Going around the icy mass potentially delayed a colony's adults from feeding its chicks

An emperor penguin colony in Antarctica had a run-in with a grounded iceberg that may have led to a disastrous breeding season.

On Coulman Island, which is home to one of the largest emperor penguin colonies on the planet, surviving chick counts declined by 69 percent in 2025 compared with the previous year. This may have been due to the sneaky shape of an iceberg grounding nearby, delaying the birds’ return from their foraging grounds with food for their young, scientists report July 23 in Communications Earth & Environment.

When emperor penguin (Aptenodytes forsteri) chicks hatch, the parents take turns crossing the sea ice to bring back food from the ocean for their babies. When penguins returned in 2025, they climbed the iceberg until reaching a steep, unsurpassable cliff. By the time they waddled back down and around the berg, their chicks may have starved.

“The shape of the iceberg itself trapped adult penguins as they were returning to the colony from the sea,” says Michael Polito, a marine biologist at University of California, Santa Cruz who was not involved in the work.

Polar researcher Jeong-Hoon Kim and colleagues counted 6,658 live chicks from 2025 aerial photos — 69 percent fewer than the 21,242 observed in 2024. The pictures also revealed something strange. “Large numbers of adult penguins were on and along the grounded iceberg,” Kim says.

Using aerial photos and satellite imagery, the researchers visualized the iceberg and recreated its movements after it split off from a large ice shelf further south. This revealed it grounded just north of Coulman Island in July 2025 and stayed there until early January 2026 — disrupting the penguins’ path for much of the breeding season. The shortest distance between the colony and open water increased from roughly 9.3 kilometers in 2024 to 14.9 kilometers in 2025, says Kim, of the Korea Polar Research Institute in Incheon.

Furthermore, the iceberg had a gentle slope that may have lured unsuspecting penguins up from the sea before facing a 20-meter drop. The 3-D reconstruction of the iceberg’s contours is a unique feature of this study, Polito says.

Both Kim and Polito highlight that, while this one-off event had a concerning impact, the iceberg has now passed by so future breeding seasons could return to normal. However, as climate change causes Antarctic ice to melt, ice shelves may become weaker. This “could increase the frequency in which large icebergs like this break off, drift away and impact penguin colonies,” Polito says.

“It was certainly a sobering experience,” Kim says. “I had always hoped we would never see something like this at one of the colonies we monitor. It is something I will not easily forget.”