Peter Frederick, a research professor of wildlife ecology and conservation at University of Florida, has made many research trips over the years to Seahorse Key, a 150-acre island five miles off the coast of Florida that for decades has been home to thousands of nesting birds. That’s how he knew, from the moment he stepped off the boat on a visit one day this May, that something was very, very wrong. Wading bird colonies are cacophonous, but Seahorse Key was almost silent. “It was just the wind,” he recalls. The birds were gone, having apparently abandoned the island in one sudden mass exodus. Thick with mangroves, carpeted with spiky cactus, and infested with venomous snakes, Seahorse Key is “a pretty nasty place to be,” says Frederick. But its lack of appeal to humans and other mammals was precisely the reason birds flocked to the island in the first place. Until this spring, it had been home to the largest bird colony on the Gulf Coast of Florida: Double-crested...