francolin vs ptarmigan

francolin

noun
  • Any of various terrestrial partridges of several genera in family Phasianidae, principally in tribe Gallini. 

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 francolin and ptarmigan occurred in a corpus of books? (source: Google Ngram Viewer )