junebuggena
Crowing
I start integrating as soon as chicks are off heat, at about 3 to 4 weeks of age. I have a grow-out coop/run for them to live in while they get used to the adults and the adults get used to them. After 2 weeks, I open the grow-out run to the main run just a bit, so the chicks can run back and forth, but the adults can't get into the grow-out coop/run. By 6 to 8 weeks of age, they are too big for the grow-out coop/run and start sleeping in the main coop all on their own. The key is to make sure there is ample room for all, so that nobody feels the need to be territorial. It's pretty easy when the chicks are young enough to not be seen as threats to the pecking order.
Your 12 week olds are old enough to be seen as a threat to the pecking order, so it's going to take a while for them to integrate. And you have to let the adults do their pecking and chasing. It's not called pecking order for nothing. It looks rough, but as long as your coop and run are large enough for them to escape the dominant hen, you just have to let it happen. They are big enough that your hen can't do serious damage. Space, multiple food and water stations, and breaking up sightlines will go a long way in reducing territorial aggression.
Your 12 week olds are old enough to be seen as a threat to the pecking order, so it's going to take a while for them to integrate. And you have to let the adults do their pecking and chasing. It's not called pecking order for nothing. It looks rough, but as long as your coop and run are large enough for them to escape the dominant hen, you just have to let it happen. They are big enough that your hen can't do serious damage. Space, multiple food and water stations, and breaking up sightlines will go a long way in reducing territorial aggression.