free spirit vs rara avis

free spirit

noun
  • One who is a nonconformist or who does not recognize the authority of others. 

  • A member of the organized heretic movement Brethren of the Free Spirit in opposition of the Catholic Church from the 13th to 15th centuries. 

rara avis

noun
  • A rare or unique, and thus outstanding or unusual, person or thing; a rare bird. 

  • A rare species or type of bird. 

How often have the words free spirit and rara avis occurred in a corpus of books? (source: Google Ngram Viewer )