Chickens push themselves into each other and gather in corners.

I was in my hardcore world with a 4x7 pen containing 3 sheep and 4 chickens. One side was connected to my base, which had walls 3 blocks tall. One day, the chickens were stuck in a corner without pushing each other and splitting, like in the screenshots. I tried to make them come and split by taking some wheat seeds and going to the other corner of the pen, but it didn’t work. After some in-game days, I decided to split them by hitting them and knocking them back. It did work, but after ~2 in-game days, when I took the screenshots, I checked the pen again, and they were exactly in the same corner, inside each other like they were before. [date]

They could still turn and look around, but not move until they receive knockback.

Update: This still works in the new version. I made a similar pen and a chicken eventually got stuck in that corner.