Biologists are trying to find the reason thousands of nesting birds have abandoned Seahorse Key off Florida's Gulf Coast.

The little blue herons, roseate spoonbills, snowy egrets, pelicans and other birds left the island all at once in May.

U.S. Fish and Wildlife Service biologist Vic Doig said what was once the largest bird colony on the state's Gulf Coast is now a "dead zone." Scientists have found no indication that disease, contaminants or predators are to blame.

The abandonment concerns biologists because it could have a ripple effect: Many bird species here return year after year to the same nesting sites.

Biologist also don't know how the disappearance will affect the island's other animals, some of which rely on the birds to survive.