Thursday, May 24, 2018
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Bird vision is a sharp sight to see


Ohio's state bird, the cardinal


The common expression about having a bird s-eye view has a lot more to it than just having a high perch or an aerial perspective.

The bird world is awash with species and families that have evolved special adaptations that allow incredible vision under a wide range of conditions.

Thus, a duck is able to see well both atop the water and when diving beneath the surface. A falcon s amazing binocular vision helps it to stay focused on a dodging pigeon while dive-bombing at 200 mph. A hawk s broad visual spectrum allows it to see even the urine trails of mice in a farm field.

A vulture s eyes can distinguish flies on a dead animal so it can swoop in and feast.

Such are just samples of sight in the avian world, one in which the eyes clearly have it.

Birds are the most visually dependent of all animal species, states Dr. Susan Orosz, a Toledo veterinarian and internationally regarded authority on bird eyes.

Even though humans are highly visual, the information transmitted to the brain is only 40 percent of that transmitted by pigeons and chickens. Birds of prey have even greater visual acuity.

Pigeons can discern subtle color differences and other avian species are able to record and remember over 6,000 images of caches where food is stored.

Dr. Orosz noted that the eyes of such species as pigeons are placed laterally (on the sides) in the skulls, giving them a wide-angle 300-degree visual field, compared to just 150 degrees for a predatory barn owl, which has eyes directed frontally. The owl s particular advantage, however, lies in having binocular vision, wherein both eyes focus on the same object and eye movement is coordinated.

B i r d s c o m e e q u i p p e d with one of three types of eyeball: flat, found in daytime-active species with narrow heads, as among swans; globular, in daytime birds with wider heads, including both songbirds and birds of prey such as eagles, and tubular, found in nocturnal birds such as owls.

What is really amazing about the shape of the bird eye is that the retina is relatively flat, so that the rays of light that hit the retina are in focus from all directions. In comparison, humans have only a small spot in the back of the retina that is in focus, so they have to scan the entire area that they are looking at to get a clear image of what they are seeing.

Another telling adaptation is that birds can change the size of their pupils at will, not just in reaction to the amount of light, using special ciliary muscles.

They also can change their focus, in a process called accommodation, using strong, quickreacting skeletal muscle instead of smooth muscle. In contrast, mammals, including humans, use smooth muscle to change the shape of the lens slowly. So while humans use skeletal muscles to run quickly, Dr. Orosz said, a falcon also uses them to stay focused on a darting prey during a dive.

Focusing in different mediums such as air and water is another problem birds contend with uniquely, the veterinarian said. When diving into the water, the change to a water medium from that of air requires a change of about 20 diopters of correction power in order to focus correctly. In some of the diving ducks, the lens is significantly softer than in mammals so that the lens can be deformed into the hole of the pupil, providing the necessary change in the curvature of the lens to maintain focus to catch the fish underwater.

Some birds cannot do this, so they may be short-sighted on land, as are penguins, which can see perfectly well underwater. Terns can see well as long as they are in the air, but when they dive into the water they are long-sighted and often miss the fish they are attempting to catch. Other birds, like kingfishers, use a structure called a fovea in the back of the retina to focus on the object while in the air, and they have an extra fovea that focuses objects when in water.

Dr. Orosz noted that among primates and humans, the eye s lens cannot detect wavelengths of ultraviolet light, but birds can visualize much shorter wavelengths, allowing them to visualize many things we cannot.

Their increased spectrum allows them to discern males from females in one-color unisex species.

In other words, we can t tell the difference but they can. They can also detect the ripeness of food items because of this quality, and hawks can visualize urine trails of mice.

The retina of nocturnal birds eyes contain mostly rods, structures sensitive to low light intensity. Thus owls eyes can gather 2.5 times more light than humans, though Dr. Orosz notes that their night-hunting prowess may result in part from their sense of hearing.

Cones in the retina are responsible for keenness of vision and color vision. In daytime songbirds and predator birds, the cones are wired to the brain in ways that permit greatly enhanced detail.

Thus vultures can see flies on carcasses and know that dinner is served.

Birds see sharper images, too, because they have relatively large eyes that can gather light and provide a large screen on the retina on which to capture images. Eye shape, among other adaptations, allows keen focus on the retina, and tubular eyes in particular actually can magnify an image.

The bird s retina also is important during migration, Dr. Orosz said. There has been much interest in understanding how birds navigate at night or on cloudy nights. It appears that they have special cells in their retinas called cryptochromes that provide a magnetic compass to help them find their way. How the magnetic information appears to the bird is unknown. But changes in color depending on the direction, like that used in cockpits of airplanes, may also occur in birds.

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