In her book Hobby Farms Chickens: Tending a Small-Scale Flock for Pleasure and Profit, Sue Weaver describes the pecking order very well:
"In any flock of chickens, there are birds who peck at other flock members and birds who submit to other flock members. This order creates a hierachical chain in which each chicken has a place.
The rank of the chicken is dependant upon whom he pecks at and who he submits to. He ranks lower than those he submits to and higher than those he pecks at.
A flock of chickens generally have their pecking order up and running by the time they're five to seven weeks old.
Pullets and cockerels maintain seperate pecking orders within the same flock; as do hens and adult roosters.
Hens automatically accept higher ranking roosters as superiors; but dominant hens give low-ranking cocks and uppity young cockerels a very hard time.
In a closed flock with an established pecking order, there is very little infighting. Each chicken knows his or her place, and except among some roosters, there is suprisingly little jostling for position. Dominant chickens signal their superiority by raising their heads and tails and glaring at subordinates, who submit by crouching, tilting their heads to one side, and gazing away - or beating a hasty retreat."