Bald eagles are one of the most readily recognizable birds in North America. They are also one of the largest, with the larger females reaching 13 pounds or more. Generally, bald eagles from the northern portions of their range are larger than individuals in the southern limits of their range. Additionally, like most raptors, females may be up to 30% larger than males. Overall, the species ranges in size from a 5.5 foot wingspan up to a nearly 8 foot wingspan depending on the sex and origin of the bird.
Bald eagles are also one of the most well-known conservation success stories in the world. In the mid-20th century, bald eagles suffered a steep decline as a result of persecution, habitat loss, and the introduction of a pesticide called DDT (dichlorodiphenyltrichloroethane) to the environment. DDT was a widely used and effective pesticide, but it persisted in the environment and traveled through the food chain via a process called bioaccumulation. Large predators, like bald eagles, ended up with high concentrations of DDT in their bodies, affecting the birds’ ability to lay eggs with properly formed shells. The thinly shelled eggs then cracked under the weight of the parents when incubated. The bald eagle’s population plummeted and by 1978, it was estimated that only approximately 400 pairs of bald eagles remained in the continental U.S.
Fortunately, this spurred the U.S. to action and the bald eagle was given full protection under the newly created Endangered Species Act. This protection, combined with subsequent reintroduction efforts and banning the use of DDT, allowed the bald eagle population to recover. By 1995, it was upgraded to federally Threatened, an improvement over Endangered status. In 2007, the bald eagle was officially removed from the federal Endangered Species list, and current populations are estimated at 10,000 pairs in the continental U.S. On the state level, the bald eagle remains listed as Threatened in Pennsylvania, although the population seems to be growing. There are currently more than 200 known breeding pairs in Pennsylvania, including two nests in the Greater Pittsburgh area (near the Hulton Bridge in Harmar, and near Keystone Iron & Metal in Hays).
The bald eagles at the National Aviary were all injured in the wild and cannot be released. Although they can no longer fly, they serve as ambassadors for their species while on exhibit and during bird shows.