The journals of many early explorers contain similar passages. The passenger pigeon would sweep across the eastern United States in vast flocks, feeding on chestnuts and acorns as they traveled. As Wilson gazed at his passenger pigeon flock, he tried to figure out how many birds it contained. From one side to the other, it was a mile wide. It streamed overhead like a feathered river for more than four hours. Based on that information, Wilson guessed that it contained over 2.2 billion birds–”an almost inconceivable multitude,” he wrote, “and yet probably far below the actual amount.”