Snake-Ridden Florida Island Provides Unlikely Haven for Birds

Blake de Pastino in Seahorse Key, Florida
National Geographic News
April 16, 2007

On a remote Florida island crawling with venomous snakes, a scientist believes he has discovered an unusual truce between predator and prey.

The tiny island of Seahorse Key on the central Gulf Coast is renowned among researchers for its teeming numbers of poisonous cottonmouth snakes.

"The population of cottonmouths on Seahorse Key is large and dense—I mean a lot of snakes," said Harvey Lillywhite, a University of Florida biologist who has been studying the island.

About 600 vipers slither around the 165-acre (67-hectare) island, Lillywhite estimates—in some areas with an average of 22 cottonmouths on every palm tree-covered acre. (See photos of the island's snakes.)

Scientists have long puzzled over how so many snakes can thrive on an island with no fresh water and only a scant number of mammals to prey upon.

The secret to the snakes' success, Lillywhite believes, is Seahorse Key's other inhabitants—tens of thousands of seabirds that nest there from spring to fall.

But the snakes aren't eating the birds, the scientist says—instead they live almost exclusively on the huge amounts of dead fish that the birds drop, vomit, and excrete every year.

"There's this disgusting carrion of fish that falls down for the snakes, and the snakes essentially scavenge on it," Lillywhite said.

In return for this fishy bounty, the cottonmouths not only refrain from eating the birds, the scientist added, they also seem to deter other would-be predators from raiding the nests.

The result is a win-win for both predator and prey that Lillywhite said he has not seen on any other island.

"There are a lot of island systems where there are birds and snakes. Of all the cases I know, the snakes are predators on the birds," he said. "At Seahorse Key, it's totally different. Here the snakes do not eat the birds, and the birds are providing food for [the snakes]. So it's a pretty cool system."

Cottonmouth Island

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