Huddles, vital for male emperors to successfully carry an egg through extreme midwinter cold, are being formed now at colonies around Antarctica.

Scientists have discovered how these highly social birds share the benefit of such thousands-strong assemblies, packed in at a density of up to 12 birds per square metre.

Rather than jamming up in an unco-ordinated mass, they gently push their neighbour in a travelling wave, said the Australian Antarctic Division biologist, Barbara Wienecke.

''It's not something you can detect by watching with the naked eye, so it has never been observed before,'' Dr Wienecke said yesterday.

Time-lapse photography by German physicist Daniel Zitterbart, published with Dr Wienecke in the science journal PLoS One, shows that emperors entering a huddle form groups that push together.

Everyone moves forward in small steps every half minute or so, and gradually the wave travels through the whole huddle.

Eventually each bird will be pushed out to the coldest edge of the huddle where it is most exposed to wind. ''Then they shuffle around from the windward side to the leeward side, and enter the huddle again,'' Dr Wienecke said.

Larger huddles, such as at the 11,000-pair Auster Rookery near to Australia's Mawson Station, will give them longer relief from the cold.

A male emperor has to survive howling winds and temperatures of minus 45 degrees carrying an egg on his feet for about 110 days before the female returns from the sea to take over breeding duties.

After 18 years work on them herself, Dr Wienecke said: ''We are still trying to understand how these stunning birds achieve such an extraordinary energy saving to survive, and this is one way.''

It's also still unclear why emperors differ from people, where a travelling wave can be seen not only in celebration but during an escape panic.

''Why these waves are unco-ordinated, turbulent and dangerous in a human crowd but not in a penguin huddle remains an open question,'' the paper said.