This question occurred to me yesterday while I was waiting for a bus and spotted some pigeons nearby. Their idiosyncratic, jerky, back-and-forth head movements sort of look like they’re continually pecking for invisible food suspended in the air – which is quite odd if you think about it, and must cause them to expend quite a bit of energy.
So why do they do it?
It turns out that the back-and-forth motion is an illusion, actually. This was first described by Dunlap & Mowrer (1930) who found that birds (specifically chickens, in this case) don’t jerk their heads backwards; in fact, they hold their heads still relative to their surroundings and move their body forwards, before thrusting their head forwards for it to catch up with the body. This is called the “thrust-hold cycle”. (See picture: the head moves relative to the body, only because the body continues to move forwards while the head stays still.)
It’s clear that this has something to do with their vision: pigeons don’t do it when they’re blindfolded and made to walk forwards, and they don’t do it when they are walking on a treadmill (such that they remain stationary relative to their surroundings) – which means the behaviour only occurs when the environment is unstable.
What’s happening is that they are keeping the image projected onto the retina still, to prevent the image blurring. Humans have a similar optokinetic reflex: try slowly scanning the room from left to right in a straight line, while keeping your head still. It’s extremely difficult to do smoothly because your eyes will keep making small, jerky saccadic movements. But hold up a finger, slowly move it from left to right, and follow it with your gaze, and your eyes will fixate on the finger and move perfectly smoothly. It’s because in the first instance the thing you’re looking at is the room, the retinal image of which is changing as you move your eyeballs. Your vestibular system does its best to stabilise the image by keeping your eyes still for as long as possible and then quickly fixating on the next image of the room. In the second instance, however, the visually important thing is your finger, and the best way to keep it in focus is by moving your eyes such that your the image of your finger is always projected onto the same part of your retina.
Rather than eye movements, pigeons make movements with their entire heads in a similar fashion to stabilise the image. This explanation is further supported by the fact that if you put a pigeon on a treadmill such that it’s only moving passively (i.e. it’s not walking or moving its feet at all), it will still bob its head: the pigeon doesn’t even need to be walking in order to produce this behaviour, it just needs to have an unstable visual environment.
There are likely to be other functions of head-bobbing, however. If nothing else, it’s a reasonable assumption to make simply because visual processing is such a complex task – the visual system always makes the most of what it’s got.
For instance, depth perception is more difficult for pigeons than for humans because the visual fields of each eye don’t overlap nearly as much as ours. We can tell a lot about the depth or distance of an object by looking at it with both eyes and comparing the retinal image projected onto each. Pigeons, with their eyes spaced more widely apart, don’t have that kind of information available to them. One source of information they can use is motion parallax, which is based on the fact that objects closer to you will move faster than objects further away as your travel through your environment.
For instance, if you’re travelling in a car, you might notice the fence at the side of the road whizzing past so fast you can’t see it clearly; the trees behind it will move quite quickly, but not as quickly as the fence; the buildings in the background will move more slowly still; and the distant mountains you can make out in the distance will barely be moving at all. You can get a pretty good idea of the relative distances between all these things in the foreground and background just from their relative motion. The same idea is shown in this GIF.
Pigeons create this motion parallax and maximise it in the thrust phase of their thrust-hold cycle: by moving their head a little bit, over and over again, they can get a rapidly updating idea of how far away things are, relative to the pigeon and to other things in the environment. Even when the pigeon is moving too fast to be able to maintain the hold phase – e.g. when it’s running or landing – it will still thrust its head back and forth to sustain the motion parallax.
Incidentally, cat owners will have noticed a similar phenomenon in their pets: before making a big leap, cats will often move their heads back and forth a few times. Although cats’ eyes overlap a great deal (even more than humans’) so they don’t face the same paucity of depth information that pigeons do, when making an ambitious jump it still helps to have all the information you can get about the distance between you and the landing spot you’re aiming for.
References
Dunlap, K. and Mowrer, O. H. (1930). Head movements and eye functions of birds. J. Comp. Psychol. 11, 99–113.
The Brain Racker 