Vision is the most important sensory system for birds, and it is more highly developed in birds than any other vertebrate, including humans. There are many reasons.
First, bird eyes are huge in relation to the size of their heads. 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 Common Ostrich are about twice as large as those of humans. In many bird species, including songbirds, the eyes are about as large as their brains.
Because bird eyes are so large, they are literally crammed into the large eye sockets (orbits), making them generally immoveable. Consequently, when a bird wants to move its eyes, it moves its head.
To compensate, in part, for the large opening of the orbit, birds have a boney “sclerotic ring” that surrounds the eyeball in the area of the lens. The ring helps support the eyeball, and it provides attachment for special muscles that help change the shape of the lens.
A massive eye allows for a large image, and the enlarged pupil permits more light to enter. This allows birds to see in much dimmer light than required by humans.
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The eyes of most birds are located on the sides of the head, providing great peripheral vision — in excess of 340 degrees — but very limited visual overlap in front (binocular vision). The area of binocularity is much larger in birds of prey, reaching 50 degrees in hawks and 70 degrees in owls, which provides much better depth perception. Birds easily turn their heads, peaking with owls at 270 degrees.
In some birds, such as cuckoos and woodcocks, the eyes are positioned far enough to the rear of the head to permit binocular vision both in front of and behind the head, creating a superb mechanism to detect predators. An unusual feature of bitterns permits both eyes to face forward in binocular fashion, even when the bill is pointed up in the birds’ familiar camouflage position.
Do birds blink?
Birds rarely blink (with the exception of owls, parrots, ostriches, and a few others) and close their eyes for sleeping by raising the lower lids. Usually unseen, however, is the third eyelid (nictitating membrane), located beneath the upper and lower lids, which moves horizontally across the eyeball. The movement of this eyelid across the eye functions like 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 retinas of both birds and mammals, including humans, consist of two types of sensory cells — rods and cones — that respond to light. When stimulated, these cells send messages to the brain, where visual centers turn them into images. Rods can be stimulated by dim light and only provide black-and-white images. Cones require greater light stimulation, allow for color vision, and provide sharper vision (visual acuity). An example of acuity is the ability to distinguish two dots that are close together, as two distinct objects, even at a distance. Because birds have so many more rods and cones than humans,
avian visual acuity is estimated to be from 2 to 8 times greater.
Vertebrate eyes have a specialized, small area of the retina — the fovea centralis — that provides the sharpest images and greatest visual acuity. The fovea is thinner and more highly packed with cones than the rest of the retina. (In reading this article, you are focusing the words on your fovea). A robin cocking its head while foraging on your lawn is not trying to hear the worm, as legend has it, but it’s trying to focus the worm on its fovea.
One can draw an analogy between the density of sensory cells in the retina and fovea of birds and the number of pixels in digital cameras. More cells and pixels mean sharper images.
Most birds have one fovea (central) that is located near the center of the retina and receives images from the side. Birds that feed on the wing, however, such as hawks, swallows, and hummingbirds, have two foveas. The second fovea (temporal) is located near the back of the eye and receives images from in front of the bird, creating binocular vision.
Unlike mammals, the retina of birds lacks blood vessels, which probably increases the transmission of light through the eye to the retina. Birds compensate for a lack of blood vessels in the retina by having a highly vascular structure (the pecten), protruding from the retina into the posterior (vitreous) chamber of the eye. Oxygen and nutrients diffuse from the pecten to the retinal cells and waste products diffuse into the pecten for removal.
The avian eye is uniquely adapted for low light, a wide visual field, outstanding visual acuity, and great distance of vision. Clearly, superior vision is another example of the amazing attributes of birds.
This article from Eldon Greij’s column “Amazing Birds” appeared in the November/December 2018 issue of BirdWatching.
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