On a cloudy day last July, a group of conservationists near St. Louis, Missouri, hopped into a small boat armed with cardboard boxes and yellow leg bands just a few millimeters in diameter. They motored toward a concrete barge anchored offshore, where they rounded up 18 squeaking, fuzzy, golf-ball-size Interior Least Tern chicks into a box for banding. “Good luck, little guys,” said Jeff Meshach, deputy director of the St. Louis-based wildlife hospital World Bird Sanctuary, during a livestream of the banding. After placing a delicate yellow band on one leg of each chick, he gently spilled them back onto the barge—their nesting habitat parked in a pond along the Mississippi River. When they grow up, it's possible they'll return to nest here; the bands let the conservationists keep track of them. “Hope all of you come back next year and raise your own kids here," he said. When Interior Least Terns were listed under the Endangered Species Act in 1985, the birds had...