Another great question from James:
When certain birds (not owls) look out at you from their left eye, they can’t see you with their right eye on the other side of its head. So what do birds with eyes on both sides of the head actually see? Two different scenes? Or some sort of panorama distillation?
Eye positioning has an inherent trade-off: you can see a wider area around you, but have less depth perception (we can perceive the closeness of objects by comparing the slightly different views of them that our eyes receive); or you can see a smaller area, with more depth perception. Generally, prey animals go for more area, less depth perception, so that they can keep a better lookout for predators: think of rabbits, horses, and most small birds. Predators like owls, cats, and humans have flatter faces with more depth perception.
Cover one eye and try to walk around, and you’ll quickly see how useful depth perception is. I tried to wear an eyepatch one Halloween and soon decided that the aesthetic piratical touch wasn’t worth the bruises from walking into things.
So what do birds with eyes on the opposite sides of their heads see? They see a much wider area, like a widescreen TV, but most of that area looks flattened compared to our vision. In the middle of that area they have overlap between the visual fields of each eye, so there they do perceive depth, allowing them to navigate while flying and judge how far to peck to grab a bug.
Bird vision differs from ours in some other ways too. They can see into the ultraviolet a little bit, which we can’t, meaning they see some colors that we can’t see. Some birds have ultraviolet coloring on their feathers that is invisible to us!
Birds integrate images over shorter periods of time than we do—if we were cameras, birds would have more frames-per-second than we do—meaning they can probably see fluorescent lights as rapid flickers where we see constant light, and they can watch the image on a TV screen write and erase itself where we see a constant image.
Humans have an area in the center of our vision where we have very high resolution, so we can see in great detail. Birds have two of these: one at the tip of the bill, presumably so they can see what they’ve picked up, and one a little ways past the tip of the bill.
Finally, both birds and humans look at the world in “saccades,” looking at steadily at one place, moving our eyes extremely quickly, then looking steadily at a second place. (This is probably why cuts between images in films don’t confuse us: we already see the world in discrete bits.)
We perform the quick motions of our eyes using muscles around the eyes, so we can look around without moving our head. Birds can do this only a little—owls’ eyes are so enormous relative to their skull that they can’t move at all—so if they want to look around, they have to move their whole heads. This is why birds are constantly cocking and rotating their heads. If you want to look like a bird, try holding your eyes still in your head: you’ll find yourself cocking your head every which way, just like a bird.
Gill, Frank B. 2007. Ornithology, 3rd ed. New York: W.H. Freeman & Company.
MF Land MF, Nilsson D. 2002. Animal Eyes. Oxford University Press.