turtle dove vs turtle-dove

turtle dove

noun
  • Any of several (species of) birds, called by this traditional name, mainly in the genus Streptopelia, of the family Columbidae (doves and pigeons, which also included the extinct passenger pigeon, dodo, and solitaire). 

turtle-dove

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