Hens will peck and fight with each other to set the pecking order. In some cases, hens in a flock will gang up on a few individuals and physically harass them each day. Hens can peck other hens to death when not kept in check.
Recognize the Problem
Your flock will set a pecking order and a few hens will take a dominant role when roosters are not present. Look for these dominant hens and keep an eye on them as they take lead roles. They will peck at other hens, which is normal to a small degree. Monitor pecking and look for hens who are losing large amounts of feathers and bleeding. These hens are at risk and you must take steps to protect them and curb the problem.
Remove abused hens to a quarantine pen where they will heal and recover. Apply bag balm to areas with broken skin and keep the hens out of reach from the aggressive hens. Keep the hens within sight of the other hens to make them accustomed to each other without the option for physical contact. This is not entirely necessary but it removes the option for aggression while maintaining an aspect of socialization.
Reset the Pecking Order
Remove the aggressive hens from the flock to reset the pecking order. With the aggressive hens gone, the remainder of the flock will work to form a new order with new lead hens. Reintroduce the aggressive hens after a week and they will earn their way back into the flock. If the hens continue acting out, place them in an isolation pen directly against your normal run. They will have a full view of the other hens but no power to act with aggression. Eventually, they will lose the desire to attack and fight with other hens.
Bring in a Rooster
Hens act with aggression in the absence of a rooster. Add a rooster to your flock and he will dominate the pecking order and check the aggression of hens. Depending on the size of your flock, you may consider multiple roosters. Figure one rooster for every 10 hens in your flock. Although the rooster will reduce aggression between hens, he does pose a risk of aggression toward humans and your other animals.