When nature made the blue-bird she wished to propitiate both the sky and the earth, so she gave him the color of the one on his back and the hue of the other on his breast. – John Burroughs
As one might expect from the amazing diversity of colors and patterns exhibited by more than 9,000 bird species found in the world, birds can see color. In fact, they can discriminate a greater variety of colors than humans; as some birds can see into the ultraviolet range.
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.
Tiny air pockets in the barbs of feathers can scatter incoming light, resulting in a specific, non-iridescent color. Blue colors in feathers are almost always produced in this manner. Examples include the blue feathers of bluebirds, Indigo Buntings, Blue Jay’s and Steller’s Jays. – All About Birds, Color