I don't have to tell you what kind of bird this is. Adult Bald Eagles are massive raptors with wing spans between 6 and 7 feet. In flight, their brown bodies can appear almost black, but their head and tail feathers are a clean white. What you may not know, is that Bald Eagles don't start out looking like this. According to various sources, it takes at least 4 years for Bald Eagles to obtain full adult plumage.
Immature birds have variably mottled wings and lack the distinctive white head and tail of adults. 1st-year birds (like above) have dark bellies with obvious white wing pits. 2nd-year birds (below) have lighter bellies and dark heads. 3rd-year birds (not pictured) are similar, but typically show more white in the head.
The 4th-year bird below shows limited white wings spots, and a mostly white tail framed with brown.
Feather color aside, immature Bald Eagles are similar in structure and size to their parents and show the same orange legs and large, hooked bills.
Note: The ages of the above birds are my best guesses -- if you have a different opinion of any of these birds, I'd love to hear it. The birds were photographed in mid-February in Bath, Maine.