ptarmigan vs thunderbird

ptarmigan

noun
  • Any of three species of small grouse in the genus Lagopus found in subarctic tundra areas of North America and Eurasia. 

thunderbird

noun
  • A mythological bird, often associated with stormy weather, especially in various indigenous North American mythologies. 

  • The golden whistler, an Australian insectivorous songbird (Pachycephala pectoralis, formerly Pachycephala gutturalis), whose male is conspicuously marked with black and yellow, and has a black crescent on the breast. 

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