More than 745 Magellanic penguins have washed up dead on the beaches of southern Brazil in the last month. The birds (Spheniscus magellanicus) died of vague natural causes according to the Brazilian Center for Coastal Studies at the Federal University of Rio Grande do Sul, reported CNN.

The Magellanic penguin normally migrates from Argentina to southern Brazil around this time of year, but there are normally fewer than 500 dead birds found on the beaches each year. The cause of this year’s high number of deaths is unknown, and they showed no signs of oil on their feathers or other injuries.

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This year may have been an exceptionally deadly year for the penguins because of natural causes, but every year the migration is threatened by oil slicks off the coast of Argentina which result from the routine small releases of oil by tankers. Although the massive spills get all the headlines, these smaller but more frequent releases of oil may actually be just as much of a threat to the birds, warned a 2005 report in the Marine Pollution Bulletin. Another study estimated that 40,000 penguins died because of this chronic oil pollution between 1982 and 1991.

Oil tankers are engineered to float properly only when they are full, so when they are emptied they must use sea water to provide ballast. A 1978 regulation established that tankers are supposed to empty the oil-fouled seawater into special holding tanks at the docks, use filtration systems or have separate ballast tanks for the seawater to prevent the release of oil into the sea, according to the United Nations Environmental Program.

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These safety procedures are sometimes ignored in order to save time and money. The result of cutting corners are many small slicks that are too small to draw media attention but big enough to kill birds.

The chronic oil slicks that coat the surface of the water can coat the birds’ feathers and remove the protective coating that keeps the cold seawater from reaching the birds skins. Without the protection, the birds freeze in the cold water. The oil itself also poisons the birds when they ingest it and causes stomach lesions which make it harder to digest food.

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Magellanic penguin (Spheniscus magellanicus), Valdes Peninsula, Argentina (David, Wikimedia Commons)