The metal bands, looped tightly around the top of the flipper where it meets the body, have long been used as a low-cost visual aid by researchers to identify individual penguins when they waddle ashore.

Foot tags are not used because of the penguin's anatomical shape.

But, says the new study, the seemingly harmless bands affect the penguin's swimming performance, causing it to waste more energy in foraging for food, sometimes with life-threatening consequences.

Publishing in the journal Nature, French and Norwegian scientists reported that they took 100 king penguins (Aptenodytes patagonicus), selected at random on Possession Island on the Crozet archipelago, a sub-Antarctic group in the southern Indian ocean.

All were tagged with a minute, electronic transponder that was implanted under the skin, which can only be read by using specialist equipment placed close to the bird. Fifty of the 100 birds were additionally given a flipper band.

The team then recorded sightings of the group over the next 10 years.

Banded birds were 16 percent likelier to die than non-banded counterparts, and had 39 percent fewer chicks, they report.

"The picture is unambiguous," researcher Yvon Le Maho told AFP. "Among banded penguins, the least-fit individuals died out in the first five years of the study, which left super-athletic birds.

"In the remaining five years, the mortality rate between the two groups was the same, but the reproductive success of banded penguins was 39 percent lower on average."

Le Maho said he had warned many years ago against banding penguins on ethical grounds but was sidelined. Opponents argued that the birds were not affected by the practice or got used to the tag after a year or so.

The latest findings, though, are unequivocal, he said.

They add to small-scale studies on captive Adelie penguins that suggest these birds -- which beat their flippers about three times a second when swimming -- lose up to 24 percent of their power when banded.

Le Maho said that banded penguins in his study arrived much later (16 days later on average) at breeding grounds compared with non-banded counterparts.

Late arrival is a known factor for poor breeding success, for chicks that are born later are nurtured in harsher weather and there are more predators around to grab them.

Birds failing in reproduction also spent five days longer at sea foraging for food for their chicks -- 21 days against 16 days -- and this again can be pinned on impeded swimming, he said.

Worried about ethical concerns, some research institutions abandoned banding in the late 1980s, but "massive banding schemes" continued, says the paper.

Penguins sometimes feature in climate research as tool for measuring the impact of global warming on cold-water wildlife.

But such studies may now have to be reviewed, for penguin population data could be skewed by flipper banding, says the paper.

"During the course of our study, when the sea temperature was low and food resources were abundant, there was virtually no difference between banded and non-banded birds," explained Claire Saraux, like Le Maho a member of France's National Centre for Scientific Research (CNRS).

"However, when there was a rise in sea temperature and food was less abundant, the penguins had to swim farther, and banded penguins stayed longer at sea to forage compared with non-banded birds."

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