folk song vs warble

folk song

noun
  • folk music 

  • A song, in any folk style, originating among ordinary people, usually non-professional musicians, and handed down by oral tradition. 

  • A song in the style of folk music, a modern genre of popular music. 

warble

noun
  • The sound of one who warbles; singing with trills or modulations. 

  • In naval mine warfare, the process of varying the frequency of sound produced by a narrowband noisemaker to ensure that the frequency to which the mine will respond is covered. 

  • A small hard swelling on a horse's back, caused by the galling of the saddle. 

  • A lesion under the skin of cattle, caused by the larva of a bot fly of genus Hypoderma. 

verb
  • To cause to quaver or vibrate. 

  • To modulate a tone's frequency. 

  • To be quavered or modulated; to be uttered melodiously. 

  • To sing like a bird, especially with trills. 

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