The pigeon has side mounted eyes unlike humans and owls which have forward facing eyes. As a pigeons have monocular vision rather than binocular vision they bob their heads for depth of perception. The pigeon’s eyes work much better with stationary images and therefore, as the pigeon takes a step forward the head is temporarily left behind. The next step jerks the head forward again and so on. This allows the bird to correctly orient itself.
Source: http://www.deterapigeon.com/21-amazing-facts-about-pigeons.htm
