• Owls can’t move their eyeballs. 
     
    That’s because owls don’t have eyeballs at all. Instead, their eyes are shaped like tubes, held rigidly in place by bones called sclerotic rings. (Human eye sockets, which hold spherical eyes, do not have sclerotic rings.)
     
    Because owls can’t roll their eyes around the way we do, they have to move their entire head to get a good look around. They frequently twist their head and “bob and weave” to expand their field of view. Owls can turn their necks about 270° in either direction, and 90° up-and-down, without moving their shoulders! 
     
    Although owls can’t move their eyes, many other adaptations help these raptors spot prey.
     
    Size
    Owl eyes are huge! An owl’s eyes can account for up to 3% of its entire body weight. (Eyes account for about .0003% of a human’s body weight.)
     
    There is one drawback to their large eyes. Owls are very farsighted. They can’t focus on objects that are too close. Instead, sensitive whisker-like bristles around their beaks help owls detect objects at close range.
     
    Binocular Vision
    Owls have terrific binocular vision compared to other birds. Binocular vision describes the ability of animals with two eyes to see an object with both eyes at the same time, giving an animal increased depth perception
     
    Like all birds of prey, an owl’s eyes face the front. This allows them to have a much greater range of binocular vision than animals with eyes situated on the sides of their heads. An owl’s field of view, for instance, is about 110°, and about 64% of that is binocular. A sparrow, on the other hand, has a much larger field of view, up to 300°, but their binocular vision is much more limited, sometimes only 10%.
     
    Night Vision
    Most owls are nocturnal, meaning they are most active at night. Two adaptations help owls see well in the dark.
     
    First, owl eyes are dominated by densely packed retinal rods. All animal eyes have photoreceptors shaped like cones and rods. Retinal cones function best in bright light and are responsible for color vision. Rods are much more sensitive and function best in dim lights. Owls have almost a million rods per square millimeter (1,550 per square inch). Humans have about 200,000 rods per square millimeter (310 per square inch).
     
    Finally, owls have “eyeshine.” Eyeshine is a result of an animal’s tapetum lucidum—a layer of tissue behind the retina that reflects visible light. This reflection dramatically increases the light available to the animal’s photoreceptors, and gives it superior night vision.
     
    Protection
    Like many animals, owls have three eyelids. The upper eyelid closes downward when the owl blinks. The lower eyelid closes up when the owl sleeps. 
     
    The third eyelid is called a nictitating membrane. This translucent eyelid moves horizontally across the eye, from the inner corner of the eye to the outer corner. The nictitating membrane is especially useful when an owl is catching prey—the owl can see still see even while keeping its eyes safe from injury.   
    1. What other animals do you think might have sclerotic rings in their skulls?

      Reptiles—including dinosaurs—have sclerotic rings holding their eyes in place.

    2. Do you think humans or owls have greater binocular vision?

      Humans! Humans have a field of view of about 180°, and our binocular vision is about 74% of that. Owls have a field of view of about 110°, and about 70% is binocular.

    3. What other animals do you think have a tapetum lucidum?

      Many, many animals have a tapetum lucidum. Cats, dogs, deer, and even a few primates such as lemurs and aye-ayes exhibit “eyeshine.” 

    4. What other animals do you think have a nictitating membrane?

      Many aquatic animals, such as alligators, frogs, sharks, and seals, have protective nictitating membranes. Other animals with nictitating membranes include camels, lemurs, and bears.

    • Great horned owls have the largest eyes of any owl, and among the largest of any terrestrial vertebrate. If a great horned owl was as large as a person, it would have eyes the size of oranges.
    • Tawny owls have the sharpest eyesight of any owl, and among the sharpest of any terrestrial vertebrate. Tawny owls may see up to 100 times better in low light than humans, although that number is probably exaggerated.
    • Not all owls are nocturnal! Some pygmy owls, burrowing owls, and snowy owls are either diurnal (most active during the day) or crepuscular (most active at dawn or dusk). The eagle owl is sometimes diurnal, and has even better daytime vision than humans.
  • Term Part of Speech Definition Encyclopedic Entry
    adaptation Noun

    a modification of an organism or its parts that makes it more fit for existence. An adaptation is passed from generation to generation.

    Encyclopedic Entry: adaptation
    binocular vision Noun

    vision in which both eyes are used to produce a single image.

    bone adjective, noun

    structure composing the skeleton of vertebrate animals.

    cone Noun

    cone-shaped photoreceptor in the retina of the eye, sensitive to color and light.

    densely Adverb

    heavily or crowded.

    depth perception Noun

    ability to judge spacial relationships between objects, especially their distance from each other and the observer.

    dominate Verb

    to overpower or control.

    eyeball Noun

    the entire round part of the eye.

    eye socket Noun

    hole in the skull where the eyeball and its associated tissues are secured. Also called the orbit and eye socket orbital cavity.

    field of view Noun

    extent of vision at any particular time.

    keen Adjective

    sharp.

    nictitating membrane Noun

    third eyelid that can be drawn horizontally across the eye for protection. Also called a haw.

    nocturnal Adjective

    active at night.

    photoreceptor Noun

    specialized cell that is sensitive to light.

    prey Noun

    animal that is hunted and eaten by other animals.

    raptor Noun

    bird of prey, or carnivorous bird.

    retina Noun

    sensitive tissue at the back of the eye that receives images and sends signals to the brain about what is seen.

    rigid Adjective

    stiff.

    rod Noun

    cylinder-shaped photoreceptor in the retina, sensitive to low light.

    sclerotic ring Noun

    rings of bone that support the eyeballs of many vertebrate animals.

    situate Verb

    to place or arrange.

    spherical Adjective

    rounded and three-dimensional.

    superior Adjective

    better than something else.

    tapetum lucidum Noun

    layer of reflective tissue in the back of the eyes of some vertebrate animals.

    translucent Adjective

    almost clear.

    visible light spectrum Noun

    light and colors that can be seen by human beings.