Paul G.'s missive, though he cites a famous horny thologist, asserts that owls and other binocular-eyed creatures don’t need to bob their heads to see well. It’s not true. Owls do, indeed, bob their heads when trying to discern prey from background. I’ve seen film of it. My cat does the same thing when he’s drawing a bead on prey. Humans occasionally move their heads to one side in order to judge distance.
My post, like the column, does not nail down the question of why pigeons bob their heads when they walk. We’re just whittling it down.
And anyway, why the hell do pigeons need to ensure that “some weight is always trailing a little abaft the port beam”? Do pigeons only step out to their right side (thus requiring a counterbalance to the left)?
Or is Cecil using terms he does not understand?
How about just “some weight is always a little aft”, eh Cecil?
I have to decline to buy the parallax theory. That is, any improvement in a pigeon’s perception of the parallax shift can only be, at best, an incidental benefit of the bobbing walk.
Yesterday while waiting for a bus, I observed a group of pigeons closely. It occurred to me that any parallax-related distortion in their perception of their surroundings would only affect objects so far away as to be of little or no consequence in their daily existence. I mean, how much parallax shift can there be within the radius of a pigeon’s ambulatory world?
It seemed apparent to me, observing these pigeons, that the problem of balance is far more likely to be the primary problem addressed by the bobbing walk. And though I’ve seen chickens walking warily, stepping slowly forward while keeping their head uncannily still in it’s spot in space, obviously in order to keep their eye trained on whatever they’re being wary of, I can’t believe that this is the primary evolutionary driver of this behavior.
The idea that moving a bit doesn’t gain an appreciable amount of parallax for anything except very close objects is quite right.
But that isn’t the reason for moving. Call it “getting a different perspective.” An animal that shifts its head trying to see things is attempting to eliminate artifacts that happen when both their eyes and the subject are stationary.
Every animal I know of has the impulse to move its head when seeing something incompletely understood. An animal is attempting to: see around things, see things in a different context, avoid inexplicable reflections and glare, and avoid defects in their own vision.
Notice that even if an animal’s shift doesn’t show them anything different, it still conveys the valuable information that what they’re looking at can’t be further analyzed from afar. If an animal moved its head, and the thing it was looking at changed radically, that would be significant. That is was probably time to make a run for shelter.
This response is based on what I gleaned from one semester of “physiological psychology” and one of “physiological mechanisms of animal behavior” back at Swarthmore (the latter class was taught by a recognized authority on bird behavior, especially orientation, migration and homing.
First of all, cats (and perhaps other predators, such as owls), have a visual system so fine tuned to discern movement, that things that are static in their environment will soon fade from their image of the world around them. They will move their heads to, in effect, cause the things around the them to “move” relative to their field of vision, causing them to be maintained as visible or even to reappear in their vision.
Pigeons, I was taught, bob their heads when walking – just as many other birds do – in order to maintain their balance. They are, like us humans, essentially walking “upright” on two legs. But they do not have the benefit of long arms at their sides to act as counter balancing weights, swinging in opposition to their legs’ motion.