murmuration vs susurrus

murmuration

noun
  • An act or instance of murmuring. 

  • An emergent order in a multi-agent social system. 

  • A flock of starlings. 

susurrus

noun
  • A whispering or rustling sound; a murmur. 

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