A bird can symbolize many things, depending on the bird. In Western culture, a dove may symbolize peace, the nightingale love and poetry. Other birds are harbingers of existential doom, like circling vultures or Poe’s raven. Sailors at sea believed a curlew singing overhead foretold a dangerous storm. In Eastern European folklore, the sight of an owl perched atop someone’s home or barn portended imminent death.
Birds of warning aren’t just poetic symbols. Delicate creatures, they warn us in real life when things skew out of balance. Many a canary has given its life for those of coal miners. A rise in bird extinctions over the last half-century warns us we’re destroying forests and wetlands. And now, according to a new study by biologists at the University of North Carolina, common sparrows are warning us about mercury levels, too.
Because of “their hydrology, acid-base status and sediment characteristics,” the study’s authors explain, distant marshlands can be just as bad as some points of direct mercury contamination. Mercury “biomagnifies” in marshy environments by converting into methylmercury—a “bioaccumulative” that, as the term implies, amasses faster in living organisms than it is shed.
Scientists used to capture and test common loons to test for mercury in our waters because they eat large fish, which also absorb mercury. But bioaccumulation rates in Loons had only been observed to increase at rates as high as 8.4 percent a year.
In contrast, researchers for the current study captured and recaptured Nelson’s Sparrows from 2009 to 2011 and measured the mercury in their feathers and blood. In areas like the marshlands near Grand Forks and Upham, ND, mercury concentrations more than doubled in some sparrows over the course of a single year. Their bioaccumulation rates were much higher, making them fantastic (if unfortunate) indicators.
They're also more versatile. Sparrows aren't restricted to fish diets. Like other songbirds they eat bugs and spiders and provide a more widely applicable means of detecting mercury methylation and mercury bioaccumulation problems in the wildlife that populates our forests and wetland (and, in some cases, our stomachs).
Nelson's sparrow, like many birds, are extremely small and light, which means that mercury accumulates in their bodies comparatively quickly. They're also cool-looking.
“Mercury is far more prevalent in our environment than anybody thought,” explained Ecologist David Evers, chief scientist at the Biodiversity Research Institute, a nonprofit lab in Gorham, Maine, in an interview with Discover Magazine . “All these decades, people were out looking for mercury and missing this key group, the little songbirds out in the yard.” (Evers is not an author of the study.)
What’s more, scientists can study the effects of mercury on birds to learn more about its potential dangers for us and the rest of the ecosystem. As the Discover article notes:
…the effect of mercury on birds provides information about how the heavy metal affects the brain. Researchers from the College of William and Mary studied song performance in four species of birds along the South River in Virginia, an area that was contaminated by industrial mercury from 1929 until 1950. They found that wrens affected by mercury sang shorter songs with fewer notes. Affected sparrows produced mating calls that had a lower tonal frequency and were less complex.
Those mid-century findings are particularly interesting in light of a separate, unpublished study announced today, which suggests that songbirds and humans activate similar genes with relation to singing and speech, respectively. Specifically, the genes associated with the vocal communication areas of Zebra Finch and Budgerigar brains are activated similarly to those activated for speech in the cortical and basal ganglia in human brains.
Zebra Finches and Budgerigars learn their songs, notes the study’s author, Duke neurobiologist Erich Jarvis, unlike doves and quails, who do not. And the molecules expressed by singing and speech genes in birds and humans, respectively, do not appear in doves and quails.
Given mercury’s negative effect on songbird songs, could mercury contamination have a similar effect on human brains? Data from the US Environmental Protection Agency suggests it does: Speech impairment is among the many neurological impairments documented in relation to mercury poisoning, a list that also includes problems hearing, seeing, and walking. Perhaps in addition to warning us about the presence of mercury, birds may also have more to teach us about what mercury can do to our cognitive abilities.
If only songbirds had teeth. They could, perhaps, teach us more about the potential neurological effects of silver amalgam tooth fillings , which are 50 percent mercury by weight and lie just inches away from millions of human brains.