Most of the basic rules written for Unity 2D are from the youtube tutorial series by Board to Bits Games
First episode of the series
Descriptions of each rule and detailed instructions for how each one should work
Flocking Project Description
Fish and Bird flocking that includes banking, independent behaviors and ray casting obstacle avoidance
Flocks, Herds, and Schools: A Distributed Behavioral Model
A multilayered human flocking simulation, using pathfinding and other behaviors that use weights to determine the steering forces
A Multi-Layered Flocking System For Crowd Simulation