Science Explained: Astonishing eyes of birds

Posted on 19th Sep 2020

Post Category: Animal Science

• Many birds' eyes are smaller duplicates of the human eye with much the same design. They are spherical, with a delicately adjusted lens system like a camera. An iris diaphragms the amount of illumination the eye, and a thin film of light-sensitive cells responds when an image is focused on it. These are the basic ingredients of an eye, whether in a bird, mammal, or reptile.
• But each type of living thing has some special modification of the simple plan: some adaptation that makes its particular eyes the best possible for its mode of life.
• The eyes of creatures that are active by day are arranged to operate under conditions of adequate light. To do so to the best advantage, these animals have dispensed with sensitivity and are almost completely blind when the sun sets. Most birds cannot fly safely among branches until daylight allows good vision. But these birds are keen-eyed by day. To locate a careless worm accurately enough to seize it, or to spy protectively hued insects, requires an eye with particularly clear sight.

Nighthawks and whippoorwills, on the other hand, lie motionless all day, relying on their protective coloration to escape notice. Their eyes, although larger than those of many day-active birds, allow no clear picture of the familiar scene in the full light of day. Anything that stands still or sways gently with the breeze is lost to view.

Such eyes are built to operate at night; to serve their owners in the dim illumination of the twilight sky. Then these birds patrol for flights of moths and beetles. Accuracy of day vision has been sacrificed in favour of the extra sensitivity required in nocturnal shadows.

Even a few visual clues mean more to a night-active bird than does clarity of vision. Sound and air currents are far vaguer than even the poorest sight, and touch would give a flying creature too late a warning. Should it be day eyes and accurate vision, or night eyes and sensitivity? that is the question that the ancestors of each kind of animal met and settled.

Only a large animal can carry around an eye big enough to work well both night and day. Man has achieved this compromise with a double system-one part like that of the robin; one like that of the night- hawk. The night-type of eye is vague, taking in general forms rather than details, and lacking any indication of colour. The presence of a tree is plain, but small branches are beyond recognition.

So far as sight is concerned, the owl can do only a little better in the dark than we can, although it catches a stomachful of mice each night. And the owl's eyes are so huge that they cannot be turned in their sockets. Instead, this bird rotates its head to change the direction of its gaze.

Smaller birds and mammals have no space in their delicate skulls for large eyes. As it is, the eyes crowd the brain seriously and limit development of the important reasoning centres.

The comparative anatomist regards it as no coincidence that small creatures base their activities upon instincts—feature of the hindbrain. Their eyes occupy the forward regions of the head, and could not be sacrificed in order to make room for a thinking nervous system.

Nocturnal birds, other than the owl, have feeding habits like those of most night-faring creatures. They use their eyes to avoid obstacles in leaving the ground, but once in the air, they depend upon “trawling" to obtain food.

The enormous gape of a night- hawk's bill is no accident. These birds fly through the twilight air with broad mouths opened wide to catch insects winging in their path. Nor are they particular in their diet. Small birds that stay up late, or fly in the early morning hours, often end their lives in the crop of a nighthawk, whippoorwill or chuck-will's-widow.

Most birds, of course, are among the day-active creatures in attaining great precision of eyesight. Many have developed one small region in the eye where the image receives special magnification. This is a shallow pit in the retina, called the "fovea,” and the cells that line it have each a private nerve fibre io the brain. Visual acuity reaches it's maximum there, and the bird turns its head or eyes until any object of interest is focused in the fovea.

Some creatures have two foveae in each eye. Thus they can examine objects to the side monocularly-that is, using an eye singly. With the other fovea of each eye they can look forward past the beak or nose to use the two eyes together-binocularly -and gain thereby an impression of distance and depth. A few, like the owl, use the eyes binocularly at all times.

The kingfisher is one of the strangest users of the two-fovea system. Its eyes can notice both an object in the air and the exact position of a fish below the water surface and also follow that fish accurately after its sudden dive into the pond. 

Vision in air and vision in water are entirely different. When water comes into contact with the clear cornea that covers the iris and pupil, it takes away all the visual functions of the cornea. But in air, the cornea is even more important than the lens of the eye in forming an image on the retina. In water the lens must act alone. Hence an eye that has normal vision in air is very long-sighted in water. And an eye that has normal vision in water is pathetically nearsighted in air.

Penguins are famous for their myopic actions on land, but they see well in water. The kingfisher does far better, through possession of an egg-shaped lens. When the bird uses its res monocularly with one of the two foveae in each eye, any prospective prey is kept in sharp focus through one end of the peculiar lens.

But when the kingfisher enters the water, and its cornea "disappears,” the image of the fish is formed through another axis of the lens on the second fovea of each eye. The fish is seen binocularly, straight ahead of the beak, in good focus, and the bird is able to complete the catch.

The kingfisher thus has two eye systems in one-an underwater visual arrangement that is hopelessly near-sighted in air but fine for binocular pursuit in the pond, and an aerial survey system with high visual acuity, that becomes useless when the bird dives after prey.

Since birds are primarily day-active, their eyes have developed remarkable acuity. Man admits this fact by lauding his fellows for having an “eagle eye" and admires the vision of the turkey buzzard, which can recognise a dying rabbit from a soaring-point high above.

In medieval times, when falconry was in vogue, the hunter took along a small bird to keep track of the falcon. The little bird-often a shrike —was caged and mounted on the falconer's saddle. Whenever the falcon was flown, the caged bird became excited and watched the hawk intently, long after the falcon had disappeared from human view. The actions of the small bird allowed the falconer to follow his trained hawk and be on hand at the kill.

Thus the eyes of birds, although built on a design so similar to our own, usually represent the ultimate refinement of visual discrimination.