Birds' and Bees' Color Vision

[Updated]

A table listing the wavelengths and frequencies
for the peak sensitivities of
• human's three types of cone,
• cat's three types of cone,
• dog's two types of cone,
and peak solar output.

Color Wavelength Frequency
infrared longer than 700 nm,
(invisible to humans; may be felt as heat) 		fewer than 430 THz
deep red: 700 nanometers,
(longest visible wavelength for humans) 		430 terahertz
red (human peak): 575 nm 520 terahertz
dog peak in yellow-green: 555 nm 540 terahertz
cat peak in yellow-green: 550 nm 540 terahertz
green (human peak): 535 nm 560 terahertz
cat peak in the light-blue-green: 500 nm 600 terahertz
solar peak: 480 – 520 nm,
(in the light blue to green range, depending on the temperature used to represent the surface of the sun, which is not clearly defined) 		620 – 580 THz
cat peak in blue: 450 nm 666 terahertz
human peak in blue: 445 nm 674 terahertz
dog peak in blue-violet 429 nanometers,
700 terahertz
violet 400 nm,
(shortest visible wavelength for humans, except for a few) 		750 terahertz
ultraviolet less than 400 nm,
(invisible to humans, although I read that in World War II the US Navy found that a few sailors could see further into the ultraviolet than most) 		more than 750 THz
Sources:
http://www.photo.net/photo/edscott/vis00010.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colcon.html

Birds can see ultraviolet and have at least four types of color sensitive cone cells. Humans have only three types of cone. (`Rods' are for dimmer, grey, night vision.)

On the other hand, bees are like humans in that they have only three receptor types. But in contrast to humans, bees are sensitive to ultraviolet but not to red.

Ronn Blankenship, who started me on this, said that dogs have two types of color sensitive cones, not three like humans. They see like certain kinds of color blind humans. Blankenship added that to help humans with red-green color blindness, the lights in traffic signals are red-orange and blue-green. This way the lights do not look exactly the same.

Update: A site for veterinarians says that

Dogs have cones that are receptive at 429 and 555 nm and are dichromats. All evidence suggests that the dog is dichromat with vision similar to a human who is red-green color blind.

Blankenship went on to say that cats have at least two types of cone, and possibly three, but that

... the number of cones per unit area of the [cat] retina is significantly less than in the human retina, so cats probably see colors as rather pale and washed out.

Update: The veterinarians' site says that cats are weak trichromats. Feline cones peak at 450, 500 and 555 nm. They live in a world of fuzzy pastels.

He added that `some types of birds have five types of cone'. I find it impossible to imagine such a bird's color vision. They see many more shades than we.

Blankenship provides more detail:

A bird's retina actually has three types of photoreceptors that `translate' light into nervous impulses:

Moreover, according to Blankenship's link,

... bird retinas, in contrast to human retinas, contain no blood vessels. This prevents shadows and light scattering, which cut down on human vision.

Update: The veterinarians' site says that acuity is 30 cycles per degree (cpd) for humans, 18 cpd for horses, 12 cpd for dogs and 6 cpd for cats , which means a resolution of

Many birds see more acutely than humans,

The denser that cone cells are, the sharper is the perceived image. The human eye has at most 200,000 cones per square millimeter, while House Sparrows have approximately twice that number. Hawks, who must spot small prey from the sky, possess about five times as many as humans! Songbirds and predators such as hawks are believed to have the sharpest vision among birds. They can see details at distances two to three times farther away than humans.

If I understand this right, hawks' resolution is 12 arc-seconds!

The veterinarians' site also says

Dogs and cats appear to respond to the blue and yellow short-wave length colors the best, but appear to have trouble with green and red. Both are also rod-dominant animals. As rods do not function in daylight these animals are dependent on their few cones for spatial and temporal visual resolution, which probably means that their blue and yellow visual world is a fuzzy blue and yellow world. What appears red to us is simply dark to the dog and cat, and a part of the green spectrum is indistinguishable from white. Colors that would appear very rich to us are more pastel-like to the cat. The cat sees a green, grassy lawn as a whitish lawn, and a green rose-bush as a whitish bush with dark flowers.

Blankenship goes on to say

I have read that an owl's eyes are around 100 times as sensitive to light as the average human's (a cat's are 6 times as sensitive).... ...while there are about 6,000 stars over the entire celestial sphere which are visible to the average human, with a cat's sensitivity it could see over 40,000 stars, and an owl should be able to see in excess of 1 million stars.

You can create star charts to show what humans, cats, and owls see. Simply set differing limiting magnitudes:

Update: According to a site on British garden birds,

Birds such as pigeons and waterfowl, whose eyes are side facing, have so little binocular vision that rely on apparent motion between close and distant objects to judge distance.
(This site also has diagrams of the ranges of human, owl, and pigeon monocular and binocular vision.)

As for why color vision developed in the first place, Mickey Rowe writes,

The advantage [of color vision] ... comes in the form of visual contrast. The lowest level of visual information processing is the recognition that something is different about a given region of space--i.e. that there is food or a predator "over there". To perform this function ... it's best to have at least two pigments, one matched to the dominant wavelengths and one offset from those wavelengths. With the matched pigment, non-reflective objects have high contrast as dark areas on a bright background. With the offset pigment, reflective objects will apear bright against a darker background. ... it's easy to imagine that if an animal has more photoreceptor classes it has a greater chance ....

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