Rocky Mountain canary vs donkey

Rocky Mountain canary

noun
  • A burro. 

donkey

noun
  • A bad poker player. 

  • A fool. 

  • A small auxiliary engine. 

  • A domestic animal, Equus asinus asinus, similar to a horse. 

  • A stubborn person. 

How often have the words Rocky Mountain canary and donkey occurred in a corpus of books? (source: Google Ngram Viewer )