Integration of new chickens to an established flock is no joke. When introducing new birds regardless of their age or size you will probably see a lot of the pecking, chasing around, some screaming and running, some chickens puffed up like a big ball, some starring and growling at each other and fighting; sometimes to the point that it brings blood if no preparations have been made ahead of time. Chickens have been known to peck each other to death. If the less dominant runs away after that peck, most of the time all is good in the flock. If the new birds challenge the pecking order then a fight will occur, and things can turn out very badly. That's why preparation is needed for bringing in the new guys.
The experience of integration of new chickens can be very irritating or fun, it all boils down to preparation. It can cause disorder and chaos to the whole feel of the farm or backyard. You'll either hear serene sounds of a peaceful flock or a flock screaming in terror. And instead of having a peaceful pleasant visit with the chickens, you can find yourself grimacing, chasing, and aggravated. Trust me, preparation will keep the peace in your flock and peace in your soul.
We move a lot of chicks at our farm. We integrate chicks that are one week old with some that are two and three-week old's; two months old and up goes with other pullets and cockerels and some 3 months and up, and hens and cocks get integrated into breeder flocks. During these times we have to keep notes on where we put all new guys and gals, or we can have some serious problems.
Problems of Integration
One problem with integration is the constant pecking on the new gals and guys. The pecking can peck the feathers off and reveal naked spots on their bodies. When flesh is revealed it's like a target for chickens, blood can be drawn very quickly from these naked places which can lead to cannibalism. Once they see blood all the other chickens alone with the dominant one will begin pecking. This is how cannibalism begins and once they pick up the habit it's difficult to change the habit.
Starvation is another symptom that can come with the integration. Whichever chicken is at the bottom of the pecking order is not allowed to eat without permission. They will always have to wait until the more dominants have finished eating, and sometimes there are not enough stations so the poor chicken at the bottom of the pecking order barely gets to eat or drink. They can be starved to the point of death.
Adult chickens will pick on little chickens. Chickens have different personalities some will be fine with the little ones, some even adopt them, but then you have chickens who find satisfaction in being boss and loves to bully over something smaller. If you're intent on mixing young smaller ones with the big guys, making preparations ahead of time will make the transition easier and peaceful for you and the flock.
Chickens have their own hierarchy and the way it's established is not always pleasant. Everyone has to know their place, and once they know their place they have to stay in their place. It actually works well if the owner of a flock knows the problems it presents and addresses them.
I have a Plymouth Buff Rock Bantam hen who thinks she's the biggest, meanest girl on the block this side of South Carolina. Trust me, no matter where she roams on our property she rules or be ready to have the back of your head chewed off by this little peep squeak bantam who just knows she's large. It's not about size, it's about attitude. Chickens read body language, they have staring contests, and if all else fails they'll jump up with both feet to let everyone know who's the boss. They sometimes growl to give a warning. Some will back off and give them their space, but you always have those that will challenge their claim. Whenever new guys come into their space they will not be welcomed. They will most likely be at the bottom of the pecking order unless they're a bit sassy.
Usually, chickens can recognize who is the dominant ones in the flock. It's the more mature ones that possess dominance due to their attitude more than size. My little 4-year-old bantam, Princess, rules the group that she lives with and they are much bigger than she is. When Princess comes to the bowl they move out of her way and if they don't, she reminds them with a good stiff peck on the back of the head. Once the pecking order is established and the birds are all mature, you won't see this happen too often.
A rooster as the head of the flock can bring an element of peace unless he's an aggressive rooster. Usually, aggressive roosters are not only mean with people but also mean to his girls. I really don't care much for those kinds of roosters. Here at our farm, they are usually replaced with one that's gentle with his girls. We have a saying here in our breeding programs; "aggressive breeder breeds aggression."
A good rooster will break up fights among the girls. When a new hen is brought among the ranks, he will protect her up to a point, but she still needs our protection when he's off in his own world.
Preparation for Integration
A section of their own wired off for safety
The best ideal is to allow the young ones to grow up beside the bigger and older established ones inside their own protective cage close enough to touch, but not hurt.Once the young chicks reach 12 weeks of age it would be normal for both the youth and the older ones to mingle because they've already had plenty of time to establish the chicken social system. Chicken society is generally not kind. There will always be skirmishes upon an integration of flocks. If chicks are integrated when they are approximately the same size as the established flock there will be less trouble.