Hunting is primarily regulated by state law; additional regulations are imposed through United States environmental law in the case of migratory birds and endangered species. Regulations vary widely from state to state and govern the areas, time periods, techniques and methods by which specific game animals may be hunted. Some states make a distinction between protected species and unprotected species (often vermin or varmints for which there are no hunting regulations). Hunters of protected species require a hunting license in all states, for which completion of a hunting safety course is sometimes a prerequisite.