a cappella vs birdcall

a cappella

noun
  • A vocal performance with no instrumental accompaniment. 

adv
  • In a manner of a choir with no instrumental accompaniment; literally, "in the style of the (Sistine) Chapel (in Rome)", such as a musical Mass done a cappella. 

adj
  • Singing solely or mainly without instrumental accompaniment. 

  • Related to a form of purely vocal music mostly associated with American college performance groups. 

birdcall

noun
  • Any vocalisation of a bird. 

  • An imitation of this cry. 

  • A device used to imitate this cry, so as to lure and catch birds. 

verb
  • To imitate the cry of a bird. 

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