Birds are near the top of the list when it comes to nature's brightest creatures. A bird's feathers are what sets them apart from other animals and gives them the chromatic advantage. We are all intimately familiar with pigments in the form of melanin, which is what gives color to our hair and skin; birds also have melanin, but they have the added advantage of structural colors.
While pigmentation is the most common form of coloration in the natural world, structural colors are what make birds one of the most vibrant species on the planet. The bright blues and greens along with the iridescence of their feathers are a result of structural colors. These colors are created by the composition of the barbs and barbules on a bird's feather. Barbs are the flexible offshoots from the central rachis (i.e. the hard backbone of a feather which runs down its middle), while barbules extend off of the barbs and interlock to create the flat surface of a feather.
In the case of blues and greens, keratin is structured in a porous manner that reflects shorter wavelengths (i.e. violet, blue and green) while longer wavelengths are absorbed by a melanin layer. A quick note on biology here as 'color' interpreted by the human eye is actually the reflection of light off a surface; so it is the keratin that creates the brilliant blues and greens that are common in bird feathers. Keratin is a fibrous structural protein that is also found in human skin and nails. The porous nature of this protein allows for air and light to interact within the keratin formations. The variation in a feather's hues is an effect of differential structural composition of keratin and absorption by melanin.
Iridescent colors in birds are the result of coherent scattering, whereby the various angles of the internal feather structures affect the directional scattering of light. This is why the iridescent colors on hummingbirds and peacocks appear black at certain angles. Iridescent feathers are made of barbs and barbules that are composed of hollow melanin granules. These hollow granules allow for increased reflection and scattering of light in specific directions. Melanin is organized in layers which are stacked on each other; a higher number of granule layers creates a more intense iridescence.
But who cares? Why does this even matter, they are just pretty colors right? In the avian world, colors are a big deal. I'm certain I would garner a lot of attention if I twirled around in an aluminum foil tutu, and birds work on a similar but far more effective principle. Biochrome pigmentation is affected by diet; in certain birds, those with the best diet typically have brighter hues of red. This is of great importance to the female population as they want a mate that will bring home the most and the best food for the female and their chicks while they are nesting.
The iridescent colors in hummingbirds, birds of paradise and peacocks are part of their mating call as the males show off their bright feathers to attract a mate. This is taken to the extreme in the case of the male Spatuletail hummingbird that is found in remote areas of Peru. While the female is nothing to write home about, the male posses two very long, curved feathers that end in large disc-like structures. The hummingbird flies around and shakes his tail feathers in the hopes of impressing the female. Birds of paradise and peacocks can be seen puffing out and shaking their shiny feathers for the same purpose. So in these cases, the male bird 'struts his stuff' using both pigmentation and structural colors in order to attract the opposite sex.
Above is a video showing the Spatuletail hummingbird showing off its tail feathers to a female.