pecking order vs social stratum

pecking order

noun
  • The natural hierarchy of social status and dominance occurring in a group of birds. 

  • The usually informal hierarchy of authority or command, often partial or approximate, as determined by the especially natural propensity for domination of different members of a specific group over each other, such as older brothers and sisters over their younger siblings. 

social stratum

How often have the words pecking order and social stratum occurred in a corpus of books? (source: Google Ngram Viewer )