plumule vs tailfeather

plumule

noun
  • The aftershaft of a feather. 

  • One of the featherlike scales of certain male butterflies. 

  • The first bud, or growing point, of a plant embryo, situated above the cotyledons. 

  • A down feather. 

tailfeather

noun
  • Any of the feathers of a bird's tail. 

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