red grouse vs willow ptarmigan

red grouse

noun
  • A medium-sized bird, Lagopus lagopus scotica, of the grouse family, found in heather moorland in Great Britain and Ireland. 

willow ptarmigan

noun
  • A willow grouse, Lagopus lagopus. 

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