Them being the same age makes it easier, believe it or not. Many of us successfully integrate different aged chickens a lot so what you are trying to do is certainly possible. That behavior at first is not that unusual either though it’s not always that bad. Sometimes it goes so smoothly you don’t notice anything and sometimes it gets violent. Usually it’s somewhere in between, some pecking and minor skirmishing. They are living animals and unpredictable.
It sounds like you’ve got two different things going on. First is what I call basic integration. Chickens can recognize who belongs in their flock and who doesn’t. Sometimes it’s not an issue but sometimes one or more will attack strange chickens invading their territory. Often it’s one or two hens that initiate the attack. The others may or may not join in. That’s where Guppy’s idea of housing them side by side for a week or so can help. It doesn’t always work but normally they come to recognize the other’s right to exist.
The other thing is the pecking order. Just like a herd of cattle or a pack of wolves, each chicken in a flock has to know its place in the social hierarchy so they know how to act when they meet. That’s called the pecking order for a reason. Once it gets worked out, the flock generally operates really smoothly, but working it out can sometimes get kind of violent. Your original five had the pecking order worked out but by adding new chickens you’ve messed that up. Now they have to work it out again. Don’t be totally shocked to see some skirmishing between your original flock members. With the pecking order being readjusted some may try to climb a little higher than they were before. They can be crafty like that.
What often happens when two chickens meet that don’t know how they rank, one tries to intimidate the other, generally by a peck but maybe something more serious. If one runs away, they are well on the way to sorting it out. There may be some chasing involved, and sometimes one goes out of its way to intimidate the other to reinforce that ranking, but usually it works out.
If one does not run away, it can get serious. That is a direct challenge and can lead to a fight. They will flare their neck feathers and jump at each other, trying to peck and claw. Even here it usually takes very little time for one to realize they are better off running away, so these normally end pretty quickly. But if they are evenly matched they may fight a while. The big danger comes in if one cannot run away and get away, even with some chasing. That’s why room is really important when you are integrating. If one gets caught in a corner or against a fence where it can’t run away, it may just hunker down and quit, but the other keeps attacking. That’s not good. Most of the time they work all this out, but occasionally you do hear a real horror story. But if you read through this forum with the thousands of people going through this process, you really don’t read that many stories of where chickens were actually killed. It really does work out most of the time, especially if they have room to run or just avoid the bullies to start with.
How long will it last? I wish I knew. Each chicken has its own personality and each flock has its own dynamics. We all have unique circumstances in how we house and manage them. I’ve never been through exactly what you are going through. I raise pullets with the flock. Mine normally integrate with the adults when they start laying. Until then, they form a separate flock.
I suspect yours will work out an accommodation in a few days at most where they get along but maybe form two separate flocks so they can avoid each other during the day. It might really help if you could house them side by side for a few days to help calm them down before they mix. Eventually they will form one happy flock but even then don’t be surprised if they form a sub-flock during the day part of the time. Each flock is different. They don’t all act the same.
I know this is long and rambling. Hopefully you can get something out of it that helps. Good luck!
