Mass whale and dolphin strandings are common in the Keys. But turkey vultures? Those scary birds that circle above their lifeless prey, then dive-bomb to feast on dead flesh, roadkill and rotting stuff in the dumpster down below? It’s rare, but vulture strandings are a thing. One happened on Monday, according to local marine mammal rescue nonprofit Dolphin Research Center on Grassy Key. Workers at the center were out conducting research in the water on the Gulf of Mexico side of Marathon, in the Middle Keys, when they noticed a vulture struggling in the water just offshore. They called the Mar…