The Perfect Design of Owl Eyes
The perfection in the design of an owl's eyes is what enables them to attack their prey with utmost precision. If a human has to hold a staring contest with an owl, then that person will be defeated without exception. But would it be considered cheating? Owls have a third clear eyelid called the nictitating membrane that enables them to blink less and at the same time keep their eyes moist and protected. This piercing stare makes them formidable in any staring competition.
The eyes of the owl are so large in their skull that they gain extra support with bony structures, called the sclerotic rings, around them. This prevents them from being able to move their eyes from side to side. Hence, they balance out this lack of movement by being able to turn their heads as much as 270 degrees. One difference between an owl's and human's eyes is that, unlike a human eye, which is spherical, an owl's eye is an elongated tube that collects more light in their vision at night. Again, they can also dilate and constrict each pupil independently, therefore controlling the amount of light reaching their retinas.
But of course, with all those eagles, hawks, and falcons present, the owls stand out at dusk. Adaptations toward nocturnal hunting include silent-flight feathers and asymmetrical ears to triangulate sound, with powerful talons, and one of its most striking features is the eyes.
Owls have faces that allow their eyes to be placed in front making excellent depth perception, a feature crucial for the timing of its attack possible. No other birds have such faces. The prey animals have eyes on the sides of their heads, which give up some of the available depth perception to gain a wider field of view in order to detect danger on many sides.
Owls have human-like, binocular visions, but an added component they have is the nictitating membrane, which functions as a "third eyelid" to help protect the owl's eyes from debris when diving down on its prey. Many owl species are nocturnal, meaning they hunt mainly in the dark. Their large eyes collect enough light to see well after dark. Owl eyes may account for up to 5% of body weight, compared with about 0.0003% for humans.
Their pupils are big and can open widely to allow as much light as possible onto the light-sensitive retina at the back of the eye. Their retinas have a higher ratio of rods (light and motion detectors) to cones (color-detecting cells) compared to human eyes, which gives owls extraordinary capabilities in detecting movement within their visual field under low-light conditions.
Another adaptation for night vision is the tapetum lucidum, a reflective layer at the back of the owl's retina. It reflects light onto the rods, increasing their sensitivity in low-light conditions. Owl eyes can be up to 100 times more sensitive to low light than human eyes. However, owls are far-sighted and struggle to focus on nearby objects. They rely on sensitive bristles around their beaks to detect small objects when they are close to their faces.
It is due to the structure of owl eyes, which are more elliptical rather than human eyes, that low-light vision works effectively. Sclerotic rings keep their large eyes in place, thereby restricting side-by-side movement. Owls compensate for this limitation by turning their heads up to 270 degrees from side to side and up to 90 degrees up and down.
There's a common myth that owls are blind during the day. The truth is, their pupils do shrink in bright light, similar to humans, and they can regulate the amount of light entering each eye. Owls are not blind in daylight; in fact, some owl species are quite active during the day. The color of an owl's eyes often reflects its activity pattern: owls with darker eyes tend to be nocturnal, while those with lighter eyes are more likely to be diurnal or crepuscular.