What is dominance hierarchy or pecking order?

The dominance hierachy is a social system to keep animals in a group from constantly fighting. It is a way for them to get along and know exactly where their place in the group is. This was first studied in chickens, so it is also called a pecking order. The alpha chicken has the right to peck every other chicken in the flock to make it move. The second chicken can peck every chicken except the alpha chicken. This goes down through the flock until you reach the omega chicken, which cannot peck anybody.