Louisiana wildlife have so far largely escaped harm from the oil spill that shut down 100 miles of the Mississippi River last week. But biologists remain nervous as the oil slick heads downstream toward the Delta National Wildlife Refuge and neighboring marshy areas, where nearly 100,000 migratory birds will alight in the fall. Barriers are being erected to keep oil away from marshes, and folks are keeping fingers crossed that currents will push the grease to the banks of the river before it reaches the wetlands. Meanwhile, the U.S. Fish and Wildlife Service has set up propane cannons designed to scare animals away from oily areas. The agency has so far found nearly 60 oil-coated ducks and other birds, as well as a beaver and a muskrat; nearly all of the animals were weakened but still quick enough to elude capture by biologists trying to get ’em cleaned up.