pigeons

I think it has something to do with keeping things rather still as they walk, because their eyes are on the side of their head. Kind of like how our own eyes are, when we're travelling in a car or train, looking to the side. If you've ever watched a person as they're looking out the side window of a vehicle, their eyes fixate on an object, and as it goes past, they stop looking at that object, and find the next object to look at. If you haven't noticed it before, check next time you're sitting next to someone in a moving vehicle. (Try not to do it if you're driving, if course! lol.) So a pigeon's eyes are on the sides of its head. So when the pigeon is walking forward, each eye will focus on an object. As it steps forward, the head stays in exactly the same position until it jerks its head forward. Then the rest of the body will move forward, underneath the head. The body will move forward smoothly, whilst the head will move backwards relatively slowly, before jerking forward to focus on the next point. That's more or less how it's meant to work. Hopefully you understand what I'm trying to explain! I think if we walked that way, we'd have the biggest headache! lol. Luckily our eyes face forwards, so we can focus on the same object with each eye, and the brain joins the two images together. So no jerky head movements for us! Yay!