Why do chicks peck each other and how do we stop them?

What you are observing is the "pecking order" which is established starting within the first week of hatch. The older stronger birds will establish dominance over the younger weaker chicks. One way to help reduce this is to use red or blue light as a heat source in the brooder box. I also hang an old CD on a wire to use as a distraction ploy, the aggressive birds will see reflection from the moving CD and peck at the disc instead of the other birds. When there is a great difference in age and size it is always best to separate the birds into groups of similar age and weight but that is not always practical in a small area.