gyrfalcon vs kestrel

gyrfalcon

noun
  • Falco rusticolus, a large bird of prey that breeds on Arctic coasts and islands of North America, Europe and Asia. 

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 gyrfalcon and kestrel occurred in a corpus of books? (source: Google Ngram Viewer )