Vertebrate vision depends on specialized photoreceptor cells found in the retina. These neurons come in two main flavors, rods, which are sensitive to low levels of blue-green light, and cones, which are much less sensitive in general, but which are attuned to different wavelengths of light. Most mammals have plenty of rods but no more than two kinds of cones, meaning that they see well in low light but are partially or completely color-blind. Humans and some other primates have red, green and blue cones, and thus can see a wide variety of colors. That might have seemed impressive, if it weren’t for birds.
Birds (and reptiles and fish) have five kinds of cones. In addition to our meager three, they have a cone that is tuned to detect violet and ultraviolet light, plus something called a double cone that may detect movement. It turns out that all vertebrates except for placental mammals have these double cones.
Joseph Corbo and his team from the Washington University School of Medicine, St. Louis Missouri have mapped the avian cones. They found that the five types of cones are evenly distributed throughout the retina, so that no two cones of the same type are next to each other. This arrangement maximizes the ability of birds to detect and distinguish colors, something that may be critical for identifying mates, or ripe fruit.
Why do birds have such superior color vision? One hypothesis is that unlike mammals, which spent part of their evolutionary history as nocturnal animals (to avoid dinosaurs), birds have always been diurnal (they were the dinosaurs).