Eyes Like A Hawk

Dr. Ivan Schwab on how animals see, via the Dish:

Birds, on the other hand, possess rich color vision, in many cases better than our own. Most birds have four cone visual pigments, although this varies. In general, birds have an additional ultraviolet pigment in their cones and many more cones than we have. Furthermore the visual pigments that would be similar to ours span different wavelengths. Their visual experience is richer than our own in ways impossible to describe or understand. Not only do they see more colors, but the interpretation of colors would be different. Think of combining different colors of paint—if you combine more colors radiating from the same object, like a flower, you will see different colors. A hummingbird, then, would see a red flower as a different color because of the ultraviolet channel input.
You may ask what good are these extra color channels in birds? Of course, it’s hard to know completely since we can’t even understand the perception of the color “ultraviolet,” but here is an example. When a mouse is being hunted by a hawk, it will often urinate out of fear and to make itself as light as possible for escape. Mouse urine radiates ultraviolet and that actually helps the hawk follow the mouse trail. Fresher urine radiates more ultraviolet light. The ultraviolet arrow will point to lunch for the hawk.