The endangered American crocodile seems to have made a strong comeback in the last 20 years, researchers say. When it was listed as an endangered species in 1975, there were only 20 nesting females living within a 20-square-mile area in Miami-Dade County and the Florida Keys. Now there are an estimated 500 crocodiles in south Florida, including up to 50 nesting females, biologists from the University of Florida announced yesterday. Still, some scientists caution that crocodiles aren’t out of trouble yet and are likely to be hurt by sprawling development in Florida.