covert vs den

covert

noun
  • A hiding place. 

  • A covering. 

  • A disguise. 

  • A feather that covers the bases of flight feathers. 

  • Area of thick undergrowth where animals hide. 

adj
  • Under coverture. 

  • Secret, surreptitious, concealed. 

den

noun
  • A squalid or wretched place; a haunt. 

  • 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 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. 

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

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