When adding new chickens to an existing flock, gradual introductions that take place when the chickens are most relaxed will reduce the chance of fights and injuries. Providing ample space, distractions for the flock and helping the newcomers stay on equal footing will all ease this transition.
Protect the Existing Flock
When bringing in new birds, the most important step is preventing disease. Illnesses in birds can spread quickly, so keep any new chickens quarantined away from the established flock for one to two weeks and watch them for signs of health problems. You also may want to consider treating all the chickens for worms and parasites during this time.
This separation period is helpful for the newcomers as well. It allows them time to become acclimated to their new environment and recover from whatever stress resulted from their transportation, giving them better health and confidence for the introductions. Never introduce a sick or injured bird into your flock.
Even the Odds
New chickens will have to establish their place in the pecking order with every bird in the flock, putting them at a disadvantage in introductions. For easiest transition, it is best to have the new birds slightly outnumber the flock.
Introductions also will be smoother if they take place on neutral ground. Either set up temporary introduction pens that are new to both groups, or place the established flock in temporary housing and allow the new chickens time to get used to their permanent home without interference.
Provide your chickens a chance to acclimate before they come in physical contact with each other by keeping them in sight of each other with a physical barrier, such as mesh chicken fencing, between the two groups.
While the chickens are separated by a barrier, feed both flocks at the same time. Eating is a distracting, calming and communal activity for chicken flocks. You also can place longer greens through the fence so both sides can pick at the same food.
When the time comes to combine the flocks, ensure their are plenty of distractions, like ample food and water in multiple locations, dust bathing materials, straw and roosting perches to encourage activities other than sparring.
Start by combining your chickens for just five minutes at a time once every half hour. While some squabbling is inevitable, you should watch closely for signs of injury. Slowly increase the amount of time the chickens spend together until they feel comfortable enough to preen and scratch at the dirt in each other's presence.
Once the chickens are spending the night together, block any windows to the chicken coop to keep it dark. This will encourage the chickens to sleep until you let them out and discourage unsupervised fights before you are up.
In most cases the new hierarchy will be established within two weeks.
Special Consideration for Roosters
Roosters have evolved to protect the flock, often making them more alert to threats and aggressive than hens, especially if there is more than one rooster in the flock. You may need to blunt the larger rooster's spurs and trim his flight feathers to reduce the chance of injury to the smaller bird.
Some roosters may never accept another rooster in their territory, requiring you to keep them separated and alternate who is with the flock and who is in a separate area.
If your roosters are fighting continuously, you may consider adding a tom turkey, goose or Pekin duck to your flock. As prey animals, these birds have evolved to dislike loud noises that could attract predators. Since they are larger than the chickens, they will often break up fights.
A broody hen, a hen who has been sitting on eggs, can be encouraged to foster chicks who don't belong to her. The process is easiest if the hen has been setting for at least two weeks and the chicks are not much more than 1 day old.
Place the new chicks under the broody hen at night so she can listen to them throughout the night and hopefully respond to them in the morning as if her eggs had hatched.
Watch for signs that the hen has accepted the chicks including clucking continuously, spreads her wings over the chicks, picking up and dropping food for the chicks or rushing to a chick who makes a distress call.
Hens will care for their chicks until they are 12 to 16 weeks old, defending them from the aggression of other birds and allowing for a smoother transition into the larger flock. If there isn't a hen raising the chicks, wait until they are approximately the same size as the existing birds and follow the gradual introduction plan to include them in the flock.