W&M biologists find link between mercury dosage, songbird losses

WILLIAMSBURG – For College of William and Mary Biology Professor Dan Cristol, tracing the path of industrial mercury through the landscape is becoming a lifelong endeavor.

Cristol is one of the co-authors of new research published this month in PLOS One, a scientific journal. The research documents the effects of sub-lethal doses of mercury in songbirds.

The study is the latest in a what has become a major research focus for Cristol, who began studying songbirds on the South River in 2005, eventually monitoring more than 600 boxes in the Shenandoah Valley.

The South River presented a unique research opportunity, as it was contaminated with mercury from DuPont's manufacture of Rayon in 1929-50. DuPont today funds much of the research on the site, including Cristol's.

For decades biologists concerned themselves with the effects of mercury on fish and fish-eating birds, such as osprey, kingfishers and eagles. Cristol and his students broke that mold in 2008 when they published the results of biological monitoring of songbirds on the South River. Cristol found that some of the songbirds, such as Carolina wrens and red-eyed vireos, had higher dosages of mercury than kingfishers.

"Nationally, we have helped put the focus on songbirds," Cristol said.

The researchers believe that forest birds are suffering high dosages because their diet either directly comes from the river, through insects such as mayflies, or else through arachnids or insects that eat river insects. Meanwhile, they found many other differences between South River's wild birds and those of reference sites, according to Cristol.

"They had 20 percent fewer babies," he said. "Their songs are sung at the wrong pitch. Their hormone levels are altered. Their immune systems are suppressed."

Because correlation doesn't equal causation, researchers couldn't draw a one-way arrow from mercury pollution to the abnormal birds. "We're assuming mercury is the cause," he said, "But we can't be absolutely sure."

It could have been that another chemical in the South River was causing the anomalies, he said. So Cristol, fellow Biology Professor John Swaddle, and then-post doctoral researcher Claire Varian-Ramos set out to measure the effects of sub-lethal doses of mercury on zebra finches in the lab.

Unsurprisingly, the researchers found that as the levels of mercury increase, the reproductive success of the finches declines. For scale, the lowest dosage was just less than what's found a can of tuna fish.

They also found that when those surviving chicks of mercury-dosed parents themselves reproduced, they did not suffer the same losses. Basically, the presence of mercury was artificially selecting for resistance in a single generation.

Dosed birds also had a much harder time recalling where food was located compared to non-dosed finches.

Other papers will deal with the phenomena confirmed in the lab – the peculiar songs, hormonal abnormalities, suppressed immune systems and even structural changes to the brains – Cristol said, but it was important to first publish the link between mercury and reproduction.

"Reproduction is the most widely known and is used by regulatory agencies to assess whether injury has occurred," he said.

"So DuPont has been extremely responsible," he continued. "They could have fought this the whole way, but instead they funded research. Basically the federal and state governments will try to collect from polluters to compensate the American people – anything from cleaning up the pollution, to replacing habitat to creating new habitat."

The research also helps to explain some of the declines in different bird populations worldwide. "The more worrisome situation is birds in pristine habitats," Cristol explained. "Some birds decline and we don't really know why."

Certain seemingly unaltered habitats, like acidic wetlands, are suffering bird losses because of chemical interactions in the habitat that convert atmospheric mercury from coal-burning power plants to a form of mercury that can be taken up by cells, Cristol said.

Climate change is expected to increase mercury levels worldwide, he said, with rising temperatures and ocean acidification. And while the United States will likely continue to draw down its use of coal, countries like India and China will probably rely more heavily on coal.

"The other thing is wildfires," he said. "They release a lot of mercury into the atmosphere that was locked up in soil and trees."

Researchers are also trying to figure out at what point exposure to mercury causes adverse effects, to determine whether there is a tipping point or at least a tolerable baseline. Right now, he said, mercury worldwide is measured at four times natural levels.

"There is mercury everywhere," he said. "You could give me a piece of a ham sandwich and I could find mercury in it. We are trying to find the lowest dosage that is a problem."

Langley can be reached by phone at 757-345-2346.