A flock of brown pelicans soars over Florida's Seahorse Key.

The island is a centerpiece of the Cedar Key National Wildlife Refuge, where more than a hundred bird species nest each year, including pelicans, ibis, and egrets.

Birds prefer this dry, viper-ridden island to other, more hospitable sites in the refuge because of the protection provided by the snakes, Lillywhite said.

This unusual relationship shows that the vipers aren't indiscriminate predators, he added, and demonstrates the crucial role that cottonmouths play in this vital ecosystem.

"[The cottonmouths] actually are an important, integral part of the system and probably are the reason that the birds keep nestingand do so successfullyon Seahorse Key," he said.

"This is unusual, but kind of cool, PR for the lowly snake."

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