The bald eagle's diet consists primarily of fish, though it can and will find other sources of food when fish is scarce. In fact, supplementing fish with other sources of food is an important part of a bald eagle's overall health. Small mammals, crustaceans and other small sea creatures are secondary sources of food for bald eagles.
Much like with humans, having a varied diet is important for a bald eagle to maintain its health. For example, an eagle can fully digest the small bones of a fish easily, but the bones of a mammal like a rabbit or mouse cannot pass through its digestive system. These bones are stored in a pouch in the eagle's throat until enough have accumulated that they form a small, hard pellet. The eagle then regurgitates this pellet. The act of regurgitation plays an important function in maintaining the health of the eagle's digestive system.
Bald eagles catch most of their prey themselves, but they have also been known to feed opportunistically on carrion. There is even evidence of eagles descending on the carcasses of animals that are being transported by hunters, or even taking food out of dumpsters and other outside garbage cans.