Unlike humans, chickens’ eyeballs don’t move in the eye socket. Therefore, when the chicken moves forward, its vision becomes blurred. Imagine looking out of a train window into the landscape while the train is moving fast forward. In order to be able to focus on a fixed point, a chicken has to keep its head completely still. The rest of the body continues to move. Once its eyes have stabilized their vision, the chicken pulls its head forward again. This way it can see a tasty worm or the grains you throw to it.