Wild waterfowl such as ducks are believed to be superspreaders of the virus in part because they travel so far as they migrate, potentially infecting other birds along the way. For the new study, scientists shot down nearly 750 wild ducks from 11 different species in California's San Francisco Bay, which is in a migratory path that stretches from Alaska to Patagonia. They then tested the ducks for mercury contamination and whether they were infected with bird flu -- or had antibodies for the virus in their system. The results, published in the journal Proceedings of the Royal Society B, showed that ducks contaminated with mercury were up to 3.5 times more likely to have had bird flu at some point over the last year or so. The study's lead author, Claire Teitelbaum, a quantitative ecologist at the USGS Eastern Ecological Science Center, said mercury contamination "can suppress the immune system, and that might make infection with anything -- including influenza -- more likely".