Activists from Mexico, the United States and Canada are asking the U.N. World Heritage Committee to include the Monarch butterfly wintering reserve on a list of sites considered in danger. UNESCO designated the 139,000-acre (56,259 hectare) reserve in the mountains west of Mexico City a World Heritage site in 2008. Monarchs from the U.S. and Canada migrate 3,400-miles (5,470-kilometers) each year to winter in the forest reserve. But the number of…