When the head of the robin goes bob, bob, bobbin'.
One day a long time ago, while watching some masked lapwings going about their daily business on the lawns of our local cemetery, I noticed something I had never noticed before. One of the birds took a couple of small steps forward then immediately reversed the action. That was odd in its own right, but what struck me was its head; while the body moved under it, the head remained locked in position, as if someone had nailed it to an invisible post. At the time I had no idea that I had observed something that ornithologists had been well aware of and had been discussing for quite some time.
Over the years I have kept a lookout for information on the phenomenon but with little success until the age of the Internet when at last, just recently, I chanced upon a brief note about it somewhere in cyberspace. A few minutes of surfing later I found that it had been given a couple of descriptive names - head bobbing and head stabilization. (If you Google on "head bobbing birds" you can now find thousands of pages on the topic.)
The first challenge students of the phenomenon have faced has been simply to characterize it. That is, to accurately describe what is happening. After that, two bigger questions have taxed scientists' imagination and intellect: first, how they do it, and, second, why. Although the challenge of characterizing the phenomenon has now been successfully met with the aid of high speed photography, the other two questions still seem to remain somewhat open to debate among specialists. Sadly, you won't find a word of praise for the brilliant inventor and master engineer of the phenomenon among the thousands upon thousands of words that have been written about it. I grieve for the experts who love studying the phenomenon but fail to recognize the glory and genius of its designer. They just don't know what they are missing!
Head bobbing (or head stabilization, if you prefer) is found in eight of the twenty seven families of birds. After watching birds do it, I tried it myself and found that my degree of success in head bobbing is about the same as my success in mimicking bird calls; I can make a rough showing of it only. Which is very demeaning; right now I am watching my chickens through the office window do it with such ease that it's almost unnoticeable. Dumb, dumb chickens can do
something I can't do! (Come to think of it, I can't lay eggs, either.) Of course they can; God gave them neural circuits that He didn't give us.
As a bob-able bird walks, the body moves forward with each step while the head remains locked in place, then the head shoots forward in a flash to a point beyond the position it takes relative to the body when the bird is standing at rest, and so on. The head and body are playing a continuous game of catch up with each other. Those who have sought to characterize the phenomenon have discovered a variation on the theme. When a bird "lands on a thin branch, or a power or telephone wire, their momentum will often set the branch or wire oscillating back and forth. Yet if one carefully observes their head, by lining it up with a static distant feature of the environment, one can see that it is likewise 'locked in space' while compensatory movements of the body and neck are made to balance the bird".1 (See pic above of stroboscopic photo of pigeon walking, demonstrating bobbing behavior.)
As to how they do it, I won't even begin to try to explain it - partly because my attempts at understanding the physiological hardware and software involved make my head reel. (I wonder if head stabilizing birds ever suffer from that.) All I will say is this: research shows that not only does the ability involve very complex processing in the brain but that it also requires "special ganglion cells in the retina"2 to begin the processing of visual information before it even reaches the brain. All very elementary stuff for the Master Designer, of course.
As to why they do it, well, most researchers now seem to agree that, "Its function is to keep the direction of gaze constant or fixed". While you and I move both eyes and head to keep our eyes fixed on that attractive-looking person sitting on the park bench while we walk by (not exactly a good idea) many birds bob their head. These "optokinetic movements" enable the bird to most efficiently detect movement, either of predators or potential prey, in its environment.
Everything in creation - and I mean everything - loudly sings the praises of Him who made it. Nothing just happened by chance. Even the stalk that holds the apple to the tree and provides passage for nutrients had to be designed down to the last minute detail.