Gentoo Penguins at Martillo Island Escape Summer Heat by Breeding Earlier

Penguins are made for low temperatures. Layered feathers provide insulation against conditions that would be fatal to almost any other animal.

Their bodies retain warmth extremely effectively, which helps in sub-zero gales but is becoming an increasing burden as the climate warms.

On a small island near Argentina’s southernmost tip, scientists followed a penguin colony over ten summers and saw an apparent contradiction emerge.

Sweltering afternoons were killing chicks, yet the birds had started to behave in a way no one had anticipated.

Penguins pushed towards their limits

Gentoo penguins breed from the Antarctic Peninsula to sub-Antarctic islands.

Their most northerly Argentine colony is on Martillo Island in Tierra del Fuego, close to Ushuaia – a chilly location that is increasingly exposed to warm summer air.

Sabrina Harris, a biologist at Argentina’s Austral Centre for Scientific Research (CADIC), has monitored this colony for several years.

She and her colleagues set out to understand how extreme heat affects a species evolved to cope with precisely the opposite conditions.

Heat-related mass deaths have been recorded from the Arctic to South Africa, while heatwaves have been associated with mortality events in numerous seabird species.

Penguins living at the northern edge of their range are close to the highest temperatures their bodies can tolerate.

A decade of penguin life

In 2013, the researchers installed a time-lapse camera in a hollow tree trunk looking towards the colony. Its wooden casing protected the lens against sunlight, rain and wind.

During daylight hours, the camera captured one image every hour. That single trail camera created a ten-year record.

Each breeding season, between 30 and 60 nests were visible, allowing researchers to follow chicks from hatching through the loss of their down to fledging. Every photograph also provided a temperature reading.

The colony itself prospered. Nest numbers increased by around 12 percent annually, while chick numbers grew at a comparable rate.

By 2023, the camera was recording roughly 60 nests and almost the same number of chicks that survived.

When heat became deadly

21 January 2015 fell midway through a three-day heatwave. By mid-afternoon, the camera reading near the colony had reached about 24°C.

Over those three days, the birds experienced 25 hours at temperatures above 20°C.

The images that followed resembled frames in a flipbook. Within 45 minutes of temperatures passing 24°C, five of the 32 chicks were dead.

Researchers excluded the most obvious causes. The photographs showed no predators, the bodies had no external injuries, and the chicks were a healthy weight for their age.

Overheating was the sole explanation consistent with the evidence.

How penguins respond to heat

Adult penguins have only a limited set of ways to lose heat. They lie down with their flippers and feet spread out to expose bare skin, and they open their bills to pant.

Chicks, which remain covered by insulating down, have fewer ways to cool themselves.

When the team reviewed photographs from each season, a clear pattern appeared. Chicks started panting above 18°C.

At temperatures over 20°C, more than half left the colony – with the camera subsequently recording them beneath nearby bushes or at the beach.

Seeking shade is a behavioural response in its own right. The chicks either worked out, or learnt very quickly, that remaining in place during hot afternoons was hazardous.

Only the chicks present in January 2015 stayed in the colony during the most severe heat.

Gentoo penguins race ahead of summer heat

Over the same ten years, another shift took place on Martillo Island: the birds began breeding earlier.

In 2015, the first eggs were laid on 20 October; by 2022, this had moved to 26 September.

The earlier timing continued throughout the breeding cycle. The final stage before the chicks fledged finished around two days earlier each year, bringing the end date forward by approximately 23 days across the decade.

Climate change itself is almost certainly responsible. Earlier springs and earlier snowmelt signal to the birds that it is time to begin breeding sooner.

What surprised the scientists was the result: an earlier start meant chicks spent fewer hot summer days on land.

Chicks now avoid the hottest days

The figures are notable. During the 2014 season, just four of 42 hot hours occurred outside the chick-rearing period, meaning the chicks experienced nearly all of them. By 2020, they avoided 28 of 44 such hours.

Since 2021, chicks at this stage have spent under 10 hours in the colony at temperatures above 20°C. More than half of every season’s hot hours now occur after the young birds have departed for the sea.

Most climate-driven alterations to biological timing have the reverse effect. Earlier breeding commonly creates a mismatch with prey availability or other ecological signals.

In this case, through a fortunate quirk of timing, the penguins are narrowly winning the race.

A limited climate advantage

The researchers are cautious not to exaggerate the finding. Earlier breeding has provided extra time rather than security.

Should summer temperatures continue to rise, the hot hours that still occur during chick rearing may exceed any additional adjustment the breeding calendar can make.

The situation elsewhere along Argentina’s coast is more troubling. A separate study recorded hundreds of Magellanic penguins dying during a single afternoon heat spike in 2019.

So far, the Martillo gentoo penguins have avoided losses on that scale.

For the first time, researchers have connected an advancing breeding season with lower heat exposure for chicks during their most vulnerable period.

The Martillo chicks are getting away just in time. If extreme summer temperatures grow more intense faster than the breeding calendar can shift, that reprieve will end.