I have six 4 year old hens (3 leghorn, 2 cornish, one sex link), and just got 25 new chicks the 3rd week of May. At 4 weeks old, I introduced the chicks at night to the old biddies, and they had their own little apartment that they could run in/out of when I opened the door just a chick-size bit after a few days of them being together. I've watched closely for signs of overt/dramatic aggression, especially since there are 3 baby silkie bantams and 4 buff polish, and have only seen some minor hustling here and there - no blood, quite civilized actually. One of the cornish hens was making a new, deeper clucking sound, and seemed to be trying to herd some of the pullets. Being relatively new to this, I couldn't tell if she was preparing to be aggressive, or wanting to be a mother hen. She tends to be broodier than the others, so I suspected it was the latter. I saw proof of it tonight when I did an after-dark bedtime check in the henhouse. While the little ones were still in a pile spilling out of the fresh hay in one of the nest boxes as they usually do at night, two of the babies were on the top roost with the others, snuggled right next to that cornish hen - in fact the little buff orpington was under the hen's belly and wing. It was so sweet I have to go back out and look at it again.