When Chris Rimmer learned that mercury deposition from the atmosphere was two- to five-times greater at high elevations than at surrounding low elevations, it inspired him to examine whether mercury was accumulating in the blood of the birds he was studying on top of Mount Mansfield in Vermont. The rare Bicknell’s thrush breeds on the mountaintop, and on other mountains throughout the Northeast, and because he was capturing the birds every year for ecological studies anyway, it would be no trouble to take blood samples for analysis.
Rimmer, the director of the Vermont Center for Ecostudies, conducted initial research in the early 2000s that revealed that every bird he tested had some level of mercury in its blood, which they probably absorbed from the arthropods they ate. But no one knew whether the mercury levels he detected were harmful to the birds, and they still don’t. But since environmental regulations have reduced the regional emissions of mercury from power plants, medical waste incinerators, and other facilities through the years, he wondered whether mercury levels in the birds have declined since his initial study.
They haven’t. An analysis of mercury levels in 302 Bicknell’s thrushes captured between 2000 and 2017 and in 58 closely-related Swainson’s thrushes between 2014 and 2017 found no trends over time. Mercury levels ranged from 0.07 to 0.1 micrograms per gram of blood for both species throughout the years, regardless of the changes in mercury deposition, which increased from 1993 to 2004, decreased from 2005 to 2010, and climbed sharply after 2010, based on measurements at the nearby Proctor Maple Research Center.
“What surprised us most was that we couldn’t find any relationship between mercury being deposited locally and what we were finding in the thrush blood,” Rimmer said. “Mercury deposition levels went up and down within and between years, and they weren’t reflected in the birds. They didn’t change in synchrony.”
His study confirmed that mercury doesn’t stay in thrush blood for long, and the birds get two to three times more mercury from their food on their wintering grounds in the Caribbean than they do in Vermont. He and his collaborators also found that the birds’ mercury levels decline through the summer months, probably because they switch their diet from spiders in June to caterpillars in July and August, and caterpillars are lower in mercury than spiders because of the differences in the food those creatures eat.
“We suspect that some mercury falling on Mount Mansfield is globally transported, not just coming from local sources,” said Rimmer. “There’s a lot of mercury in the atmosphere, and plenty of inputs from distant sources.”
The biologist’s primary take-home message is that there is still a great deal to learn about how mercury cycles through the terrestrial system, how it’s transferred within the food web, and how it is processed in the birds’ bodies. Studies of common loons with elevated mercury levels have found them experiencing a variety of physical and behavioral impairments, but similar studies have not yet been conducted on wild songbirds. “There’s no way of knowing what’s a problematic concentration of mercury in a Bicknell’s thrush,” Rimmer said, “but we know that all of them have some. So there’s still work to be done.”