den vs warren

den

noun
  • A small cavern or hollow place in the side of a hill, or among rocks; especially, a cave used by a wild animal for shelter or concealment. 

  • Synonym of fort (“structure improvised from furniture, etc. for playing games.”) 

  • A comfortable room not used for formal entertaining. 

  • A group of Cub Scouts of the same age who work on projects together. 

  • A squalid or wretched place; a haunt. 

verb
  • To ensconce or hide oneself in (or as in) a den. 

warren

noun
  • The system of burrows where rabbits live. 

  • The right to maintain and hunt an area of small beasts, similar to a free warren, but with certain limitations, such as restricting the right to hunt on parts of the land held by freeholders. 

  • A mazelike place of passages and/or rooms in which it's easy to lose oneself; especially one that may be overcrowded. 

  • A place legally authorized for the keeping, breeding and hunting of beasts of warren, especially rabbits. 

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