An old lie, often repeated.
Bald Eagles are amazing animals. They take care of their offspring better than most humans are they mate for life and are faithful, unless their mate is killed or dies.
Just as this Bald Eagle stole the fox's successful dinner-catch, Bald Eagles are infamous for chasing down an Osprey that'd caught a fish and bullying it for release—always with success—I've seen it myself.
Franklin's reference to the Kingbird also rings true. The little Kingbird is fearless. The letter:
“For my own part I wish the Bald Eagle had not been chosen the Representative of our Country. He is a Bird of bad moral Character. He does not get his Living honestly. You may have seen him perched on some dead Tree near the River, where, too lazy to fish for himself, he watches the Labour of the Fishing Hawk; and when that diligent Bird has at length taken a Fish, and is bearing it to his Nest for the Support of his Mate and young Ones, the Bald Eagle pursues him and takes it from him.
With all this injustice, he is never in good case but like those among men who live by sharping & robbing he is generally poor and often very lousy. Besides he is a rank coward: The little King Bird not bigger than a Sparrow attacks him boldly and drives him out of the district. He is therefore by no means a proper emblem for the brave and honest Cincinnati of America who have driven all the King birds from our country…"
“I am on this account not displeased that the Figure is not known as a Bald Eagle, but looks more like a Turkey. For the Truth the Turkey is in Comparison a much more respectable Bird, and withal a true original Native of America… He is besides, though a little vain & silly, a Bird of Courage, and would not hesitate to attack a Grenadier of the British Guards who should presume to invade his Farm Yard with a red Coat on.”
—Ben Franklin