Pigeons’ eyes bulge out from the sides of their heads.
A pigeon just landed outside my window and walked along as pigeons do. He took his steps with his head popping forward about two inches with each step and then his head remained fixed in space while his body moved forward with another step. I paused for a moment to consider what he would be seeing. A pigeon’s eyes are located to the sides of his head facing out and not forward as our human eyes do. That would give him a very different view of the world from ours. We probably see in our peripheral vision only ten percent of the total view possible from a given location and only about one square degree really sharply. That would be only one part in 41,253 square degrees of the sphere surrounding us that we could see at any given moment. It is amazing what a tiny portion we actually see of what is available for us to see. However, a pigeon’s eyes bulge out of his head slightly and when his head was in any position the view from both eyes would be nearly 360° around horizontally and vertically. That would be good for spotting and avoiding swooping hawks. That much visual space is far too much to attend to in detail so he must only pay attention to very specific things. His predator detection system must work well because I don’t recall ever seeing a hawk capture a walking pigeon, even though, over the years, I have seen a great many pigeons on the ground and many hawks in the air. Obviously the hawks could see the pigeons, but they don’t bother with an attempted capture, apparently because they know it would be a futile waste of time and energy. The pigeons’ defenses are more effective than the hawks’ offenses.
Why do pigeons pop their heads forward with every step?
A second feature of a pigeon’s eyes on the sides of his head, and about 5 inches above the ground, is that it gives him a good closeup view of the surface of the ground. That is a fine adaptation for finding seeds and other tiny things lying about for them to eat. The frequent popping forward of his head about two inches at a time might give him a stereo 3D mental image of the sharp still pictures of a local bit of ground. It gives him a strong stereo vision of those things on the ground on both sides of his head. This two-picture view from each eye would give the pigeon about the same degree of 3D effect as you would have by looking at your finger held about eight inches in front of your face, but on both sides of his head, and not just straight ahead like us. The pigeon probably has a hard-wired capability for comparing a still picture of one head-pop eye picture to the next eye picture from the next forward pop. Since he is only interested in eating things lying on the ground, it would be unnecessary for him to have this 3D stereo analysis ability for things outside of this very limited area where he typically interacts instantly with the seeds on the ground. There is no need to dedicate a portion of his rather diminutive bird brain to this sophisticated 3D analysis of everything else within super wide-angle view of his eyes. The patch of ground which is available to his pecking range, about a two inch by two inch area on either side of his beak, is all that is needed to be analyzed in detail. For a pigeon the rest of the view which fell to his eyes would be visual noise and lead to confusion, so it isn’t observed with the high degree of 3D analysis. Perhaps his actual viewing angle is much smaller, and perhaps he really only looks at a one inch square area. That could be determined by careful analysis of videos of pigeons eating grain scattered on the ground to determine where, relative to their eyes, they actually pecked for seeds.
The goal is to eat and not be eaten.
The rest of his visual processing would be devoted to predators, like hawks, snakes and cats, and that, instead of being 3D vision devoted to finding food, would be peripheral vision devoted to not being food himself. He would watch for movement and hawk shapes high in the sky and for subtle movements and eye detection of close-by cats on the horizontal, and for snakes a little lower on the surface. I wonder what pigeons’ defense against snakes would be, perhaps avoiding grassy places where snakes might lurk invisibly and keeping to flat open spaces where snakes could be easily seen and avoided. A pigeon’s world view is very different from ours, and probably one human’s view of the world is as different from some others as is a pigeon’s from an average human’s.
We expand our view of the world by looking through another’s eyes.