Keeping chickens is an addictive hobby and once you get a few birds, often you start thinking about others you'd like to introduce to the flock. The big problem is that few people realise the problems that potentially can occur.
Chickens are best kept in small social groups. This is how they live in the wild and just about everyone has heard of the phrase 'pecking order' or 'hen pecked' and it's just that. There is a definite order within a flock of hens, every hen knows her place. If you are lower down the 'pecking order' and you are feeding, you'll get a short sharp peck, a kind of 'what do you think you're doing, I'm senior to you, now clear off'. Every hen has their place and in the wild, nature is performing its own selection, ensuring the future survival of the species. Over thousands of years of evolution, hens have learned to avoid predation by not fighting over their food.
Introductions can be made and are sometimes necessary, it's often very hard on us the owners to see such 'cruelty' happening in the pen but often it is necessary for the first week or two until the 'pecking order' is established again.
Many birds are reared in pens with the same colour and type of bird. So for example, a breeder keeps 300 Black Rock hybrid hens together in a pen until they are sold. They go to their new home and for the first time in their lives, meet Buff Orpingtons who have never met anything other than a Buff Orpington before. This is a huge shock for them, they have only ever seen the same type and colour as themselves. Understandably, the birds are shocked and will be aggressive towards the new birds who themselves are stressed at being moved from their own environment into a new place. If birds have been mixed with other colour birds, they won't find it so difficult to accept a different colour bird.
Young growers that are introduced to a flock are normally very submissive. They don't really know how to react and can get a real pecking from older hens. It is not a good idea to mix different age groups of chicks and growers as it can cause disease in the younger birds who haven't yet built up immunity but also, the older birds (often twice the size if only a month or two apart) can easily kill the youngsters.
One of the best ways to introduce new hens is to put them with your existing flock in a house and run that is new to both of them with lots of space and lots of food distractions. Neither of them has an established territory and this can sometimes work well, especially if they have lots of space to stay away from one another at first. This isn't always possible though.
The second way for a more 'normal' set up is to put newcomers in a run / small fenced off area within the existing run so that they can see one another and get used to one another but with the safety of the wire between them. Once a week has passed, the newcomers can be popped into the coop at night, but make sure you are there at first light in the morning to rescue them if blood is drawn.
Here is my top ten list of things to remember when introducing new birds:
Expect a certain amount of bullying. It's hard to watch but often after a week, sometimes a little longer, things suddenly click into place and the pecking order is re-established.