by Helen Fields and Alanna Mitchell / Environmental Health News
It was the summer after her freshman year at the College of William and Mary, and Hallinger was working with her professor, ecologist Dan Cristol, to investigate the effects of mercury left behind by a factory. Over and over she recorded birdsong, visiting various sites in the woods and along the shore, some polluted, some unpolluted.
When she got back to Williamsburg with her tape recorder, Hallinger sorted through the hours of bird songs. She turned them into digital files in the computer, then analyzed them. The differences were striking: The wrens and sparrows along the contaminated South River were singing simpler, shorter, lower-pitched songs.
Scientists have long known that mercury is a potent toxicant: It disrupts the architecture of human brains, and it can change birds’ behavior and kill their chicks. But after extensive research in Virginia, scientists have shown that mercury also alters the very thing that many birds are known for – their songs.
Emitted by the burning of coal, mercury in the atmosphere has quadrupled since the days before industrialization, according to a recent study published in Nature. And the amount of methylmercury in animals throughout much of the world is rising, too.
After extensive research in Virginia, scientists have shown that mercury alters the very thing that many birds are known for – their songs.“There’s a global decline in songbird populations,” and while the causes are unexplained, “I can’t help but think that mercury and other chemicals and organic compounds are partly implicated in this,” said Nil Basu, an environmental toxicologist at McGill University.
Children exposed to methylmercury in the womb, usually through their mothers’ consumption of seafood, can have impaired speech, language development, learning and memories. So understanding why mercury-contaminated songbirds can’t sing their songs could help scientists learn more about how human brains are damaged by mercury, too.
“Methylmercury can affect the very motor functions that control speech – something that may have a parallel in the bird world,” said Philippe Grandjean, a Harvard University environmental health scientist who led pioneering research on the IQs of mercury-exposed children.
The songbirds in the woods downstream of the chemical plant “are affected profoundly by mercury in a way that suggests learning problems,” Cristol said, “and this has indirect implications for humans.”
Why birds sing
Cristol is standing on a boat ramp at the confluence of the South River and the North River, two of the tributaries of the Shenandoah. High above, the spring’s new arrivals tweet, chirp and whistle from the trees. “That’s a blue-gray gnatcatcher, making all that noise,” he says. Another bird calls. “That’s an eastern wood-pewee.” A moment later: “That’s a great crested flycatcher.”
When you’re a bird, your song matters. Songbirds sing for two basic reasons: sex and turf protection. Whether at a backyard feeder or in the canopy of a rainforest, their songs are critical to their survival and reproduction. That’s why songbirds put so much evolutionary effort into learning how to sing. Some, like wrens and sparrows, learn their songs from their fathers, a feat of memory, mimicry and advanced brain development that’s akin to humans learning to speak. Others, like the Eastern phoebe, are born knowing how to sing.
Across the globe, the breadth of sounds that birds make is awe-inspiring. Each species has its own rhythm, or tempo. The Cornell Lab of Ornithology notes that marsh wrens always sing as if they’re in a hurry, while white-throated sparrows take their time. Birds sing across all four parts of the chorus, from the high soprano of a tiny cedar waxwing to the low bass of a raven. Some songs run up or down a scale, like the canyon wren; others stick to a single note, repeated, like the chipping sparrow. Some birds whistle, others trill, some sound like flutes, others caw. Each song is a unique identifier of a species. Yet to the human ear, these songs are so complex that recently a British scientist developed a computer program to decode songs by species.
Cristol and Hallinger aren’t the only ones who have shown that contaminants are changing the songs that have inspired poets and musicians like Shakespeare and Handel. Around a smelter in northern Europe with a lot of heavy metal pollution, a study of birds found that they knew fewer songs and sang less at sunrise than birds at two less-polluted sites. One small study of Nelson’s sparrows at sites with high and low mercury in Maine found that their songs were measurably different, although they weren’t the same differences that Cristol’s group found. Last year another study showed that chickadees along the Hudson River exposed to polychlorinated biphenyls sing strange songs.
In rural Virginia, 20 miles upstream of the confluence where Cristol was listening to songbirds, a DuPont factory opened in 1929 to manufacture rayon – the world’s first synthetic fiber. Mercuric sulfate was part of the process that turned wood pulp into wearable thread. The company stopped using it in 1950. But, by then, mercury already was polluting the river. In 1984, DuPont funded a 100-year monitoring program for mercury and its effects on the river ecosystem, which later launched Cristol’s investigation of the area’s birds.
Understanding why mercury-contaminated songbirds can’t sing their songs could help scientists learn more about how human brains are damaged, too.Most birds known to be harmed by mercury are fish-eating seabirds or waterfowl. So Cristol set out to find out if mercury made birds have fewer chicks by studying three species of local birds that are closely connected to the aquatic food web: kingfishers, tree swallows and screech owls. Kingfishers nest in holes they dig into riverbanks, so Cristol and his students dug pits next to the nests, then carefully scooped their way in from the back to count eggs and young, and take blood and feather samples. For tree swallows and screech owls, they put up nest boxes. Other birds nested in the boxes, too, including bluebirds and wrens. “We sampled them just like everything else,” Cristol said.
What they found surprised them: Even birds that they thought had no connection to the river, such as bluebirds, had high mercury levels. Cristol knew he was onto something unusual. “We began systematically netting every species we could find,” he said. Over several summers, they took samples from 70 species of birds. They discovered that the songbirds got their mercury from eating spiders, and that some were more highly exposed than fish-eating birds. Swallows in the mercury-contaminated areas laid about as many eggs as uncontaminated birds, and the eggs hatched, but many of the young died in the first week outside the egg, according to the Cristol team’s research.
Then Cristol’s team started looking for other effects. That’s when Hallinger took her tape recorder to the river.
Originally, Hallinger and Cristol didn’t know whether mercury had made the river’s birds bad singers, or whether it had altered something in their brains. Maybe it made them lethargic, for example. So in her third summer at the river, Hallinger investigated another bird: The Eastern phoebe, which is born with its song. She recorded their songs at contaminated sites and reference sites, and found that they sang the same tune wherever they lived.
This implied that the wrens and sparrows had something wrong with their brains so they couldn’t learn their songs properly. Given mercury’s neurotoxic effects on developing human brains, that was quite plausible.
Still, the song research was “kind of a fishing expedition,” Cristol said. He knew that sometimes scientists find interesting results that don’t hold up.
To test the hypothesis, Cristol took the question indoors – to the laboratory.
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