According to scientists, birds have been evolving in a very specific way. And it looks like, given their abilities, they'll continue to evolve. We just won't be able to see it.
As we find out more about animals, we understand that the human being is a sadly hobbled beast. We lack the ability to sense the electric signals of muscles contracting, the way sharks do; and although we may have the capability to sense magnetic field lines, we certainly don't use it the way flying and swimming animals do. We also lack vision. There are huge ranges of color that birds, bees, and other animals can see that we don't consider 'visible light.'
Birds have an additional color cone in their retinas that allows them to see light well into the ultraviolet range. Their plumage is already remarkably colorful to human eyes. To bird eyes, feathers should take color to a whole new level. And yet when scientists tested to see what wavelengths of light bird plumage reflected, they saw that, in the ultraviolet spectrum, birds are drab. Though some birds, like the Saw-Whet owl pictured above and to the left, are bright red under UV light, the colors that their plumage reflects does not come close to the many different colors that they can see.
Plenty of birds stay drab as a way of blending in with their environments, but sexual selection has driven some to develop plumes of red, green, electric blue, and bright yellow. Scientists believe that, as birds evolved, the search for resplendent mates caused them to slowly dial up the amount of colors they display. And, although they have all the colors that human eyes can see, sexual competition hasn't yet started selecting for plumage in the ultraviolet range. Considering their increasing brightness, it probably will. Somewhere, some young hatchling will get a lucky gene that lets it dazzle a mate with a color human eyes can't see, and it will get the opportunity to spread its genes around. And when that happens, most people won't be able to see it.
Evolution will continue driving the diversity of birds, but we'll need ultraviolet-sensing goggles to appreciate it.
Image of Saw-Whet owl wings via Ojibway Nature Center