The end of South Litchfield Beach is a sprawling spit of sand, where tides and waves combine to create a wide beach that is popular with people and nesting birds. So Robert Purkerson wasn’t completely surprised when he found a bustling colony of Least Terns there on a beach walk last spring.   But unlike many other tern nesting sites on the South Carolina coast, this colony hadn’t been fenced off yet. The tiny terns numbered in the hundreds and were nesting directly on the beach, not far from people walking the shoreline and dogs chasing balls through the surf.  Purkerson and others who had discovered the colony reported it to Audubon and the South Carolina Department of Natural Resources (SCDNR).…