sandwich Islands.

Analysing new helicopter and satellite images from 2015 and 2017, Henri Weimerskirch at the French National Center for Scientific Research in Villiers en Bois and his colleagues found that vegetation has overrun much of the penguins’ breeding ground. The team estimates that the colony’s numbers have dropped by 88% in 35 years, to 60,000 breeding pairs.

Nearly one-third of the world’s king penguins have disappeared in the collapse. The researchers have no conclusive explanation for the colony’s decline, which may be ongoing.