bird's nest vs birdhouse

bird's nest

noun

  • A structure built by a bird as a place to incubate eggs and rear young. 

  • Any of several plants having some resemblance to the typical nest of a bird, eg, wild carrot (Dauca carota) 

  • Any tangle of fibers resembling a typical nest of a bird. 

birdhouse

noun

  • A small house for birds. 

  • An aviary. 

How often have the words bird's nest and birdhouse occurred in a corpus of books? (source: Google Ngram Viewer )