Chickens are dumb. Plain and simple. They have no idea that they are even standing there. We are talking about animals that will walk into the rain and are too stupid to tuck their heads down and they drown.
If you draw a line that splits like a Y and lay the chicken down at the lower part of the Y it will not move. It somehow gets hypnotized and cannot understand why the line split. Cover its head with your jacket and it thinks it is night and falls asleep with a min or so.
These are chicken things to do in Vet school 101.
But in defense of a chicken, no matter how dumb they are they can feel immediate pain. Even if they forget they were in pain a few min later they can still feel it.