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Avian vision
Not all birds see like a hawk, but every bird’s eyes are amazing
Contributed by Eldon Greij
Published: December 22, 2010 Vision is the most important sensory system for birds, and avian vision is the most highly developed among vertebrates — animals that have a backbone or spinal column, including humans. There are many reasons for this.
First, bird eyes are huge. They are proportionately much larger than human eyes. The eyes of eagles and owls are about the size of human eyes or larger, and the eyes of the Ostrich are about twice as large as those of humans. In many bird species, including songbirds, the eyes are about as large as the brain.
A massive eye allows for a large, sharp image, and an enlarged pupil permits more light to enter the eye. This enables birds to “see” in much dimmer light than humans require.
The retinas of both birds and mammals, including humans, consist of two types of sensory cells that respond to light — rods and cones. When stimulated, they send messages to the brain, where visual centers turn the messages into images. Rods react to dim light but provide only black-and-white images. Cones require more light, allow for color vision, and provide greater visual acuity.
Visual acuity refers to sharpness — the ability to perceive as separate objects two dots that are close together, even when the dots are seen at a distance. The acuity of bird eyes is at least two or three times greater than that of human eyes.
In vertebrates, concave depressions in the retina known as foveae provide the sharpest images and greatest visual acuity. (You are focusing words on your foveae as you read this article.)
Foveae are more densely packed with cones than the rest of the retina, and birds’ foveae have more cells than our own. The human fovea, for example, has about 200,000 cones per square millimeter. The fovea of House Sparrows has about 400,000, while that of hawks and vultures has about 1,000,000.
You can draw an analogy between the density of sensory cells in the retina and foveae of birds and the number of pixels in digital cameras. More cells and pixels mean sharper images.
Looking ahead
Most birds’ eyes are set on the side of the head and have one fovea. Located near the center of the retina, it receives images from the side. Birds that have forward-directed eyes and feed on the wing, such as hawks, swallows, and hummingbirds, have a second fovea, known as the temporal fovea. It receives images from the front of the bird.
Unlike the retina of mammals, the retina of birds lacks blood vessels. (The absence probably increases the receptor cells’ efficiency by reducing material that might interfere with light transfer.) Birds compensate for the lack by means of a highly vascular structure that protrudes from the retina into the eye cavity. Oxygen and nutrients diffuse from the structure, known as the pecten, to the retinal cells, and wastes are removed.
Because bird eyes are so large, they are literally crammed into big eye sockets called orbits, making them generally immoveable. Consequently, when a bird wants to move its eye, it moves its head. A robin cocking its head while foraging on a lawn is not trying to hear the worm, as legend has it, but trying to focus the worm on its fovea.
Another aspect of immoveable eyes is that birds can turn their heads freely. Owls, for example, can rotate their heads about 270 degrees.
To compensate, in part, for the large opening of the orbit, birds have a boney “washer,” known as the scleral ring, that surrounds the eyeball. The ring helps support the eyeball and provides attachment for muscles that change the shape of the lens.
Binocular vision
Situated on the sides of the head, the eyes of most birds provide great peripheral vision — in excess of 340 degrees — but only limited visual overlap in front. The area of so-called binocular vision is much larger in birds of prey, reaching 50 degrees in hawks and 70 degrees in owls. Such birds consequently have much better depth perception.
In some species, such as cuckoos and woodcocks, the eyes are positioned far enough to the rear of the head to permit binocular vision both in front and to the rear, a superb mechanism to avoid predators. An unusual feature of bitterns allows both eyes to face forward in binocular fashion even when the bill is pointed up in the birds’ familiar camouflage stance.
With the exception of owls, parrots, Ostriches, and a few others, birds rarely blink, and they close their eyes for sleeping by raising the lower lid.
Usually unseen, however, is a third eyelid, known as a nictitating membrane, that is located beneath the upper and lower lids and travels horizontally across the eyeball. Its movement resembles our blinking and serves to keep the eye moist. In some diving birds, the nictitating membrane has a transparent window that permits vision underwater while the membrane is closed.
The avian eye is uniquely adapted for low light vision, outstanding visual acuity, and the ability to see in many directions simultaneously. Clearly, superior vision is another example of the amazing attributes of birds.
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