Approximately four weeks old, this young eaglet has a panoramic view of the lakes and ponds at Seney National Wildlife Refuge. Already its large sharp eyes and large feet are impressive. Peering into eagle nests looking for offspring is a common practice for Dr. Michael Wierda of Clemson University, who is studying eagles in Michigan, including those nesting near Mill Creek Discovery Park outside Mackinaw City. (Photograph courtesy of Teresa McGill of McGill's Nature in Motion of Central Lake) High above inland swamps near the top of a 100-foot tall aspen tree, a pair of bald eagles have built a large platform nest. This spring two baby eaglets kept their parents busy bringing food to them from Lake Huron and nearby beaver ponds. One year earlier, the bald eagles hatched three eaglets. That is when Jeff Dykehouse, curator of natural history at Mackinac State Historic Parks, took Michael Wierda, a Ph.D. student from Clemson University, to the site. Mr. Wierda was studying eagles in Michigan.

An old baseball cap covers the eyes of this eaglet while Dr. Michael Wierda takes a blood sample of the young bird. Tests from the blood are showing a decrease in DDT and other chemicals in the birds in Michigan, but of concern is a rise in mercury levels. (Photographs courtesy of Teresa McGill of McGill's Nature in Motion of Central Lake) The remote nest is south of Mill Creek Discovery Park on state forest land outside Mackinaw City, said Mr. Dykehouse, who had been watching the nest for a few years.

In June 2009, Mr. Dykehouse led Mr. Wierda and his research assistant, Katherine Leith, to the nest. Mr. Dykehouse had visited the nest in the winter when it was easier to traverse the land, over frozen swamps and minus pesky insects. Travel to the spot offered an entirely different set of conditions in warm weather. The group waded through about a mile of marshland, fighting off aggressive mosquitos all the way to the site.

Carefully, Dr. Michael Wierda slips an eaglet from a special bag used to transport the bird to the ground. During the process, the bag is attached to the ropes seen hanging from the tree. The ropes act as an “eaglet elevator” that will lower the baby to the ground where it is banded, measured, and a blood sample taken for testing. When they reached the base of the tree, Mr. Wierda's goal was to visit the nest. He used a tree-climbing strap and special boots to shimmy up the tree. Once there, Mr. Wierda found three curious, yet scared baby eagles. He gently placed one of the eaglets in a blue cloth bag, tied it to a rope, and carefully lowered the bag to the ground. Researchers call the process using the eagle elevator.

At the base of the aspen, Ms. Leith took the arriving eaglet from the bag and placed a baseball cap over its eyes to help keep it calm while she took a blood sample. The blood samples will help determine what, if any, contaminants are in the baby birds. Next, the young birds, estimated to be about four weeks old, were banded, measured, and weighed.

The process took about 15 minutes per bird, said Mr. Dykehouse.

When the group first reached the nest and for about 20 minutes, the parents flew overhead, before flying off. The parents, said Mr. Dykehouse, would come back later. They would abandon the nest only if the group were there every day.

Eagles, the largest of any nesting bird in the world, make the tallest tree their home. Nests have a flat platform on the top, and when the eagles begin constructing the nest it is about five to six feet across, and about two feet deep. The birds may use the same nest for 20 years.

“Every year they use the same nest, they simply add more materials,” said Mr. Dykehouse. “More sticks, more soft material, branches, grasses, things like that are added and it continues to rise. There are some nests that measure 10 feet across and 20 feet high.”

Eagles are magnificent predators, Mr. Dykehouse said, but they also are like canaries in a coal mine.

Years ago, he said, coal miners would take canaries into the mine with them. The small birds have a high metabolism rate and quickly can detect changes in the air quality. They always are singing, and if that singing stopped when they were brought into a mine, it was gave miners an early warning that something wasn't quite right with the air, said Mr. Dykehouse, and they would quickly leave the mine.

Since then, researchers have learned eagles are early warning indicators of pollutants in the environment and can be used as a measure of contamination.

At one time, experts estimate, about 500,000 eagles inhabited North America, but those numbers dropped to about 50,000 by the 1950s and the birds were declared an endangered species.

The rapid decline of the eagles left Michigan with fewer than 100 active nests by the 1960s.

“This is primarily because of DDT and chemicals like it,” said Mr. Dykehouse.

Also at that time, the number of young produced in those nests dropped. Then, about every two nests were producing only one young bird and the population could not grow.

In 1972, DDT was banned in North America. The chemical, which was used to reduce mosquito populations, caused problems for many animals, including eagles. The chemical delayed breeding in eagles and the young were born too late in the year to survive through the winter. DDT also caused the shell of eagles' eggs, which are about twice the size of a chicken's egg, to thin and the eggs cracked during incubation.

Now, eaglets serve as a good measuring tool for contaminants in watersheds and lakes, said Mr. Dykehouse. The process where a chemical travels from one animal to another up the food chain is called bioaccumulation. The small fish are eaten by bigger fish, which are eaten by eagles or, more importantly, are fed to young eagles. In the first four weeks of life, an eaglet is fed food gathered within 10 miles of the nest.

Blood tests from the eaglets show there is still DDT in Lake Huron. It's very hard to measure because the parts per billion have gone way down, said Mr. Dykehouse.

“We're not putting DDT in the water anymore, but it is a long lasting contaminant,” he said. “Scientist tell me that they can't go out there and take a water sample in the Straits of Mackinac and find DDT, but we know it's out there. We can look at animals that fish there, so they're using eagles as a way to measure contaminants in the Straits of Mackinac and all over Michigan to see if they are going up or down.”

The eagle population began to return in the 1980s.

Today, eagles have become a common sight at Mill Creek as they fly from the nest to Lake Huron to feed.

“The Eagles at Mill Creek, we see them all the time,” he said. “When I'm working on site I'm always looking up and whenever we see an eagle we have our radios and we tell all of the adventure tour guides, 'eagle, 10 o'clock,' and the people can see an eagle fly by.”

Again, however, the eagles are early indicators that something negative is going on in the waters of the Great Lakes, he said. The eaglets tested in 2009 helped further that study.

The good news is that DDT contamination continues to drop, said Mr. Dykehouse, but the bad news is mercury levels are climbing. Mercury levels are rising for eagles feeding from the Great Lakes, he said.

Mercury is a harmful chemical, especially for children, who have developing nervous systems, said Mr. Dykehouse.

“We know that mercury affects your nervous system, especially if your nervous system is growing or developing, in other words young kids, young animals,” he said.

People should not throw mercury thermometers in the trash, rather have them recycled like batteries, he said, and replace mercury thermometers with digital ones.

There are other sources for mercury contamination. Smoke coming from coal plants and incinerators emits a mercury vapor that eventually makes its way into the watersheds and lakes. To help reduce this effect, he said, people should try to reduce energy consumption. Many types of lightbulbs have mercury and consumers should avoid using mercury bulbs. If a bulb breaks, children should be kept out of the room until it can be cleaned up. A broken bulb emits a mercury vapor, he said.

How long it will take before mercury becomes a serious problem, he does not know.

“We know that a large amount of mercury in your system is bad for you whether an eagle or a person,” he said. “The fact that it is increasing is scary. My concern is it is increasing in the last 10 years, where most toxins in the environment are decreasing.”