duck hawk vs kestrel

duck hawk

noun
  • peregrine falcon (Falco peregrinus) 

kestrel

noun
  • Any of various small falcons of the genus Falco that hover while hunting. 

  • The common kestrel, Falco tinnunculus. 

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