Yellow or red
feathers usually get their colors from pigments in the foods birds eat. However, blue pigments in foods like blueberries are destroyed when digested. So how do birds with bluebirds end up with blue feathers?
Dr. Richard Prum, and ornithologist at Yale, figured out that the color blue results from the way the particular structure of these feather interacts with light.
Dr. Prum and his colleagues analyzed hundreds of feathers from birds that look blue to us. As these feathers grow, stringy keratin molecules inside the cells separate from water. When the cell dies, the water dries up and is replaced by air. A pocketed structure of keratin protein is left. When white light strikes a feather made this way, the pattern causes red and yellow wavelengths to cancel each other out. On the other hand, the blue wavelengths are reinforced, amplified, and reflected back. The size and shapes of the air-filled cavities in the keratin impact the shade of blue seen by the eye.
Bluebird feathers are not iridescent like the feathers of a hummingbird, so the hue looks the same regardless of the viewing angle. The feathers of a male bluebird reflect a lot of UV light which makes them brighter. Feathers of females look more subdued because their structure is different. Humans can't see UV wavelengths, but birds can.
References and More Information:
Source: Why So Blue, By Helen Fields, Smithsonian Magazine, March 2012
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