More than 500 penguins have been found dead on beaches in Brazil's southern Rio Grande do Sul state.
Brazil's Centre of Coastal and Marine Studies says veterinarians are investigating the deaths of the Magellenic penguins.
Officials say the veterinarians are puzzled by the large number of deaths, and by the fact the birds appear well fed, not exhausted and without injuries or oil stains.
Magellenic penguins, named after the Magallenes region in which they breed, mate in large colonies in southern Argentina and Chile.
They traditionally migrate north between March and September along the Rio Grande do Sul coast up to Sao Paulo.