Adding chickens to an existing flock requires more than just placing new birds in the coop and hoping all goes well. That's a recipe for disaster. Careful introduction means relative peace and harmony can prevail. For best results, bring in only birds that are the same size as your current chickens. Otherwise, the smaller chickens will be bullied and possibly killed.
Before adding any chickens to your existing flock, make sure the newcomers are free of disease or parasites. Always quarantine new birds, ideally for two weeks or more, before putting them in your flock. Feed and clean the new chickens' pens or cages after you've taken care of your existing birds. That way, you don't inadvertently bring any viruses or bacteria the new chickens may harbor into your birds' coop.
The Pecking Order
The pecking order governs chicken flock life, even if you keep only a few birds. That pecking order, from boss bird to most subservient chicken, doesn't usually change unless birds die, are removed or become sick. Adding new chickens upsets the pecking order, and it must be re-established in the flock.
Place new chickens in a cage with food, water and nesting material, and place the cage in your coop. Put them in there at night, when your other birds are roosting and getting some shuteye. They'll discover the new chickens in the morning, investigating them through the cage bars. If you let your chickens free-range, don't let the new birds out for a couple of days. If your chickens stay inside all the time, or you want to leave them in the coop for introduction, let the newcomers out the morning of the second day. Put out extra food and water, as members of the established flock will chase newcomers away from these essentials. Keep an eye on the flock after releasing the new birds. There might be some scuffles, but after a week or so the new pecking order should be in place.
If you've kept hens without a rooster, you and the girls are in for a new experience. The rooster mates with the hens and he protects them. Quarantine and introduce a rooster the way you would any other chicken. Once you let the rooster loose with the hens, he finds his role in the flock fairly easily. Putting more than one rooster in a flock can work, depending on the personality of the cocks and the number of hens. If you have sufficient hens that both guys have their own harems, it's a possibility. If you don't have enough hens or if one rooster is particularly aggressive, a cockfight to the death could occur. If you find roosters fighting, separate them and keep them separated.
It's possible that you've introduced dominant hens into your flock. That's especially likely to happen if you're merging one established flock with another. If fighting leads to bloodletting, remove the injured chicken immediately. Once chickens spot blood, they will peck a bird to death. In a large coop where hens have reasonable amounts of space, the behavior should improve. Since the dominant hens will attempt to keep their victims away from food and water, provide additional feeding and drinking options. If dominance gets out of hand, remove the bully, not the victim, for a few days. It's like a "time-out" for chickens.
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