The din created by thousands of nesting birds is usually the first thing you notice about Seahorse Key, a 150-acre mangrove-covered dune off Florida's Gulf Coast. But in May, the key fell eerily quiet all at once. Thousands of little blue herons, roseate spoonbills, snowy egrets, pelicans and other chattering birds were gone. Nests sat empty in trees; eggs broken and scattered on the muddy ground.