A keto- gem. There are also several variations in the structure of birds' feathers or the presence of so called powder particles which can influence their colouration.
Melanin is present in tissues as granules.
Melanin granules are found in melanocytes and melanophores. In melanophores the movement of melanin granules may change the colour of the animal or produce different shades. The dark skin is observed when the granules are diffused whereas a pale colour of the animal is produced if the granules are concentrated. A screen of granules and the light scattering due to little particles produce the Tyndall blue which we admire in eyes , skin and feathers of many animals. In conclusion blue and green colours are see in Nature only if melanin is present.