18 month old hens keep picking on 18 week olds.

[Flixflaxnoob]

I expected the two younger pullets to get picked on when I first introduced them to the flock but it's been a month now of sharing the big, new coop, and even longer of letting them free range together. They can coexist fine when free-ranging but when confined to the run and chunnel, 3 of my 4 bigger hens will chase them away from wherever they are. The two that were previously lowest in the pecking order are the worst to the younger ones. Last year's top hen has actually been the nicest to them, just giving them gentle pecks to move them out of the way. But the others peck at them hard! So far, they haven't injured them. How long can this behavior last? Should I consider putting peepers on the bigger bullies?

I feel so bad that they went from enjoying life to being so skiddish after I integrated them.

 

[50-45-1]

This sounds normal to me. It only looks cruel to us.
I would suggest extra feeding and water stations so they have options.
Put some clutter in your run so they can escape. Like limbs to fly up on, a kitchen chair laid on its side, a couple plastic
milk crates to hop up on and hide behind.
It will take awhile but the flock will eventually get the pecking order figured out. No peepers necessary.