Feathers are the most diagnostic feature of birds. All birds have them, and no other animals, living or extinct are known to have feathers, although there has been considerable speculation since the 70's that some dinosaurs might have evolved them for insulation.

Feathers form a strong, light, warm and flexible integument and are a marvel of evolutionary form and function. In the physiological view of feather evolution, feathers evolved from scales primarily to protect early birds from cold, and they later became elongated into flight-related structures.

The colors seen in bird plumage are produced by a variety of pigments and structural adaptations of the feathers. Melanin produce black, grays, and brown. Carotenoids produce intense reds and yellows. Porphyrines produce a range of reds, browns and greens, notably the intense green of the great frigate bird male that nests on Tower Island, where we spent the day.

These green porphyrin pigments are related to hemoglobin and other bile pigments formed by the break down of hemoglobin by the liver.