Human eyes have 3 cones, where-as birds have 4 cones. It is believed they can see in UV light... which I suppose is why some species of parrots have yellow heads!
Evolution of Retinal Structures
http://www.csulb.edu/labs/bcl/elab/a...sion_intro.pdf
The Bird Eye and Bird Vision
Ultraviolet Vision [SoBS - UoB]
What do birds see? (speculation)
Tetrachromacy - Wikipedia, the free encyclopedia
To make a point, I took a black light and shone it on 4 budgies and 1 cockatiel (okay, I shone it on bourkes and a conure, too, but they didn't show up!) You can see the results below!