When is a blue bird not blue? The answer to this question is always. There actually is no such thing as a blue bird. To find out why, Smithsonian asked Scott Sillett, a wildlife biologist at the Smithsonian Migratory Bird Center.
“Red and yellow feathers get their color from actual pigments, called carotenoids, that are in the foods birds eat,” Sillett explains. “Blue is different―no bird species can make blue from pigments. The color blue that we see on a bird is created by the way light waves interact with the feathers and their arrangement of protein molecules, called keratin. In other words, blue is a structural color. Different keratin structures reflect light in subtly different ways to produce different shades of what our eyes perceive as the color blue. A blue feather under ultraviolet light might look uniformly gray to human eyes.”