Three miles off Florida's west coast, the Milky Way blazes over Seahorse Key. Thousands of stars reflect in the glassy surface of the Gulf of Mexico, but Mark Sandfoss keeps his eyes fixed on the ground. He’s watching for snakes.

Checking the path ahead for cottonmouths is a good idea for anyone visiting this uninhabited island in the Cedar Keys National Wildlife Refuge. But Sandfoss isn’t hoping to avoid snakes. He’s hoping to find them.

And lately, they’re getting harder to find.

His headlamp rakes the wrack piled up at the high-tide line. At night, some of the island’s cottonmouths leave the wooded interior and head for the beach, possibly to forage for dead fish. Fish used to rain down on them from the trees courtesy of the nesting birds above, whose regurgitated offerings often missed the open beaks of their young and fell to the snakes below. For decades, the snakes feasted on these leftovers, growing up to a meter long and thick as baseball bats, proportions unheard of in the rest of the state.

Nearly everything about these cottonmouths, in fact, is unheard of. What is a snake that lives in swamps and rivers doing three miles out in the Gulf? How did a creature armed with venom for killing live prey come to rely on carrion? Why are the snakes not eating the birds, and vice versa? How do they survive on an island with no fresh water source? In the 1930s, famed ecologist Archie Carr wrote that "there is little about these island cottonmouths that I do understand." University of Florida researchers like Sandfoss have been studying these questions ever since, and the university has maintained a lab on the island since 1951. But now a new question has supplanted all of the others: What will the snakes do next? Because in April 2015, the birds roosting on Seahorse Key left, and they haven’t come back.