The colors in the feathers of a bird are formed in two different ways, from either pigments or from light refraction caused by the structure of the feather. Red and yellow feathers get their color from actual pigments (carotenoids) that come from the bird’s diet. Blue, however, is a structural color, created by the way light waves interact with the feathers and their arrangement of protein molecules, called keratin.
Thus, no birds have blue feathers made from pigments – they are blue strictly from the structure of the feathers. Different keratin structures reflect light in subtly different ways to produce different shades of what our eyes perceive as the color blue.
If you observe the blue feather of an Eastern Bluebird, Blue Jay or Indigo Bunting in normal lighting conditions you will see the expected blue color. However, if the feather is back-lit, and the light is transmitted through the feather, it will look brown. The blues are lost because the light is no longer being reflected back and the brown shows up because of the melanin in the feathers.
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