It's probably different in each case, but I just had a problem with integrating my second mom with chicks. One (the first) mom with chicks that are 4 weeks old was with the flock all the time, even while sitting on the eggs, and is doing mostly fine. The second mom was separated in a dog cage with her 2 weeks old chicks but I started letting her out. She was in a separate area but found her way back to the main coop and flock. And she started a horrible fight with the other chickens, attacking everything that moved. I've never seen something like that - she was just a hissing and growling ball of feathers with beak and claws tearing in all directions. She attacked all the other hens including the other mom and the rooster. I was afraid that she had killed the chicks of the other hen but I found them later, shell-shocked and huddled in a hiding place. I had to take her out by force (wearing thick gloves) and threw her back into the dog cage. Then I scooped up her chicks one by one and put them back in with her. Now I'm building a second small run just for her because I'm not going to re-integrate her any time soon.