Hundreds of baby penguins swept from the icy shores of Antarctica and Patagonia are washing up dead on Rio de Janeiro's tropical beaches. More than 400 of the birds have been found dead on the area's beaches in the past two months, and more are being found in a distressed or sickly condition.

While it is common to find a few dozen penguins swept here from the Straits of Magellan by strong currents, there have been more this year than at any time in recent memory. Experts are divided over the possible causes. Thiago Muniz, a veterinarian at the Niteroi zoo, said he believed overfishing has forced the penguins to swim further from shore to find fish to eat, "and that leaves them more vulnerable to getting caught up in the strong ocean currents". Niteroi, the state's biggest zoo, has already received about 100 penguins for treatment this year, many of them drenched in petroleum. The Campos oil field that supplies most of Brazil's oil lies offshore.

But biologist Erli Costa of Federal University suggested weather patterns could be involved. "I don't think the levels of pollution are high enough to affect the birds so quickly. I think we're seeing more young and sick penguins because of global warming, which affects ocean currents and creates more cyclones, making the seas rougher," he said.

Download the new Independent Premium app Sharing the full story, not just the headlines

Most the penguins found are too young to out-swim the currents they encounter while searching for food. Every year, Brazil airlifts dozens of penguins back to Antarctica or Patagonia.