An online resource based on the award-winning nature guide

Eyes

Owl Night Vision

1-5-15  barn owlIMG_6003Like humans, birds have a sensitive retina in the back of their eyes that absorbs incoming light, senses it, integrates the information in it, and sends this information on to their brain. An avian retina is much thicker than ours and contains more rod cells (for dim light vision) and cone cells (for acuity and color vision).

As most owls are active at night, their eyes must be very efficient at collecting and processing light. The eyes of owls are disproportionately large compared to the size of their skull, and enable them to collect as much light as possible. In addition, the retina of an owl’s eye has an abundance of light-sensitive rod cells — owls have almost a million rods per square millimeter compared to humans which have only about 200,000. Barn owls can see a mouse at 6 – 7 feet with an illumination of .00000073 foot-candles – the equivalent of humans seeing a mouse by the light of a match a mile away. Eye size, an abundance of rod cells and additional neural mechanisms provide owls with vision greater than that of most of their prey.

Since owls have extraordinary night vision, it is often thought that they are blind in strong light. This is not true, because their pupils have a wide range of adjustment, allowing the right amount of light to strike the retina. Some species of owls can actually see better than humans in bright light. (Photo: barn owl, in captivity)

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Nictitating Membrane Provides Moisture & Protection to Eyes

12-4-14  crow-nictitating membrance 109You and I have two opaque eyelids, one above the eye and one beneath. When we blink, they meet in the middle of our eyes. Some birds, amphibians, reptiles, fish and mammals have three eyelids – two similar to ours, and a third translucent or transparent eyelid, called a nictitating membrane. This membrane moves horizontally across the eye from the inside corner to the outer edge of the eye, much like a windshield wiper, when needed for protection, to clear debris or to moisten the eye. Although this American Crow’s nictitating membrane looks as if it was blinding the crow, it isn’t. Because of the membrane’s translucency, the bird can still see when the membrane is covering its eye.

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Nictitating Membranes

12-17-13 crow 054If you look closely, you’ll see that part of this American Crow’s eye appears white. This white (actually transparent) part of the eye is referred to as a “third eyelid,” or nictitating membrane. Many animals, including some fish, mammals, reptiles, amphibians and birds, have nictitating membranes. Instead of moving up or down like the upper and lower eyelids, they move horizontally across the eye, much like windshield wipers. These membranes lie under a bird’s eyelids. When not in use, they are curled up in the inner corners of a bird’s eyes, but can be quickly drawn across the eyes to protect, clean and moisten them without shutting out the light.

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Dragonfly Eyes

7-8-13 dragonfly eyes2 381Dragonflies (and bees) have the largest compound eyes of any insect, each containing up to 30,000 facets, or ommatidia (house flies have 6,000). Each facet points in a slightly different direction and creates its own image, and the dragonfly’s brain compiles these thousands of images into one picture. This eye structure enables dragonflies to be extremely sensitive to motion. Because a dragonfly’s eyes wrap around its head, it can see in all directions at the same time (though its forward-looking vision is the sharpest). When capturing prey, a dragonfly doesn’t chase it – it intercepts it in mid-air, and it’s successful nearly 95% of the time. This hunting technique entails calculating the distance of its prey, the direction it’s moving and the speed that it’s flying – an impressive feat any tennis, baseball or football player would especially appreciate!

(Photo is looking down on the eyes of a Common Green Darner. Three simple eyes, or ocelli, are located in the black section below (above in the photo) its two tan compound eyes. The short, thin black lines are its antennae, which can detect wind direction and speed. The yellow section is part of the upper half of its face, or frons.)