s are popularly known for their vision, and -- along with other birds of prey
s, etc. -- do
have the best visual acuity
of all animals. Bald Eagle
s swoop down from 500 meters or more in the sky to pick a fish out of the water, and Golden Eagle
s can track a rabbit
from a mile away. There are several features that aren't individually unique to aquiline
eyes, but when put together make them a powerful system.
- Eagles have a second, translucent eyelid called the nictitating membrane. An eagle's regular eyelid is made of regular skin which closes from the bottom during sleep, and to blink while not hunting. During hunting and flying, when even a moment's lost vision could mean escaped prey, dangerous flight, or attack from other birds, the nictitating membrane closes every three to five seconds, wetting the cornea and clearing dust. With the membrane in place enough light is filtered out that an eagle can see an attacking bird through glare, even if the sun is directly behind it. Also, eagles close their nictitating membranes when feeding their young to avoid having their own eyes pecked out.
- With the protection of the cornea and lens given by the nictitating membrane, eagles have been able to evolve to be very flexible (but thus less resistant to damage). This evolution allows for better and faster accommodation, the natural muscular focusing of the lens on objects at different distances. Even when something is moving very quickly towards or away from an eagle, their focus can accommodate quickly enough not to lose sight of it. Since binocular vision isn't always used when eagles are looking at something, eagles judge distance by the amount of accommodation they must perform to see it.
- All of that protection and focusing equipment wouldn't be very useful to the eagle if it didn't have a high enough density of rods and cones on the retina to make out what was being focused upon. Humans have pretty wicked vision as animals go, probably somewhere in the 98th percentile, and we only have 200,000 rods and cones packed into each square millimeter of our fovea, the area of the retina where we see best. Eagles have around 1,000,000 rods and cones in the same areas. Thus, their visual resolution is five times what we have -- it's the difference between playing Quake at 320x240 and 1600x1200, almost an unbelievable jump in accuracy.
- Coolest yet, in each eye eagles have not one but two fovea, a central one for looking forward in tandem with with the other eye, and lateral one for looking monocularly outward. Thus, an eagle can track prey with one eye while tracking a rival bird of prey with the other, etc. Since each eye can only foveate using one fovea at a time, the eagle probably cannot attend to more than two objects in its visual field at once. Still, that's one more object than we can. Also, having two fovea in each eye gives the eagle a 200° field of vision, as opposed to our 140° or so field.
- Finally, to support all of this structure and keep it oxygenated and healthy, eagles (along with all other birds; reptiles and fish, too) have a structure named the pectin. The pectin is a piece of very convoluted tissue that extends from the retina to the lens, and is filled with capillaries and fed by its own blood vessel. It's job is to keep the vitreous humor of the eye full of oxygen, which in turn keeps the lens, muscles, rods, cones, and so forth from dying. Compare this to the human retina, in which capillaries grow over the rods and cones, and you'll see why it helps the eagle's vision be better than ours.