By D'Vera Cohn
Washington Post Staff Writer
Monday, April 17, 2006
The dramatic aerial battle this month between two bald eagles contending for territory on the Potomac River south of Washington is a sign that their population rebound has been so successful they are running out of habitat.
The number of breeding pairs of eagles in the Chesapeake Bay region grew from fewer than 100 in the late 1970s to about 1,000 this year. Eagles are crowding together more closely, and a growing number of birds are being treated for injuries suffered in turf battles.
The nonprofit Wildlife Center of Virginia in Waynesboro treated six eagles last year that had wounds consistent with fights with other eagles, compared with two the year before, said its president, Ed Clark. At Tri-State Bird Rescue & Research in Newark, Del., another nonprofit facility, the bald eagle injured near Washington was the fifth one brought in this year from Maryland with fight injuries.
"All of this relates to the fact that the population is reaching some level of capacity at this point," said Bryan D. Watts, director of the Center for Conservation Biology at the College of William and Mary. "The bay population has increased tenfold in the last 30 years. It doesn't take that many years to go to saturation. The bay has produced more chicks in the past five years than it has in the previous 25.
"The population is still growing but beginning to level off," he said. "We've been expecting these kinds of encounters to start."
Watts co-wrote a paper last year that analyzed eagle population growth in the tidal reach of the Chesapeake Bay and predicted it would reach saturation within a decade. As the human population grows, the acreage of waterfront habitat for eagles probably will shrink, the report predicted. The size and stability of the region's eagle population, it added, will depend on how well eagles adapt to a more urban landscape, as well as how much prime waterfront can be protected from development.
The bald eagle population is also booming in other parts of the country, and in some places it is showing signs of reaching capacity. In Florida, which has the largest number of eagles in the lower 48 states, the Audubon Society's Center for Birds of Prey in Maitland treats a growing number of eagles each year for wounds incurred in fights over territory, said Lynda White, who heads its EagleWatch monitoring program. Jody Millar, the U.S. Fish and Wildlife Service monitoring coordinator for bald eagles, said Forest Service data from northern Minnesota suggest that the number of young per nest has been declining there in response to competition for nesting space among a growing eagle population.
Millar said she would not be surprised to see the growth rate of the bald eagle population level off in areas with prime habitat such as the Chesapeake Bay region, Florida, Maine, the northern Midwest and the Northwest, where numbers have skyrocketed in recent decades.
The bald eagle's recovery -- from fewer than 500 pairs nationally three decades ago to more than 7,000 now -- has been credited to a ban on the pesticide DDT, which had thinned eggshells of many bird species to the breaking point, as well as to expanded protections for eagle habitat. The Interior Department issued a formal proposal earlier this year to remove the birds from the endangered species list.
Environmentalists have praised the rebound of the eagle as a success story, though some expressed concern that taking it off the protected list would enable more homes and commercial buildings to go up on the waterfront land that the eagles prefer. "The bald eagle population is growing into a shrinking habitat, and eventually the habitat will be filled," said Clark, of the Virginia Wildlife Center. "As the habitat diminishes, so will the population."
Maryland stopped doing annual eagle-nest surveys in 2004 because the population had met recovery goals, but Glenn Therres, associate director for wildlife at the state Department of Natural Resources, said he believes the number continues to grow. Birds are nesting in new territories -- last year, for instance, breeding pairs settled in two mountainous western Maryland counties for the first time in recent decades.
In prime habitats such as those along the Potomac River, eagles are "packing in denser" than biologists had thought possible, occupying nests every mile or two. In the 1930s, Therres said, biologists recorded that eagle nests were generally three miles apart. This trend "may be an indication that we are getting close to carrying capacity," he said.
In Virginia, the 2005 bald eagle survey by the Center for Conservation Biology said the number of breeding pairs grew 5.8 percent from the year before. The average annual rate of growth has been 10 to 12 percent in recent years.
"Growth in the Virginia breeding population appears to be slowing in recent years, possibly reflecting some threshold in capacity within the lower Chesapeake Bay," said the survey report, funded in part by state and federal wildlife agencies.
A record number of chicks were born in Virginia last year, though Watts predicted that this, too, could level off. Virginia bald eagles also are ranging into new inland territories as the prime coastal habitats fill up.
The one known eagle nest in the District, located on National Park Service land, did not produce chicks this year for the second year in a row, said Park Service resource management official Susan Rudy. The reason is unknown but could be related to parasites in the nest, she said.
The April 5 fight between two eagles near Washington began near a nest on the Maryland shore of the Potomac River that has produced chicks since the late 1990s. The pair occupying the nest were nicknamed George and Martha by construction workers on the nearby Woodrow Wilson Bridge along Interstate 95. Martha was attacked by another eagle, believed to be a female seeking to take over her territory, and is recuperating at Tri-State Bird Rescue.
The fight and its aftermath made news reports nationwide, in part because bridge construction workers see the nest every day. But Craig Koppie, a Fish and Wildlife Service biologist, said there is no doubt there are many more fights out of public view.
The male, George, guarded the nest for three days after the attack, but the eggs in it did not survive. Koppie said that a decade or two ago, biologists would have dismissed the idea that bald eagles would be willing to nest within earshot of a construction project, as these two did.
"Isn't it ironic," Koppie said, "that they could handle the man-made features and not the rivalry of their own species?"