forktail vs swallowtail

forktail

noun
  • Any of various small insectivorous birds in the genus Enicurus. 

  • A salmon in its fourth year's growth. 

swallowtail

noun
  • Any of various butterflies of the family Papilionidae, many of which have a long extension to each hindwing. 

  • A type of tailcoat with two long tapering tails.ᵂᵖ 

  • The forked tail of a swallow. 

  • Anything, such as a burgee, of a similar forked shape. 

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