Biochrome Pigments - These are natural chemicals that absorb certain wavelengths of light and reflect others. There are three main types:
Melanins - produce grays, blacks, browns, and buff colors.
Melanins are present in almost all birds, and are made from an amino acid by pigment cells.
The distribution of melanin particles in barbs and barbules create bar or speckled pattersn in the feather.
Melanin can make a feather more resistant to wear or fraying (hence the dark wing tips of gulls), and can protect against damage from sand abrasion (in desert species).
Melanins can help resist the damage to feathers by bacteria - which is why birds living in wet climates tend to be darker than those in dry climates.
Melanin also absorbs radiant energy (sunlight), which aids temperature regulation and drying of wet feathers.
Carotenoids - produce bright yellows, oranges, reds, and certain blues and greens (except in parrots, which produce psittacofulvins).
Carotenoids come from a bird's diet and are then modified by cells in the feather follicle, as the feather is forming, and depoisited in barbs and barbules.
Porphyrins - produce bright browns and greens and a unique magenta color.
Porphyrins tend to contain iron (brown) or copper (green).
They are easily destroyed by sunlight, so are mostly found in new feathers.
Structural Colors - These result when the structure of the feather alters the components of the light that hits them.
Incoherent reflectance - all light waves are scattered to produce white feathers. Light bounces off of air spaces in the cells of the feathers. (Rock Ptarmigan)
Coherent light scatteringby melanin granules in the barbules. Hollow melanin granules, arranged in stacks or crystal structures, create iridescent colors that change when looked at from different angles. (Hummingbirds)
Coherent light scatteringby air bubbles in the keratin of the rami (the shafts of the individual barbs). This is similar to the previous type, but the bubbles are not arranged in any structure, so they look the same form all angles. (Blue Jays and Bluebirds)