Vision is almost universal within the animal kingdom but seeing abilities vary greatly in different animal species. Birds and a few other vertebrate animals (humans for one) see in color. Many other species see only in shades of gray or, more correctly, in shades of brightness. The blind animals, such as some cave dwellers, lost their vision as they adapted to a life of total darkness but they probably still detect some changes in brightness. With so much diversity in the physical and biochemical characteristics of vision throughout the animal kingdom, one could surmise that vision originated independently along several evolutionary pathways.
What is color vision? The simple answer: color vision requires photo receptors that detect light as a function of wavelength. An animal needs at least two kinds of photo receptors operating at the same time to have color vision. We've all heard of the rods and cones in the eye. The rods are for night vision and don't go to great lengths at differentiating color (the spectral or wavelength content of light). The cones are much more color discriminating and operate mostly in daylight or in artificially lighted environments. Wavelength filters and other adaptations to the photo receptors of the cones facilitate color vision.
To me, the biochemical process that changes the light received into different neural inputs based on wavelengths and allows the brain to perceive color differences is a real miracle. For readers with infinite curiosity, read up on the functioning of spectral sensitivity, carotenoids, chromophores, isomerization, retinal, opsin, and rhodopsin. For the rest of us, avoid a vitamin A deficiency and have faith. In earlier lessons we explored many different ways color is used to either advertise oneself or to blend into the environment. Changes in a predator's ability to see go hand-in-hand with adaptations in a prey's ability for camouflage. Advertising for a mate and avoiding an enemy are practical reasons for colors but this lesson is more concerned with the actual detection processes.
As background, imagine a rainbow. Why do we see discrete bands of color when, if we measured the radiation with a spectrophotometer, the wavelengths would decrease continually and smoothly from the outside to the inside of the bow? Our neural processor (brain) perceives a discreteness based on the input of the photo receptors (chromophore plus opsin). Our perception may not be reality, but it is much more useful to perceive distinct colors and not just gradients. We learn to distinguish between colors and even make up names for colors and shades of colors, but many mysteries exist when we attempt to understand the perception of another animal.
Generally speaking, bird vision is highly developed and in many ways surpasses human sight capabilities. Diurnal birds have many more cones than we do and are much better at color detection. Bird's eyes not only differ from humans, but differences also exist between species. For example, many hawks have a flat lens placed far in front of the retina. By the time the image hits the retina, it is enlarged, like the picture hitting your eye when looking through a spotting scope. Placement of the eye also varies. Each eye has a specific field of view and, where these views overlap, depth perception is enhanced. Frontally placed eyes are great for depth perception, but peripheral vision is lost. Eyes on the side of the head provide great peripheral vision, but not much in depth perception.
In man, binocular vision is about 140 degrees out of a total view of 180 degrees--we have great depth perception but must turn our head for peripheral views. In the Rock Dove the binocular vision is only about 25 degrees, but the field of view is approximately 320 degrees--good for seeing a falcon approaching. Owls need binocular vision to locate and capture their prey and have about 70 degrees of binocular vision with a total of 110 degrees of view. Owls cannot move their eyes in their head, but can rotate their head some 200 degrees. American Woodcock eyes are placed on the back region of the side of their head. Woodcocks feed by probing the mud with their sensitive beaks -- no need for binocular vision when feeding. However, even with their bill in the mud, a woodcock can scan the area above, behind and in front of it for danger.
There is more to superior eyesight than magnification and eye placement. The real key is the cones. Many species of birds have cones sensitive to ultraviolet light, which is invisible to humans. In addition to cone density and cone sensitivity, two more factors contribute -- foveae and oil droplets.
Foveae are extra dense concentrations of cones in special retinal pits. Over a million light-responsive cells can be packed into a single fovea. These foveae are thought to fine tune the image, giving it sharpness, improving definition and better detecting movement.
The oil droplets are red, orange, yellow, green, or clear. Oil droplets filter out certain wavelengths making it possible to detect subtle hues. Bird feeding behavior often provides clues to the abundance of these oil droplets. Birds like kingfishers, needing to see through the surface of water, have an abundance of red and yellow oil droplets. Birds that spend much time underwater, like goldeneyes, also have an abundance of yellow droplets. Have you ever put on a pair of yellow sunglasses or yellow swimming glasses to see past the reflective surface of water and to have a clearer view through water? Night feeding birds tend to have only clear oil droplets, as the colored ones reduce brightness.
It's exciting to think about the different adaptations and possibilities as we watch an Osprey see past the sparkling veneer of a stream and monitor several trout, hoping one will swim closer to the surface. Or to think about a hummingbird's view of the ultraviolet range of flower color to better identify nectar producers. We wonder why the Vermilion Flycatcher male is so red and wonder if finding a mate is more important than avoiding a predator. We love the blue of the Mountain Bluebird but know many bird species don't perceive blue as we do. Are they just seeing a dull gray bird?
Birds and humans are both very visually oriented. It would take several books to adequately discuss vision and behaviors related to vision acuity. For now, I hope I sparked your curiosity.