For decades scientists have been following penguins by puttingbands around their flippers.
This allows individual birds to be identified at a distance.
The latest study, reported in the journal Nature, confirms it. Scientists from Strasbourg University followed a colony of king penguins for ten years.
But there have been concernsthat flipper bands might harmthe birds by slowing them down as they swim.
Birds fitted with bands died younger, started breeding later in the year, took longer to forage for food and over all raised about 40 per cent fewer chicks.
The researchers suggest that using flipper bands would now beunethical in most situations.
Scientists in the field would now have to find other tagging methods but in the meantime there are also concerns that some data gathered on penguins down the years, in this ecological crucial part of the planet, may now be worthless.