Why do many kingfishers have blue or blue-green feathers?

The blue and blue-green colors observed in many kingfishers are not caused by blue pigments. Instead, these colors are structural, resulting from the physical interaction of light with the microscopic architecture of the feathers.1

Blue coloration is produced by a thin, porous layer of keratin that overlies a dark layer of melanin pigment. When light strikes these feathers, the shorter blue wavelengths are scattered by the fine structure of the keratin, while the longer wavelengths (such as red and yellow) pass through and are absorbed by the underlying dark melanin. This phenomenon is often referred to as the Tyndall effect.123

Most green feathers in birds, including those of some kingfishers, are not the result of green pigments. Rather, they are created by the combination of this structural blue color with an overlying layer of yellow pigment. If the yellow pigment is removed, the feather will appear blue.13