By Alberto Barrera

CERRON GRANDE RESERVOIR, El Salvador, May 17 (Reuters) - An artificial lake in El Salvador brimming with sewage and industrial waste is mystifying scientists by attracting thousands of migratory and sea birds.

Built in 1974 to drive El Salvador's biggest hydroelectric project, the 33,360-acre (13,500- hectare) Cerron Grande reservoir collects some 3,800 tonnes of excrement each year from the sewage pipes, as well as factory run-off and traces of heavy metals like chromium and lead, the government estimates.

So scientists are puzzling over the fact that some 150,000 seabirds from more than 130 species have chosen to make the reservoir their home. At least 90 of the species are migratory birds arriving from as far away as Alaska.

"It's one of the most contaminated lakes we have, which is why we should carry out a study on why the birds are here," said marine biologist Oscar Molina.

Waste from 54 industrial plants, 55 coffee processing plants, seven sugar mills and 29 sewerage systems flows into the reservoir, the environment ministry found in a 2004 study.

Yet the birds attracted to the lake even outnumber the roughly 100,000 people living in villages around it.

On one of 28 islands dotting the reservoir, biologists discovered 46 nests made by migrating American storks who produced some 100 chicks at the start of the year.

Biologists have been monitoring bird numbers at the reservoir since 2001, but Molina said El Salvador lacked the funds to carry out a full investigation into why so many are attracted to the area.

Environment ministry ornithologist Ricardo Ibarra said birds may be attracted by the sandy beaches, crawling with insects, that appear around the edge of the lake in the dry season.

"The most relevant thing is why the stork is nesting here for the first time," Ibarra said.

But the contamination is bound to be harming birds that feed and nest there, possibly making them too weak to be sure of making their migratory flights or affecting the strength of the shells of their eggs, he warned.

In a worrying sign, agronomist Edgardo Erazo, who studies wildlife on the reservoir, said scraps of plastic and metal -- and even plastic clothes pegs -- had been discovered in bird nests around the lake. (Additional reporting by Luis Galdamez)

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