As we've discussed, the human eye has three different types of color receptors that respond three basic colors: red, green, and blue. Or other colors are seem to combinations of these different types of the receptors. Birds, however, have receptors for these three basic colors and for ultraviolet or UV light. Thus they have much greater color range. That's really interesting though, is the role that UV light plays in a bird's life. Scientists have discovered that the bird's eye is very extremely sensitive to ultraviolet light. In one experiment, robins were able to distinguish between two objects. One completely unlit and one lit by very dim ultraviolet light. Even with the very dim UV light, the object was radically?readily apparent to the robins. In another experiment, female birds were exposed to two male birds. One was behind the filter transparent to the UV light, and one was behind the filter that blocked UV light. The female birds preferred the male birds behind the transparent filter. Those birds' feathers may have appeared more colorful. Hunting bird use UV light to track down small animals. This is possible because small animals use urine to mark their trails and urine observed?absorbed UV light. The hunting birds can use the UV light to follow the trail to their prey.