covert vs down feather

covert

noun

  • A feather that covers the bases of flight feathers. 

  • A covering. 

  • A disguise. 

  • Area of thick undergrowth where animals hide. 

  • A hiding place. 

adj

  • Under coverture. 

  • Secret, surreptitious, concealed. 

down feather

noun

  • A feather of the down of a bird 

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