pecking order

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A hierarchy among a group, as of people, classes, or nations.
• noun The social hierarchy in a flock of domestic fowl in which each bird pecks subordinate birds and submits to being pecked by dominant birds.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the organization of people at different ranks in an administrative body