In the early 1800s, a naturalist named Alexander Wilson was traveling in Kentucky when the sky suddenly became dark. Wilson believed, he later wrote, that it was “a tornado, about to overwhelm the house and everything round in destruction.”

When Wilson got his wits back, he realized the sun had been blotted out by passenger pigeons.

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