A bald eagle was circling not far above the cupola on George Washington’s Mount Vernon when we visited this week. From the driveway at the end of the house tour, a sharp call could be heard, something like an osprey’s. It was a cold, early spring day, toward closing time, and most of the guests were ready to move on. A federal security guard standing by a wrought-iron stanchion did not look up. Nor did anyone else, as the national bird wheeled around several times and headed down river, still calling.