High atop Mt. Mansfield, with the sun barely breaking the eastern horizon, I hold a small songbird called a Bicknell’s thrush in the palm of my hand. I carefully push back the feathers on the underside of her wing and prick a very small vein with a very small needle. I collect a few drops of blood that ooze from the vein in a small glass tube. The bleeding stops almost immediately. I also clip a piece of feather from the inner part of her wing and place it in an envelope.
Biologists at the Vermont Institute of Natural Science (VINS) have been studying this rare bird species for over a dozen years. The blood sample will reveal how much mercury the bird has ingested in the last few weeks, while the feather sample will give an indication of how much it has accumulated over its lifetime.
Meanwhile, in Maine, biologists from the Biodiversity Research Institute (BRI) are conducting similar studies with loons on lakes. When the sun creeps below the western horizon, biologists board a small boat with powerful lights and a large dip net. They find the loons floating on the lake at night. With a bright light, they ease up to a bird in the boat and, with a quick dip of the net, they capture it. Like the thrush, they take a small sample of blood and a feather and release the bird back to the lake.
Although mercury is a naturally occurring element, humans have greatly increased its prevalence in the environment through manufacturing, fossil-fuel burning, and waste incineration. Mercury these days is deposited in the Northeast at a rate that is two to four times higher than it was before the industrial revolution.
Most of the mercury in the air today is in an inorganic form that is only slightly toxic at low doses. But this form is converted into much more toxic methyl mercury by anaerobic bacteria found mainly in aquatic ecosystems. The mercury enters the food web via small aquatic organisms and is passed on to larger and larger predators. At each level in the food chain, the predator absorbs more and more mercury from its prey. Predators at the top of the food chain, like loons or raptors, can have very high concentrations of mercury in their bodies.
“Birds are good indicators of environmental quality, like the canary in the coal mine,” says Dave Evers, director of BRI.
In New England, 15 to 25 percent of loons are considered to be at high risk from the methyl mercury in their diet. At moderate levels, mercury affects loons’ nervous systems and causes them to behave oddly. At higher levels, their reproductive success is reduced.
“In our Maine and New Hampshire study population of around 200 loon pairs, the high-risk pairs produce 40 percent fewer fledged young than the low-risk pairs. This creates a negative population growth-trend, according to the population models I’m constructing,” says Evers.
That Bicknell’s thrushes on Mt. Mansfield are also turning up with high levels of mercury has come as a surprise to biologists. Bicknell’s thrush has no association with aquatic ecosystems where methyl mercury is created. The evidence seems to suggest that high-elevation soils, even if they are frequently saturated, may provide a suitable anaerobic environment for the creation of methyl mercury.
Chris Rimmer, director of conservation biology at VINS, says, “Considering how poorly we understand the effects of mercury toxicity in insectivorous birds like Bicknell’s thrush, and the fact that we know deposition of mercury is four to five times higher in mountaintop forests of the Northeast where this bird breeds than at lower elevations, there is a real need to investigate levels of this toxic element in montane wildlife populations.”
Both birds and mercury know no borders. Bicknell’s thrushes migrate to the Caribbean each fall, and VINS biologists follow them there. Tiny blood samples once again show that the story is far from over. Birds are gaining mercury while eating on winter vacation, too.
Several other Northeast bird species have also been found to have elevated mercury concentrations: Nelson’s and sharp-tailed sparrows found in coastal salt marshes; belted kingfishers in lakes and rivers; barn and cliff swallows along lakes; and American woodcock in upland areas. On the higher peaks, elevated mercury levels are being found in white-throated sparrows, blackpoll warblers, and myrtle warblers.
Back atop Mt. Mansfield, the Bicknell’s thrush in my hand is anxious to be released. I take a few more body measurements, open up my hand, and, in a flash of brown feathers, she disappears into the forest.