Evers isn’t scratching his head anymore. A decade later, in the most sweeping analysis to date, a newly released report by the Biodiversity Research Institute and the Nature Conservancy found that a wide range of insect-eating songbirds, as well as little brown bats, can concentrate mercury in their blood at rates that can stunt reproduction.
More stunning was the habitat range of mercury exposure spread all the way from coastal estuaries to mountaintops. Researchers were particularly alarmed about birds such as the rusty blackbird. It has already declined 90 percent in its boreal habitats since the 1960s and breeding pairs in the northeastern United States have particularly high levels of mercury.
“It is a game-changing paradigm shift,’’ Evers said. “For years, we’ve understood the notion that birds like an eagle can obtain toxins by eating a bass, which has eaten a perch, and the perch has eaten a fly. Now we understand the same kind of analogy can be applied to a water thrush, which eats a spider, which has eaten a smaller spider, which has eaten a fly.’’
Mercury most notoriously is emitted into the atmosphere by coal-burning power plants, with the capability of traveling hundreds, even thousands of miles before falling to earth, often in some form of precipitation. The mercury that wafts down into forests can be absorbed by leaves. When the leaves fall to earth, bacteria transforms mercury into the organic toxin methyl mercury, which magnifies in impact as it moves up the food chain.
Wood thrushes, blackbirds, and sparrows consume mercury by eating critters on the ground. Warblers and vireos get mercury in the treetops by eating the spiders and insects that had consumed critters on the ground. Bats get mercury by snaring spiders and flying insects. Mercury can make adults too lethargic to raise their young.
