Why they fight
An established flock considers their present housing to be THEIR home. They will fight to maintain dominance over the "intruders".  Existing flocks have an agreed "pecking order".  The chicken "society" has been established and each bird "knows" where they fit in this pecking order.  The dominant hen gets the first food, the best grass, the best spot on the roost, her favorite nest box, etc.  The lowest hen on the pecking order usually gets picked on by all the others.  This sound mean, but it's the way chicken society functions.  Roosters consider the hens in the flock to be "his" and will protect them from new roosters.

Here are a few things you can do to make the operation less difficult:

1. Put them ALL in a new enclosure
If possible, put the existing flock and the birds to be introduced in a NEW enclosure that neither can claim to be theirs at the start. In our new barn, we have sixteen 10ft x 10ft pens. We always keep ONE pen empty just for this purpose. We combine flocks in that pen, then use one of the pen(s) they came from as the new "empty" pen. We recently placed 15 excess bantam cockerels of various breeds in one pen (new to all of them) to free up needed cage space. The fact that the pen wasn't anyone's home turf limited fighting to about 24 hours, after which they all get along pretty well. 

2. Partition an existing enclosure
If you don't have an open pen to combine the birds, divide the existing pen and place the older birds in one side and the new birds on the other side. Use wire for the divider so the two groups can see each other and interact.  Make sure that each partition has their own food and water available.  Leave them like this for at least a week, or until the birds quit trying to fight with each other through the wire, then remove the partition. Watch the birds to identify the "fighters". If they haven't stopped fighting within two weeks, they never will and it would be best to cull the aggressive birds before combining the flocks. 

3. Use a temporary cage within the main enclosure 
If you only have one or two birds to introduce to an established flock, put them in a cage large enough for them to be comfortable with food and water.  We have several 2ft x 3ft wire "rabbit cages" we use for this purpose. Place the cage in the pen or other enclosure so that the existing flock can see them and interact with them but not hurt them until they get used to the newcomers.

4. Keep age and physical size as close as possible
If the new birds to be introduced are younger than the existing flock (the usual situation) and there is really no rush, wait to introduce them until the younger birds are close to maturity and about the same physical size as older birds. Any time you introduce young immature birds to a mature adult flock there will be fighting. Generally, the closer they are to the same age/size, the less fighting there will be. 

5. Avoid combining cockerels/roosters
If possible, don't combine roosters. Hens will fight to reestablish the pecking order, but roosters fight for dominance and may kill each other and forget about servicing the hens. If either flock has enough roosters to service the number of hens in the combined flock, remove the roosters (cull, giveaway, sell, whatever) from one flock before combining them. We've found that one rooster does a good job with about 10 hens. They only need to copulate once per week to maintain constant fertility. This is true for most production breeds.  ONE rooster is NEVER enough though, always consider at least TWO, one as a spare.  If you are adding new roosters to your flock, consider replacing all of them at one.  For instance, if you have 2 roosters and decide you really need 5, consider removing your existing roosters (cull, give away, stew pot, etc.) and add 5 new ones all at the same time. 

6. Keep them busy or distract them
While introducing new birds (or the morning after), feed the entire flock treats such as fruit, fresh greens, hen scratch, or the like to keep them busy and take their mind off the new additions.

7. Do it in the Dark
Whatever method you use to combine the flocks, DO IT IN THE DARK. Get the enclosure as dark as possible before introducing the new birds.  The best way we've found of doing this is waiting until they all go to roost for the night.  We close the coop, shut off the light and use a flashlight to place the new birds amongst the existing birds on the roosts.  If they all wake up together in the morning, it's much less obvious that their numbers have increased and they have new company.

If anyone has a method that works for them that's not listed here, please send us a description and we'll include it on this page.