How exactly is the vision ability of a hawk different than that of an average human?
Neuroscience Graduate Student Julie Desjardin and Biology Graduate Student Brandon McLaughlin weigh in via Quora:
Hawks and other birds of prey actually have 2 fovea. The fovea is the spot on the back of your eyeball where you have the highest density of rods and cones. Beneath these you have a greater number of ganglion cells and so a higher representation in the retina and eventually on the visual cortex. Hawks have both a central and a peripheral fovea. Humans only have a central one.
A great demonstration of this is to open up some document full of text on your computer and without moving your eye, try to see how many words you can read starting at the center and going out. You’ll discover that our visual acuity drops dramatically outside our central fovea. That’s why we have to move our eyes so much.
If you’re looking for a more specific answer, I would recommend checking out this review article:
Great answer by Julie. I’ll just add a little here on a more “systems” level, rather than cellular.
Most vertebrates have what is referred to as “binocular vision”. I’m sure that you have heard something about how binocular vision works. Binocular vision is referring to the fact that we use two eyes working together to perceive images. From an anatomical/physiological perspective this offers several advantages. First, it gives a wider view of the world around us. Befoe I get to hawks, check out the well-studied binocular vision in frogs:
You can see the binocular field and the monocular fields in the right picture. Notice the extremely small blind field directly behind the frog. The frog is simultaneously an animal of prey and also predator. Being a prey animal, it is important to be able to see as much as one can from every angle at any given position. You don’t want predators sneaking up on you. For this reason animals that are predated typically have eyes that are situated further apart, more towards the outside of the head area. In the case of a predator, eyes are typically closer together and in the more frontal area of the head (hawks, and humans, although humans are somewhere in the middle too). This provides a superior ability to focus in on a target and go to it swiftly and accurately. Hawks don’t have any predators to my knowledge so they don’t necessarily need to see a particularly wide view, unlike the frog. In the case of a predator like a hawk going after this frog, there is very little room for error when it comes to stealthiness.
The picture on the left shows the overlap of the visual fields of each eye. This is the binocular field. The left eye can see a bit of what the right eye also sees, and vice versa. So in this picture the right eye sees areas A, B, and C, but not the unmarked area. So why does this matter and why am I talking about frogs? Well, have a look here. There is also something referred to as a “topographical map” that is projected from the eyes to the brain. In the frog, this map of what the retina “sees” is on the optic tectum. Like a road map or a map of a city, landmarks are in some way spatially associated with the way they actually are in real life. So if the Empire State Building is in location x in relation to Times Square, so it is on a map as well.
This is a rough outline of the visual system of the frog and is very similar to humans. Notice that images from the left eye project to the right tectum in some spatially related way to how they are projected on the retina. From there, information is fed to the nucleus isthmi (superior colliculus in humans) where there is some feedback to the tectum, BUT, some of that information is projected back up to the opposite tectum. Interesting. Well, it turns out that this is where binocular vision is processed.
So what about hawks? Unfortunately hawks are not nearly as well studied as frogs. But we do know something. Check out these schematics comparing some different types of animals and how their visual information is processed:
Without going into too much of the experimental details that implicate nucleus isthmi in highly contributing to behavioral response I can tell you this: In frogs the nucleus isthmi enhances neurotransmitter release from retinotectal axons that enable them to reach their “behavioral threshold” and act according to the type of stimulus (Dudkin, Myers, Ramirez & Gruberg 1998). Now, have a look at the bird schematic vs the mammal one. See all of those highly developed and segregated nuclei compared to the mammal, or any other of these animals?
Just to add to how complicated and developed this system is in birds, have a look at this:
This is the anatomy of the bird isthmotectal system. The blue tracts are excitatory input from the tectum, green is excitatory feedback from the Ipc & SLu to tectum (those nuclei), and the orange is inhibitory output from the Imc to Ipc, Slu & tectum everywhere but to the visuotopic location.
And here is the electrophysiology:
I’m not expecting for anyone to fully understand what these two images reallymean. I don’t even fully understand them. The point is, look at this beautiful, highly complex, highly organized system. These animals have done something truly amazing throughout their course of evolution and as a result are EXTREMELY good at what they do: seeking prey and getting them.