We’ve all seen them do it.
The head bob has two phases: hold and thrust. For the hold phase, the body moves forward as the bird walks, but the head stays still, creating the illusion that the head is moving backward. You need a bird’s long neck to do this properly.
In the thrust phase, the head moves forward faster than the body, so the head gets thrust out in front. Let’s see it again:
There are two reasons a bird might do this. One: it might help the bird walk. When birds hop, they hunch down, then launch their heads (and the rest of themselves) forward. This isn’t all that different from the head bob, so it could be that the thrust step gives them extra forward motion while walking.
The second possibility: head bobbing lets birds see better. Seeing while moving is tricky – if your eye moves too fast, the image is blurry; and even if the image is sharp, it’s hard to judge the motion of things you see if you’re moving too. This is why birds keep their heads still if you move them. Head bobbing lets the head stay still for a moment (hold), then move quickly (thrust) to the next hold point. If they didn’t head bob, their heads would be moving more or less constantly, which would make vision harder.
In work on Elegant-crested Tinamous, who also head bob, Hancock et al. (2014) found that the head motions weren’t exactly synchronized with the leg movements. This makes it highly unlikely that they’re head bobbing to walk better. However, the head bobbing does affect the mechanics of the walking birds—it shifts around their center of mass, for one.
It turns out that not only is vision the primary reason for head bobbing, but birds actually adjust their walking in order to accomodate their head bobbing and achieve the best vision. So that head bobbing pigeon? He’s doing it to keep an eye on you.
Reference: Hancock JA, Stevens NJ, Biknevicius AR. 2014. Elegant-crested Tinamous Eudromia elegans do not synchronize head and leg movements during head-bobbing. Ibis 156:193-208.