I copy and pasted this from an article I found on a website:
When it comes to color vision, these farm fowl have bested humans in many ways, a new study finds.
The superior color vision comes down to a well-organized eye, structurally speaking, the researchers say.
They mapped five types of light receptors in the eyes of
chickens. They discovered the receptors were laid out in interwoven mosaics that maximized the chicken's ability to see many colors in any given part of the retina, the light-sensing structure at the back of the eye.
"Based on this analysis, birds have clearly one-upped us in several ways in terms of color vision," said study author Dr. Joseph C. Corbo of Washington University School of Medicine in St. Louis, Mo. "Color receptor organization in the chicken retina greatly exceeds that seen in most other retinas and certainly that in most mammalian retinas."
Birds likely owe this exceptional color vision to not having spent a period of evolutionary history in the dark, according to Corbo.
Night-vision relies on light-sensitive photoreceptors in the retina called rods, while daytime vision relies on receptors called cones.
During the age of the dinosaurs, most mammals became nocturnal for millions of years. Birds, now widely believed to be descendants of dinosaurs, never spent a similar period as nocturnal animals. As a result, birds have more types of cones than mammals.
"The human retina has cones sensitive to red, blue and green wavelengths," explained Corbo. "Avian retinas also have a cone that can detect violet wavelengths, including some ultraviolet, and a specialized receptor called a double cone that we believe helps them detect motion."
In addition, most avian cones have a specialized structure that Corbo compares to "cellular sunglasses," or a lens-like drop of oil within the cone that is pigmented to filter out all but a particular range of light. Researchers used these drops to map the location of the different types of cones on the chicken retina. While the different types of cones were evenly distributed throughout the retina, no two cones of the same type were located next to each other.
There's more to the article, but I think this is enough info. I hope it helps!