American bison vs North American bison

American bison

noun

  • A bovine mammal, a species of bison, Bison bison. 

North American bison

noun

  • Bison bison (“B. bison”); Synonym of American bison (“American buffalo”) 

How often have the words American bison and North American bison occurred in a corpus of books? (source: Google Ngram Viewer )