partridge vs ptarmigan

partridge

noun
  • Any bird of a number of genera in the family Phasianidae, notably in the genera Perdix and Alectoris. 

ptarmigan

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

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