This is hard to describe. I was in my world & below me was a cave pond? with 1 chicken & 2 baby chickens in it. I kept hearing the baby take damage. It eventually died. I thought the water was causing the damage, but they swim in water just fine. I think when they are on a 1 block high block and pushed into a wall or block they take damage. When I was in my other world i had chicken following me & i broke dirt above me & water came through the cave & killed the chicken. I think the water pushed it into the wall? I haven't seen this happen with any other mobs.