“Men still live who, in their youth, remember pigeons; trees still live who, in their youth, were shaken by a living wind. But a few decades hence only the oldest oaks will remember, and at long last only the hills will know.” —Aldo Leopold, “On a Monument to the Pigeon,” 1947 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...