feather one's nest vs improve

feather one's nest

verb
  • To achieve benefits, especially financial ones, by taking advantage of the opportunities with which one is presented; to amass a comfortable amount of personal wealth. 

improve

verb
  • To make (something) better; to increase the value or productivity (of something). 

  • To become better. 

How often have the words feather one's nest and improve occurred in a corpus of books? (source: Google Ngram Viewer )