Maryland’s Bald Eagle Success Story

The Chesapeake Bay is now home to one of the largest populations of bald eagles in the US. Here, two eagles rest after eating a fish. (Photo by Bill Mish, submitted for the 2024 Maryland DNR Photo Contest)

For 18 years, Glenn Therres watched the recovery of Maryland’s bald eagles from the window of a four-seater plane.

The longtime bald eagle biologist for the Maryland Department of Natural Resources, Mr. Therres had counted nests for the department’s annual bald eagle survey since 1985, when there were only 62 breeding pairs in the state. Every year, he would cross the watery parts of Maryland from 200 feet above in a Cessna, scanning the treetops for bald eagle nests. A few months later, he would come back to count the nestlings.

“Every year we would find additional nests and map the new locations,” said Mr. Therres, who retired from DNR in 2021.

For a while, a few more active nests would appear each year, but by 1990 they found 123 breeding pairs, and 200 only six years later.

“In the early days, we flew three days, then another three [for the young eagles],” he said. “By the end, we were flying nine or 10 days each survey period just to cover all the nests.”

By 2004, DNR found 390 breeding pairs of bald eagles in Maryland. Mr. Therres ended the department’s survey after that—the species had well surpassed its recovery goals in the Chesapeake Bay region. In 2025, the Maryland Bird Conservation Partnership estimates there are over 1,400 breeding pairs in Maryland.

It was a long way to come for a bird of prey that had struggled for decades in the state and throughout the country. Across much of their range, bald eagles had fallen to near-extinction levels due to the effects of pesticides, as well as other factors like habitat loss and illegal shooting. In 1963, only 417 nesting pairs had been documented in the lower 48 states.

Scientists have attributed the large part of the recovery of bald eagles to the banning of DDT, a common pesticide that built up in the bodies of many birds and resulted in reproductive failure and the thinning of eggshells. The amounts of pesticide in the eggs of Chesapeake Bay area eagles were some of the highest in the country. In 1972, after decades of advocacy and debate, the Environmental Protection Agency banned DDT from most uses, and the populations of bald eagles, ospreys, and other birds slowly began to bounce back.

Bald eagles and other raptors are often considered a success story of protections like the Endangered Species Act, as well as an example of society collectively taking action to address a pressing problem.

“Bald eagles are a very good example of what happens when you find the solutions and take action, and now you can look at how they’ve come back,” said Dave Brinker, a DNR conservation ecologist. “If we could do the same thing with something like climate change, we could start to see changes. But we’re in denial. We were in denial about eagles for a long time.”

Gwen Brewer, a DNR science program manager, said the recovery of the iconic bird of prey is a testament to changes people were willing to make across broad sectors of society.

“To recover a species that is so widely distributed and that declined to such a serious level—that is something to celebrate,” Ms. Brewer said. She also noted the symbolic importance of taking action for a bird that holds a lot of cultural significance, both to the United States and to many Native American traditions.

Aside from pesticide restrictions, actions to restore the Chesapeake Bay and protect nest sites helped the eagles in Maryland. The Chesapeake Bay Protection Act of 1984 established protections for “critical areas” within 1,000 feet of tidal waters, which safeguarded bald eagle nest sites from development and disturbances.

DNR biologists and staff like Mr. Therres assisted both in on-the-ground nest protection and in collecting the data to track how the population was adjusting.

The department began its aerial surveys in 1977, taking over from earlier surveys conducted by the Audubon Naturalist Society (now Nature Forward). That year, scientists found only 44 breeding pairs—the lowest on record.

The survey ran for nearly 30 years, with one biologist and one pilot on each flight. They patrolled the areas within a few miles from large bodies of water, including the Bay, major tributaries, and reservoirs.

“It took a strong stomach and inner ear because we were flying donuts around the nests,” Mr. Therres said. The biologists would rely on their eyesight to see the nests—binoculars in the small moving plane would only increase the chances of making yourself sick. Luckily the nests were large, and the planes had high wings to increase visibility. The pilot was always someone skilled in flying low.

After an initial run in February or March, they would come back in May or early June to count the young eagles, when they were almost the size of adults but not yet flying. That made it “a little easier to see than when they’re a puffball the size of a grapefruit in a nest,” Mr. Therres said.

Bald eagles now nest in every county in Maryland and in Baltimore City. Nationally, bald eagles had been reclassified as “threatened” rather than “endangered” in 1995, and they were delisted in 2007.

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This article was written by Joe Zimmermann, science writer with the Maryland Department of Natural Resources.