Story highlights More than 700 washed ashore in Rio Grande do Sul, biologists say

Birds were decomposed but otherwise seem unharmed

Penguins migrate from Argentina about this time of year, biologist says

Researchers at the Brazilian Center for Coastal Studies at the Federal University of Rio Grande do Sul in southern Brazil believe that 745 penguins found washed up along the state's coastline since June 15 have died of natural causes.

The center, known as Ceclimar, said in a release this week that it reached the conclusion after analyzing the conditions in which the animals were found. Most of the birds were young, according to biologist Mauricio Tavares. The lean birds showed no signs of external injury or oil in their plumes.

"Birds in the first year of life are inexperienced," he said, adding that the pattern of dead birds is common for this time of year and that the deaths are a result of "the process of natural selection."

In 2010, more than 550 penguins washed up on Sao Paulo state's Atlantic coast. In that case, necropsies showed that the penguins died of starvation.

The Peruvian government launched an official inquiry this year when close to 3,000 dolphins and more than 500 pelicans washed up along that country's northern Pacific coast.

This case is the first in which Ceclimar is conducting a more intensive monitoring of deaths. According to Tavares, July is historically the month when more birds arrive at the center for rehabilitation after washing up on the beaches weakened, injured or covered in oil.

"The animals usually migrate from Argentina around this time of year in search of food and warmer weather, and each year, some do wash up," he said. "But over 500 is a very, very high number."

Ceclimar set up weekly monitoring along the Rio Grande do Sul coast to study the dynamics of the Magellan penguins.

The biologists working at the center also collected 30 birds for a more detailed analysis; that report should be completed within a month. The release says the physical state of the birds is such that researchers could designate the deaths natural.

Tavares said earlier that "the animals were a lot smaller in terms of size and weight than normal penguins, so we think it will be natural causes, but it is certainly very strange."

Penguins have also shown up along the famous beaches of Rio de Janeiro. CNN affiliate TV Record reports that since the end of June, almost 40 penguins have been rescued along the beaches. In those cases, the birds are being cared for by SOS Aves and the Chico Mendes Institute for Conservation of Biodiversity.