King penguins on Possession Island breed 19 days earlier as ocean temperatures rise

A new study has found that king penguins on a sub-Antarctic island now start breeding 19 days earlier than they did in 2000, with more of their chicks surviving winter.

For now, that shift has given the species a short-term benefit from a warming ocean, although the advantage could still be lost.

King penguins on Possession Island

Researchers studied a colony on Possession Island, a remote island in the southern Indian Ocean between Antarctica and Madagascar, where breeding begins noticeably earlier than it once did.

Using 24 years of records from tagged birds, Gael Bardon of the Monaco Scientific Centre (CSM) linked the earlier start to a substantial increase in chick survival.

By 2023, 62% of chicks survived, compared with 44% in 2000, demonstrating that the change in timing has had meaningful biological effects.

The improvement has not eliminated the underlying threat: it will last only while the penguins can reach the food on which it depends.

Why earlier breeding helps

The timing of life events determines whether a king penguin chick can accumulate enough fat before winter arrives.

Each pair rears a single chick. While the young bird remains on land, its parents make trips to sea and bring back fish.

Starting earlier gives adults a longer period to feed their chick, allowing it to build reserves before a lengthy winter fast consumes its energy.

If breeding starts later, many chicks are smaller when the most demanding months arrive, making starvation a far more likely outcome.

King penguins’ food source

Well south of the colony is the polar front, a zone where warm and cold waters meet and food becomes concentrated.

As the waters mix, nutrients rise towards the surface and feed plankton, which in turn support lanternfish, the penguins’ principal prey during breeding.

Warmer water can benefit these fish, but warming also pushes the productive feeding zone farther south.

That extra distance matters: longer foraging trips cost parents time and energy that could otherwise go towards feeding their chicks.

Signals from seawater

Near the feeding grounds, sea temperature and chlorophyll a levels changed alongside breeding timing. Chlorophyll a is a pigment used to monitor plankton near the surface.

Earlier breeding coincided with lower levels of surface plankton and water close to approximately 4.4°C, probably because lanternfish were more available.

The relationship may initially seem counterintuitive, but plankton left over at the surface can indicate that grazers and fish below are out of sync.

Rather than a straightforward story of warmer water being beneficial, the ocean appears to be driving a chain of delayed responses.

Benefits arrive later

Breeding success reflected ocean conditions from one to two years previously as well as the weather the parents encountered in the current season.

The delay is plausible because penguins catch older fish and squid. The numbers of those animals depend on how successfully their offspring grew and coped with changing seas.

Adults may carry a benefit into the following year too: a bird that feeds well in one year may be in better condition to breed the next.

These lags suggest that a good feeding season today may still be partly determined by ocean conditions in the past.

King penguin hunting styles

King penguins already use different hunting strategies, and that diversity may be giving them more time.

Some travel south to the most productive fronts; others remain closer to the colony and change to prey such as squid.

With no breeding islands located south of Crozet, longer hunting trips may be the only option if the front continues to move.

Such flexibility can cushion the impact, but it cannot remove the threat of the richest waters moving ever farther away.

Why numbers stalled

The increase in surviving king penguin chicks did not automatically make the Possession Island colony larger, as it already appears to be very close to its practical population limit.

When breeding space and local resources become scarce, additional young birds may establish themselves elsewhere rather than crowd into a full colony.

That is plausible for a species distributed across scattered sub-Antarctic islands, where an increase in one location may later become apparent in another.

Population health may therefore be more difficult to assess from one beach than across the wider archipelago.

A rare bright spot

Penguins have broadly been breeding earlier, but this result is distinctive because a change in timing does not benefit every species to the same extent.

Penguin research at Oxford Brookes University helps explain why ecologist Tom Hart found the result unusual.

“This is a rare win,” said Hart, who was not directly involved in the research. Yet even a rare win may not last while the ocean system supporting it continues to change.

King penguins in a warming world

Earlier research had already warned that king penguins can struggle when their fishing grounds move too far from their breeding islands.

A 2008 study linked warmer seas with lower adult survival because parents had to travel farther to find food.

Bardon’s paper points to the same future risk, with the most favourable conditions centred on waters near 4.4°C.

If that sweet spot becomes much warmer or shifts much farther south, the benefit of earlier breeding could quickly vanish.

King penguins have maintained breeding success through changes to their timing, diet and routes that currently fit the warmer Southern Ocean.

The adaptation is real, but its scope is narrow. The coming decades will test whether that flexibility can keep pace with a shifting food web.