Sunday, August 20, 2000		Nature

Pray, where’s the prey ?
By Nutan Shukla

BIRDS, like people, are essentially visual creatures, and the hunters amongst them use their eyes to find a meal. Birds of prey have big eyes; the ratio of eye size to brain size is much greater than in mammals. Even smaller hawks have much large eyes than our own. They can also see a large picture and it in more detail. The transparent cornea at the front of the eye is flat, unlike the curved cornea that we have. This allows more of the field of vision to be in focus, unlike the limited centre-spot focus that we can see. At the back of the eye, on the light-sensitive retina, birds have a greater number of light receptor cells than we do. They are know as the ‘rods’, and we have about 200,000 per square millimetre of retina. Even the tiny sparrow, which is not a bird of prey, does better than that — it has 400,000. But the hawks have an amazing 1,000,000 per sq. mm., giving them a high definition view of the world about which we can only dream.

Another secret weapon in a hawk, falcon or eagle’s eye is the fovea, of which raptors have two. These are the points of sharpest vision, where the density of cones is greatest. They give a bird a visual image that it, perhaps, eight times as acute as our own. They help a hunting bird to perceive distance, an essential function for a bird, like a peregrine falcon, hurtling a prey at 100mph or more.

With eyes like these, a bird of prey is a formidable predator, capable of amazing feats. In test, the kestrel has been found to be able to spot a mouse from a height of a mile (1.6 km) above the ground. And a martial eagle — one of the largest of the Africa’s eagles — was once seen to take off from a hill 4 miles (6.4 km) from unsuspecting guineafowl, only to swoop down and carry it away. The best a person could do, without binoculars, is to see the same sized bird at a mile and no further.

Birds that dive into water after prey, like the kingfisher, also have two foveas and they enable the birds to adjust its eyes above and below water. Light behaves differently in air and water. This is why a stick, standing in water, looks bent. For a bird, diving at high speed to catch a small, thin, well-disguised fish, this could be a serious drawback. To overcome the problem, a second ‘aquatic’ fovea compensates for the charge in conditions as the birds enters the water, ensuring that it is successful in capturing its prey.

The mammalian equivalent of the bird’s sophisticated visual system is known as the ‘visual streak’. Lions and other cats do not have a central fovea, as we do, but a broadly horizontal band of ultrasensitive light receptors. This increases visual acuity horizontally so cats are better able to see the position and movement of prey on the ground. They do not need to be that much aware of things above and below, only events happening ahead. The cheetah takes this to an extreme with a very narrow strip of nerve cells across the retina, enabling it to see it prey clearly against the horizon.

Other smaller animals with well-developed eyes include the praying mantis, the similar-named mantid shrimp, and the chameleon. Each has the ability to move each eye independently of the other and in all directions. But when prey is located, both eyes are brought to bear on the target, giving binocular vision greater accuracy in securing a meal.

For some animals, sight is essential for capturing prey, but a precise and detailed picture seems unnecessary. An angel shark lies dormant on the bottom of the sea until the movement of a fish nearby triggers it to rise up like some gigantic monster with an enormous open mouth and swallow the passer-by whole. Frogs, toads and day-hunting snakes also respond only to movement. As long as a slug remains still, a toad will ignore it. If it is in gulping distance, however, and it moves, the toad will grab it. The European whip snake relies on the movements of its prey in order to catch it. The snake will follow a frog intently through the reeds as long as it is moving. If it stops, the snake stops; it if moves, the snake pursues until it can reach out and grab the fleeing prey.

Some species of snakes rely on wavelengths of light that we ‘feel’ rather than see. Pit-vipers and rattlesnakes, for example, have pits in the nose containing membranes sensitive to infra-red radiation or heat. They can appreciate changes in temperature as minute as 0.005 degrees C. In pitch darkness, a rattlesnake can detect the presence of a mouse, just as long as the unfortunate rodent approaches to within 6 inches of the snake’s snout. Boas and pythons have the same kind of heat receptors on the lips.