Quoted from the How Stuff Works website (www.howstuffworks.com)
The U.S. national bird, known for its dramatic wingspan of up to 8 feet (2.4 meters) and its 200 mph (320 kph) flight speed, has another superlative ability: nest building.
Bald eagles, like most other birds, build their nests in trees. Unlike most other birds, bald eagles build nests that can break those trees.
A typical bald eagle nest is big: up to 5 feet (1.5 meters) in diameter and up to 6 feet (1.8 meters) deep. But that's nothing compared to the largest nest ever discovered. One found in Florida in the 1960s had a weight of more than 2.2 tons (2 metric tons), a diameter of 9.6 feet (2.9 meters), and a depth of 20 feet (6 meters). Another that fell out of a tree in Ohio in the 1920s was 8.5 feet (2.6 meters) across, 12 feet (3.6 meters) deep, and weighed about 2 tons (1.8 metric tons).
How does a pair of birds build a nest that massive? Slowly. Bald eagles use the same nest year after year, sometimes for decades, and they're constantly adding twigs, branches, moss, feathers and other nesting material. The Ohio Department of Natural Resources estimates that bald eagles, likely more than one mating pair over time, had been building and living in that 2-ton Ohio nest for about 35 years before it fell.