Why are some feathers blue? (2012)

Scientists theorized that birds look blue for the same reason the sky looks blue: Red and yellow wavelengths pass through the atmosphere, but shorter blue wavelengths bounce off of particles and scatter, emitting a blue glow in every direction. Richard Prum, an ornithologist at Yale, discovered that birds make blue feathers in a different way. Building such precise nanostructures is an exceptional evolutionary feat of engineering, yet the color blue has popped up independently on many different branches of the bird family tree, especially in males.

Source: www.smithsonianmag.com