Belongs in a 'nature' forum, but this is as close as it gets. Why do some birds (pigeons, starlings) bob their heads (front and back) when they walk? I have two hypotheses: 1] They have lower retinal retentiveness, and must hold an image on their eye longer to resolve it. They move their heads so as to spend a maximum amount of time with their eyes motionless with respect to the environment. 2] It's a centre-of-gravity balance/counterbalance thing.