By Linda Fine Hunt
We all know birds are amazing. Studying bird vision reveals one of their most valuable assets. A bird gains more information about its surroundings through its eyes than through all its other sense organs combined. Bird vision detects direction, distance, size, shape, brightness, color hue, color intensity, three-dimensional depth, and motion of an object. Through evolution, bird vision has reached a state of perfection not found in other animals.
Eye position in a bird’s head shows close correlation with its life habits. Inoffensive vegetarians, such as ducks, quail, and pigeons, have eyes laterally placed where they can view possible enemies coming from any quarter. The eyes of hawks and other predators, intent on their next meal, are directed more toward the front. Owls nearly match humans with their frontal eyes. Unlike humans, owls’ eyes are almost immovably locked in their sockets. To compensate, owls can twist their heads at least 270 degrees.
All birds have three eyelids. Two of these are external, an upper and a lower lid, of which the lower one is more movable than the upper one. Birds are also equipped with a so-called “third eyelid,” known as a nictitating
membrane. This thin, retractable layer swipes across the eyeball horizontally, providing protection against damaging airborne particles and offering lubrication and cleansing functions. The nictitating membrane also protects the eyes of birds of prey from injury while hunting. It helps diving ducks keep their eyes open and able to see underwater, preventing impurities from getting into their eyes and damaging their vision.
Light enters the eye’s cornea and is focused through the lens deeper into the eyeball. In the diagram, the comb-like object behind the lens, known as the pecten or pecten oculi, is something human eyes don’t have. It is a structure of blood vessels believed to both control the pH inside the eye and nourish the retina.
The human retina has two kinds of light-receiving receptor cells: cone cells for daylight vision and rod cells for low light levels. Bird retinas have a third kind of cell, known as double cone, enabling them to see more colors than humans. They may be able to perceive ultraviolet or near ultraviolet light, which humans cannot. A retina with more densely packed cone cells yields a sharper image. The human eye has at most 200,000 cones per square millimeter, while House Sparrows have approximately twice that number. Songbirds and predators such as hawks are believed to have the sharpest vision among birds. They can see details at distances two-to-three times farther away than humans.
Stay tuned! Next month continues with how birds navigate their environment through vision and cognition.