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The Rio Madre de Dios basin was an ideal location for studying this question because it provided a natural gradient from the heights of one of the tallest mountain ranges down to the floodplains of the world's largest river by volume.

Hotspots for mercury contamination included Madre de Dios, Peru, and Ayapel, Colombia - centres of artisanal gold mining.

"It is widespread throughout the Amazon Basin, it's widespread throughout the Congo Basin and Indonesia — this is a global tropics issue," said ecotoxicologist Chris Sayers at the University of California Los Angeles, who has studied the impact of mercury on birds in Madre de Dios.

Los Amigos lies in the rainforest of southeastern Peru's Madre de Dios region where some 46,000 miners are searching for gold along river banks in the country's epicenter of small-scale mining.

Reuters accompanied the researchers in Madre de Dios over three days in late May and reviewed their previously unreported findings.