Date of this Version
That the visual range of at least some birds extends into the ultraviolet region has been known since the early 1970s, when it was first discovered in hummingbirds and pigeons. The ultraviolet region is that energy consisting of light waves shorter than Violet, and thus beyond human perception. Although UV perception has long been recognized to occur in many insects and other invertebrate groups, scientists had doubted that either birds or mammals have this ability. Instead, it was generally assumed that the vertebrate cornea provides a protective block to ultraviolet waves, perhaps because UV energy can do damage to the living cells of the retina. Similarly, melanin in the dead mammalian cells of the skin's surface layer provides such a block for preventing UV light from reaching living cells underneath.