In May 1850, a 20-year-old Potawatomi tribal leader named Simon Pokagon was camping at the headwaters of Michigan’s Manistee River during trapping season when a far-off gurgling sound startled him. It seemed as if “an army of horses laden with sleigh bells was advancing through the deep forests towards me,” he later wrote. “As I listened more intently, I concluded that instead of the tramping of horses it was distant thunder; and yet the morning was clear, calm, and beautiful.” The mysterious sound came “nearer and nearer,” until Pokagon deduced its source: “While I gazed in wonder and astonishment, I beheld moving toward me in an unbroken front millions of pigeons, the first I had seen that season.”

These were passenger pigeons, Ectopistes migratorius, at the time the most abundant bird in North America and possibly the world. Throughout the 19th century, witnesses had described similar sightings of pigeon migrations: how they took hours to pass over a single spot, darkening the firmament and rendering normal conversation inaudible. Pokagon remembered how sometimes a traveling flock, arriving at a deep valley, would “pour its living mass” hundreds of feet into a downward plunge. “I have stood by the grandest waterfall of America,” he wrote, “yet never have my astonishment, wonder, and admiration been so stirred as when I have witnessed these birds drop from their course like meteors from heaven.”