What is the ‘Pecking' Order?

The bird at the top of the pecking order will have first access to water, food, the best roosting place and so on. The bird at the bottom of the pecking order has the least ‘rights' in the flock and will usually be the last to the food and will ‘skirt' around food that is scattered for them, nipping in to grab a beak full when possible. She (or sometimes he) has last rights to food and other resources.
Once pecking order is established and birds are fully grown, it usually remains unchallenged.
When introducing new chickens to the flock, the pecking order is upset and needs to be re-established. This is very stressful for the flock and it is usually best to keep birds of similar ages together to prevent younger birds getting bullied too much or older birds being constantly challenged for their position in the pecking order as younger birds mature.