@Wikipedia asked @smithsonian an intriguing question inspired by #NationalBirdDay:
— Wikipedia (@Wikipedia) January 5, 2018
And the Smithsonian replied with a winner:
Blue birds: not really blue. While red & yellow feathers are colored by pigments in what birds eat, no bird species can make blue from pigments.
— Smithsonian (@smithsonian) January 5, 2018
From the Smithsonian Insider:
“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.”