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Winter 2013

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

bald eagle nest on the Potomac River

February 26, 2013

Bald eagles: The comeback bird

In the nation's capital, the national bird gets an early start raising a family. Bald eagles court and mate in February and typically lay eggs before March arrives.

The Washington area is prime real estate for breeding eagles, which prefer the upper reaches of tidal freshwater rivers, such as the Potomac, where American shad and other anadromous fish swim upstream from saltwater into freshwater to spawn. This huge concentration of fish in early spring coincides with the hatching of eagle chicks, whose diet is 94 percent fish, says Bryan Watts, director of the Center for Conservation Biology at the College of William and Mary.

Watts studies eagles in the Chesapeake Bay region, where the bird's numbers reached a historic low in the early 1970s. The pesticide DDT accumulated in the food chain, causing eagle eggshells to thin, which resulted in numerous broken eggs and dead embryos. After the pesticide's ban, the bald eagle population began a slow recovery, and it "has been growing exponentially since the early 1980s," says Watts, who believes that the resident eagle population within the tidal reaches of the Chesapeake Bay now exceeds 1,500 pairs, "making it the largest breeding population in the eastern United States."

The Chesapeake Bay area also hosts 17,000 migrating eagles and local juvenile birds. One migrant group arrives from New England and northeast Canada for the winter, returning north by late March. In late spring, young, non-breeding eagles fly up from the southeastern states for the summer. They head south for the winter, where some will establish breeding territories.

At about age 5, young eagles begin to breed — and need space to do it. "If all is full at the inn," Watts says, "they can either migrate out of [the Chesapeake Bay area] to some other location to breed, or contest existing territories."

Eagle contests can be violent. "We are getting increasing numbers of reports annually of bloodied birds in the early part of the breeding season," says Watts, who sees eagle combat injuries and deaths as a sign of a population reaching saturation. "It is the behavioral mechanism that allows the population to come into balance with available space."

Eagles prefer to nest well away from human activity, but with prime spots already filled, eagles "are increasingly colonizing residential areas and are using man-made structures for nesting," says Watts, who cites about a dozen nests on transmission towers in the bay area. Eagles have also nested on water towers, cellphone towers and observation towers.

But most eagles prefer to lodge their massive nest of sticks into a sturdy fork below the crown of a tall, mature tree near a large waterway. Undeveloped waterfront properties along tidal freshwater "represent the core of eagle activity," Watts wrote in a research paper, "and should be the focus of future conservation efforts."

Sources: Waterbirds, Journal of Wildlife Management, The Auk

bald eagle nests in the Chesapeake Bay tidal region

Bald eagle breeding pairs
in the Chesapeake Bay tidal region