Most birds are tetrachromats or have four types of cone cells; red, green, blue and ultraviolet.* These four types of cones are what allows them to process a wider wavelength spectrum than humans. Their spectrum extends to the UV range of 300-400 nm.
Although birds are not the only ones who have UV vision (fish, amphibians, reptiles, and insects also have the ability), it does provide them with evolutionary advantages. One of the most important functions is orientation. UV vision gives birds an advantage over trichromats and dichromats in being able to identify shapes and objects which facilitates in travel. Another important function is the increased ability to look for food. UV vision allows birds to more easily spot insects, berries, seeds and mammals. Another function is signaling, especially during mate selection. The UV allows birds to differentiate between the desirable and undesirable candidates for mating.
* Humans have only three types of cone cells: blue, green, and red.
All of these images are from This Is How Birds See The World As Compared To Humans And It’s Pretty Amazing , Bored Panda.
Image credits: Klaus Schmitt
On the left is what we humans see, on the right is (probably something like, I mean, who knows, we should ask the bird) what birds see:
Image credits: Cynthia Tedore
Image credits: Joel Sartore