In the back there is a mature chicken, that's where the chickens are supposed to be. But I found out that this little fella was all over my crops. He never got out through a door due to my pen not having any doors whatsoever but a ladder that only I can use. I also notice that they often go to the corners of the fences and stay there for a long time. It's probably how this chick got out.
I would use nether brick or use that three block hole farm trick. Three block deep farm that animals can walk into as a trap. Plus nothing can escape and you can get out with a ladder.
In the latest version of minecraft they have changed the hitboxes of baby animals in minecraft. Since they have been change baby animals can go under fences. (I think this only happens on baby chickens)
In the back there is a mature chicken, that's where the chickens are supposed to be. But I found out that this little fella was all over my crops. He never got out through a door due to my pen not having any doors whatsoever but a ladder that only I can use. I also notice that they often go to the corners of the fences and stay there for a long time. It's probably how this chick got out.