The standard way of tagging penguins for science - putting bands around their flippers - affects their survival and reproduction, a study has found.
French researchers, reporting their work in the journal Nature, found king penguins had 40% fewer chicks if they were banded, and lived shorter lives.
They say continuing to use the tags would in most situations be unethical.
Flipper bands have been used for decades to identify individual penguins so they can be tracked on land and sea.
They allow for easy visual identification of individual birds from a distance.
Some studies down the years had suggested they harmed the birds - for example, by creating extra drag when they swam, or by reflecting sunlight in a way that could attract predators.
But others had suggested there was no problem.
"There was a debate about whether bands have an effect or not - and you could find studies and some would say 'yes' and some would say 'no'," said Claire Saraux from the University of Strasbourg and the French Centre National de la Recherche Scientifique (CNRS).
"So our idea was to try to make sure - instead of doing one-year studies, to try to find out what's going on over 10 years," she told BBC News. BBC News
