A small group of Adélie penguins return to their colony. A mega-colony of these penguins was hit by a big die-off in early 2024. Ralph Vanstreels hide caption

Ralph Vanstreels

Sailing toward the Danger Islands, Dr. Ralph Vanstreels had a good idea of what he might find: A lot of dead Adélie penguins.

It was March 2024 and a virus had been sweeping through the area, leaving deceased birds in its wake.

Vanstreels got the crime right but the culprit wrong. There were dead penguins but a virus was not to blame.

As his vessel pulled up to a landing ramp on one of the islands — located just off the Antarctic Peninsula — Vanstreels and his fellow researchers immediately noticed bumps in the snow.

"Was each bump a rock or was it a carcass?…