Eagle eyesight is one of the strongest in the animal kingdom, with eyesight estimated to be 4 to 8 times stronger than that of the average human. It is said that an eagle can see a rabbit from 3.2 km away. Although an eagle may only weigh 10 pounds (4.5 kg), its eyes are about the same size as those of a human.
As the eagle descends from the sky to attack its prey, the eye muscles continuously adjust the curvature of the eyeballs to maintain sharp focus and accurate perception throughout the approach and attack.
In addition to eagles, birds such as hawks, falcons and robins have extraordinary vision that allows them to pick up their prey with ease. Their eyes are said to be larger in size than their brains, by weight. Color vision with resolution and clarity are the most prominent features of eagles’ eyes, which is why those with sharp vision are sometimes referred to as “eagle-eyed.” Eagles can identify five different colored squirrels and locate their prey even if they are hidden.
The eagle’s weight varies: a small eagle might weigh 700 grams (1.5 lb), while a larger one weighs 6.5 kilograms (14 lb); an eagle weighing about 10 kilograms (22 lb) might have eyes as large as those of a human being weighing 200 pounds (91 kg). Although the size of an eagle’s eye is approximately the same as that of a human, the shape of the back of the eagle’s eye is flatter. An eagle’s retina allows for a higher Nyquist limit. (see article: Arctic Tern).
Its retina is more pronounced with rod cells and cone cells. In the eagle, the retinal fovea has one million cells per mm2 compared to 200,000 per mm2 in humans. Eagles have a second fovea and three eyelids (two of which are visible). The second fovea in eagles gives them better and sharper vision, while the long, narrow ribbon-like area connecting the two eagle foveae is inferred to be a third fovea.
The phenomenon of an eagle turning its flexible head almost 270 degrees, while sitting or flying, is attributed to the fact that when its large head is fully turned its eyes are also turned, unlike a human.
It is said that an eagle in flight can see a rabbit two miles away. Claw-eye coordination is a hunting imperative. From its treetop perch, the eagle can dive at speeds of 201-322 km (201-322 miles).
Eagles blink because their upper eyelid is larger than their lower eyelid; the extra inner eyelid is known as the nictitating membrane, which “grows at the inner corner of the eye, next to the tear duct.” Eagle tears moisturize the eyes and contain the chemical lysozyme that protects against salt water and also destroys bacteria, thus preventing eye infections. The nictitating membrane acts as a sweeping wiper across the eye.
The eagle iris is pale yellow, much lighter in color than human eyes. Both eagles and humans have a white area called the sclera, but in the case of eagles, it is hidden under the eyelid. The eyelid openings are oval in humans, while they are round in the case of bird eyes.
Most eagles have excellent vision. Generally, eagles do not suffer from myopia (nearsightedness) and hyperopia (farsightedness); those with these defects cannot hunt easily and eventually starve to death. Eagles have the unique characteristic of pectin.
Its function is not clearly understood, but the general belief is that it helps nourish the retina, keeps it healthy without blood vessels, facilitates fluids to flow through the vitreous body at an appropriate pressure, absorbs light to minimize any reflections within the eye that might affect vision, helps perceive motion, creates a protective shade from the sun, and senses magnetic fields.
340-degree field of view
Despite their ability to see over long separations and see a greater number of nuances than we do, hawks also have twice our field of vision. A bird’s eyes are arranged on a 30-degree edge from the focal point of the face, giving them a relatively unobstructed back perspective.Hawks have a perspective field of 340 degrees in contrast to the human perspective field of 180 degrees, giving them a preference for both chasing prey and escaping predators.
The eyeball of each hawk moves independently, and the eyeball is so extensive and finely tuned that it barely rotates inside the eye attachment. The birds stop people in their tracks to find prey and different objects of excitement by moving their zoom focal points (fear) across their entire field of view.
When it distinguishes its prey, it turns its head toward it and uses its stereoscopic vision (the gaze of both eyes all the time) to quantify the separation it must cover and the speed at which it must approach it.
Falcons’ eyelids close when they rest. In fact, they also have an inner eyelid considered to be the nictitating layer that runs back and forth across the eye every three to four seconds, expelling dirt and debris from the cornea. The layer is translucent, so the hawk can see through it as it glides back and forth.
The researchers trust that since hawks and different winged prey animals abuse their erudite limit in handling visual signs, they have created less scent and taste sensation than different groups of creatures.
Wood (1917), The Fundus Oculi of Birds, Especially as Viewed by the Ophthalmoscope: A Study in Comparative Anatomy and Physiology, describes the anatomy of the eagle’s eye in detail:
- “Bald eagle (Haliaeetus leucocephalus). The predominant color of the back of the eye of this bird is dark reddish brown, the lower half changing to a dull orange-red. The entire eyeball is covered with choroidal capillaries and dotted with grains of brown pigment, giving it a rough, granular appearance. A gray sheen permeates the upper part of the fundus.On the temporal side and at some distance from the upper end of the optic nerve is a round, white, shiny spot surrounded by a small, light green reflex ring, which in turn is enclosed in a very bright, narrow green ring: the muscular region. On the nasal side of the disc, and at the level of this macula, there is another area, gray in color, surrounded by a fan-shaped luminous reflex. The optic nerve entrance is distinctly white, and along its center there are a large number of tiny pigmented dots. The outer margin of the disc is bordered with black pigment, as if the pectin cast a shadow upon it. In this respect and in others, this eye-bottom resembles the eye of the sea eagle.” (see article: Yellow-legged Gull).
- “White-bellied eagle (Haliaeetus leucogaster). The eye coloration is mostly dull brown, the lower quadrants of the field are covered with orange-red, opaque, evidently choroidal capillaries. The optic disc is a long white oval, the center of which is tinged with orange and covered with small pigmented dots. The papillary margins are white bordered with black pigment. The upper half of the fundus is covered by a mass of dull gray dots.There is a well-defined reflex near both maculae, each similar in position to that seen in the lesser kestrel. These areas are evidently very sensitive to light, as the bird becomes very restless and irritable when the reflected rays from the mirror are thrown directly on one or the other fovea. The pectin is very large and is directed toward the posterior surface of the lens. Both extremities of the organ are clearly visible through the ophthalmoscope. There are very opaque nerve fibers visible anywhere in the foreground.”