Outside the mainland US, Hawaii also has feral pigs introduced to Oahu soon after Captain Cook's discovery of Hawaii in 1778, where they prey on or eat endangered birds and plants. The population of feral pigs has increased from 2million pigs ranging over 20 states in 1990, to triple that number 25 years later, ranging over 38 states with new territories expanding north into Oregon, Pennsylvania, Ohio, and New Hampshire. Some of these feral pigs have mixed with escaped Russian boars that have been introduced for hunters since the early 1990s.