Adelie penguins – the most common species of penguin in Antarctica – are happier when there is less sea ice, researchers say.

The animals enjoy more favourable foraging conditions during unusual ice-free conditions, a new study suggests.

In ice-free conditions, the flightless birds are able to travel more by swimming, than by walking, and access food more easily.

“For penguins, swimming is a whopping four times faster than walking,” lead researcher Yuuki Watanabe at the National Institute of Polar Research said.

“They may be sleek in the water but are pretty slow waddlers overland.”

In recent decades, Antarctica has experienced a steady increase in the extent of its sea ice, even as the Arctic has suffered through a marked decrease, researchers say.

But this is not expected to last much longer as the climate changes, with Antarctica also projected to see a decline in its sea ice.

Polar biologists have known for some time that Adelie penguins tend to see population increases during years of sparse sea ice and suffer massive breeding failures in the years with the greatest growth of sea ice.

But until now, researchers did not really know why.

Researchers with Japan’s National Institute of Polar Research electronically tagged 175 penguins with GPS devices, accelerometers and video cameras across four seasons with different sea ice conditions.

This allowed them to track penguins on their trips, categorise walking, swimming and resting behaviour, and estimate the amount of prey captured during dives.

Dr Watanabe said: “It turns out that these penguins are happier with less sea ice.

“This may seem counter-intuitive, but the underlying mechanism is actually quite simple.”

When there is heavy sea ice, the penguins have to walk – and sometimes toboggan – a long way to find cracks in the ice in order to access the waters where they hunt, taking sometimes quite lengthy rests along the way.

But when there is less sea ice, the birds can dive anywhere they want, often just entering the water right by their nests.

Scientists say this is more energy and time efficient and it expands their foraging range.

It is also likely to reduce competition with other penguins for prey and allows them to catch more krill – the penguin’s main prey.

Less sea ice also means more sunlight entering the water, leading to larger blooms of the plankton that the krill feed on.

But this only applies to the penguins that live on the main “continental” part of Antarctica.

The opposite happens to the penguins that live on the thin Antarctic peninsula that sticks out from the continent or live on its islands.

The study published in Science Advances covered the breeding seasons in 2010/11, 2011/12, 2012/13 and 2016/17.

The researchers found that the penguins may have expended an average of 15% to 33% less energy per trip compared with ice-covered seasons, putting that saved energy into growth and reproduction.