conspirator vs stool pigeon

conspirator

noun
  • Part of a group that agree to do an unlawful or unethical act. 

  • One of a group that acts in harmony; a person who is part of a conspiracy. 

stool pigeon

noun
  • An informer to the authorities, especially one who gives up criminal associates. 

  • A pigeon tied to a stool by a piece of twine, used as a decoy to entice free-flying birds to the ground to be shot or trapped. 

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