Bird’s eye views
BIRDS have the largest eyes, relative to their size, of any animal and have many adaptations which give them superior sight. They can see ultraviolet, which we can’t, while raptors have five times the number of light receptors of humans, enabling them to see greater detail. Birds can also see rapid movements better than we can.
A bird’s eye is very similar to that of a reptile’s, with a flatter shape than the spherical human eye. Most birds have eyes which are fixed, with no muscles to move them around, so they have to move their heads to see in different directions.
Diurnal and nocturnal birds have different eyes, with the latter having a higher density of rods for good vision in poor light. Owls have tube-shaped eyes with a large retina to enable them to see well in poor light conditions. An owl’s eyes can account for up to 3% of its entire body weight and it is said that Tawny Owl has the sharpest eyesight of any owl, while Great Horned has the largest eyes.
The size of birds’ eyes relative to their body varies considerably. Ostrich eyes are the largest of any living bird (50 mm across) and the largest eye of any flying bird is that of Wedge-tailed Eagle (33 mm). While large birds tend to have large eyes, as do nocturnal species, there are exceptions. Kiwis, which feed at night, might be thought to have large eyes, but they use their sense of smell to forage. Consequently, they don’t need large eyes, and so have very small ones with a diameter of only 7 mm. Incredibly, small numbers of totally blind kiwis have been found surviving well in the wild.
Little research has been done into the size of birds’ eyes, but a recent study, using measurements of the eyes of 4,000 bird species taken in the 1970s but never published, has thrown up some interesting facts. The data from more than 2,700 species was correlated with information about their habitat and behaviour.
It was found that birds with the largest eyes live close to the equator where the rainforest habitat is largely quite dark. Birds which feed close to shaded forest floors have bigger eyes than those which live in bright sunlit places. Birds which eat insects have larger eyes, which help them spot small moving prey, and flycatchers have good long-distance binocular vision. Finches and tanagers, which feed on seeds and fruit, have smaller eyes and the smallest eyes mainly belong to nectar-feeders which probably rely more on colour than shape.
Birds use their left and right eyes to do different things. Peregrines mainly use their right eye to hunt, while pigeons use their right eye when feeding and their left eye to watch for predators. Tits and jays find cached food better with one eye, compared to the other. Studies of embryos within the egg show that the right eye is usually positioned close to the shell and so gets exposed to light coming through the shell, while the left eye is deeper inside the egg in relative darkness. If eggs are kept in darkness, the birds show no difference in eye use.
Birds are also known to use their eyes for more than what we understand as sight. Research into how birds detect magnetic fields has shown that a protein in the eye, called cryptochrome, is activated by light to become magnetically sensitive and that birds sense magnetic fields with their eyes. Chris Harbard