sparrow hawk vs windhover

sparrow hawk

noun
  • The American kestrel or American sparrowhawk (Falco sparverius), a small falcon found in North and South America that preys on grasshoppers and small animals. 

windhover

noun
  • The common kestrel (Falco tinnunculus). 

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