Birds see what we don't

When I was a teenager, a park ranger leading a nature hike happened to refer to some small birds seen only in fleeting glimpses as ‘little brown jobs’. It wasn’t to say that one bird was the same as another or that the ranger couldn’t have told them apart, but just that, to the human eye, many species of small brownish birds look quite similar. It shouldn’t come as a surprise that the birds themselves can tell each other apart quite easily. The interesting thing is how they do so.

Vertebrate vision depends on specialized light receptors found in the retina. These receptors come in two main flavors: rods, which are sensitive to low levels of light, and cones, which are much less sensitive, but which are attuned to specific 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 have at least one extra cone, allowing them to see in ultraviolet. Unlike mammals, they also have oil droplets in their cone cells that help filter and magnify differences in color. Taken together, birds have much better color vision than we do.

The four bird cones. Human eyes can only see light from wavelengths longer than 400 nanometers.

This means that the same drab brown plumage might look completely different to birds. In particular, bird species that were assumed not to have any sexual dimorphism, that is, in which the male and female were thought to be identical, turned out to be quite different. 

http://wikis.evergreen.edu/m2o1112/index.php/Birds

You can learn more about the ways birds use their UV vision from The National Wildlife Federation.

By the way, birds are far from the color-vision champions of the animals world. Mantis shrimps have at least twelve different cones.