Do farm animals understand the concept of "fences"?

A fence is a barrier that you can see through, or over, or under, that is continuous as in attached to itself from one end to the other. A wall is a barrier that you can’t see over or under or through, that has at least one end or one opening.

Animals understand barriers. They look at them, sometimes test them, and decide whether they can get around, under, over or through. A fence is just one kind of barrier, that humans happened to put there, which animals don’t care about.

All that’s important is “It’s in my way”. Animals understand that.

Except barnyard fowl, especially chickens. Barnyard fowl will try to fly over the top of things that have no upper edge because there is a horizontal barrier up at the top of the fence / wall, like a roof! Or a horizontal fence!

Chickens take it one step further. Here are the chickens; there is a very non-solid visible fence in their way. They encounter this fence over and over, they walk up and down along it looking for a way to the other side of the fence. They look up at the top of the fence and decide that no, it’s too high to fly over. They mostly accept the existence of the fence, but it never hurts to check, you know?

And then … somebody removes the fence. There is no barrier to keep the chickens from proceeding right on into whatever has been beyond the fence. They get to the fence that is not there, and they pile up against each other. They scream, and flap, and step on each other and scream some more. They do not go through. They know the fence is there, they know they can’t go through it, and … they scream some more. They can’t see or feel the fence - because it is GONE - but it is still there in their minds. They have chicken hysterics when the human comes along and shoves them “through” or picks them up two at a time and lifts them through the fence they are hallucinating.

Coming back is no problem. They’ve never seen the fence from the Other Side, so it doesn’t exist from that direction.