I forgot to add that the younger one is in a small dog kennel outside the main run during the day. They stay in the same coop at night but there's a piece of netting separating them.
It sounds like it's going better now. But I'm not sure I'm understanding your set up. If the young one is/was in a kennel outside the main run, how is/was the older one getting to her? Or maybe you're saying you didn't keep the younger one in the kennel for very long...?
One thing you've got is a pretty big difference in age, 12 weeks vs. 25 weeks. This often (not always) means that the integration may be tougher/more difficult. So, more caution is probably advised and the best/least risk way to go. Even if they are currently getting along, you may still want to backtrack and employ the methods discussed in that article. Keep in mind that the more you allow the young one to be stressed, the more susceptible she is to illness. There are some pretty bad things that are naturally in chickens that rear their ugly heads when the chicken is stressed. Like, Marek's, which is a virus that lays dormant in almost all chickens and if they're not vaccinated for it, the virus can go from dormant to active when the chicken is stressed. Marek's is fatal. Lots of other illnesses are activated by stress as well but I've had Marek's so I just happen to know about it. Mine was brought on by stress, I'm almost positive. You probably already know all this, but it's something to consider.
I have done only one integration myself and it went smoothly. I created a make-shift fenced area inside my coop where I put 5 pullets that were just 2 weeks younger than my existing flock of 10 chickens (mix of pullets and cockerals). The 10 members of the existing flock use the coop as well but because I free range, the existing flock only came in and out of the coop periodically. I left the 5 new pullets behind this barrier for a full week. I put a little bench in their little pen so they could roost by themselves. They had their own water and food, etc. The existing 10 would try to peck at the 5 through the fence, but it was big enough that the 5 would just back away from the fence. After a week of this, I let them all out to free range. The 5 pretty much kept to themselves and free ranged where the 10 were not. There was some chasing but honestly, this was almost 100% the cockerals chasing the younger, new pullets more as part of their juvenile mating "I'm a young, bumbling cockeral and have no idea what I'm doing yet" behavior. I eventually moved all the cockerals to their own bachelor pad, but this is a different story. Anyway, having lived side by side for a full week, everyone knew everyone else was allowed to be there so there was no real issue. After many changes (I prepared 6 cockerals for the dinner table, for example), I now have 1 integrated flock.
Hope this helps,
Guppy
