There's a fascinating article in the July 2006 issue of Scientific American, but alas, it's not available online without a subscription. Entitled "What Birds See", it describes the ability of birds to see and respond to colors beyond what we mere humans can see, into the near ultraviolet.
Humans have three different types of cones in our eyes. We detect color when our brains receive signals from two or more of these types of cones, and our brains compare the signals to form the sensation of color. Color itself is not actually a property of light or of objects that reflect light; it only exists once the brain has processed the data from the cones. That's why, incidentally, that we can use three colored lights in televisions to simulate all the colors; red and green lights, when mixed in the proper proportions, stimulate the cones in the same way that a yellow light would.
Mammals used to have four types of cones, way, way back, millions of years ago, but in those days, mammals were mainly nocturnal critters, so they had more need for the rods that enable vision in dim light. At some point mammals lost two types of cones; happily for us, the primate lineage regained one at some point.
But birds still have four different varieties of cones, so they can perceive many more colors than we can. Think of the color range that humans can sense as being represented by a triangle; to obtain all the additional colors that birds can see, we would have to expand that triangle into three dimensions to make a three-sided pyramid. That's a lot of colors!
Reminds me of the very early story by Isaac Asimov called The Secret Sense where (I'm writing this from memory, as I haven't read the story in over thirty years) a human is temporarily given a sense available only to an alien race; when he loses the sense, he feels like he has been blinded.
Remember that the next time you see a bird; they can perceive far more about this world than you can.
Posted by jt at July 9, 2006 04:31 PM