Brazilian scientists are investigating what has led to more than 500 dead penguins being found on beaches in the last week.
Autopsies are being carried out on some of the birds, which washed up on the beaches of Rio Grande do Sul state, in the south of the country.
Biologists at the Centre of Coastal and Marine Studies said the penguins appeared to have been well-fed, with no apparent injuries and no oil on their bodies.
Similar incidents in the past have been blamed on shifting ocean currents and colder temperatures.
Magellanic Penguins migrate to southern Brazil from Patagonia every year during the southern winter.
Last week dozens of young penguins were rescued from beaches in Rio de Janeiro after straying far beyond their normal range.
The birds delighted beach-goers, but scientists said their health was suffering in the tropical waters.
Brazil’s environment agency is preparing to fly those penguins back to the south.