Many birds with colored dorsal plumage have white underparts and among waterbirds extensive white plumage is common. Feathers are made of keratin, which is naturally transparent and colorless. They appear white because keratin foam or fibers scatter all frequencies of the visible spectrum. Colored feathers have keratin structures that reflect light selectively or mechanisms for depositing pigment within feathers. Color is not essential for flight, streamlining, insulation or waterproofing; some of the largest birds fly and breed successfully with all-white plumage. Adaptive scenarios concerned with camouflage, communication and thermoregulation have been based upon the visual properties of white feathers. These are discussed, but few examples are convincing enough to falsify the hypothesis that feathers may be white by default. Arctic land birds provide evidence for cryptic white plumage, but some white feathers are retained after they cease to be cryptic. White plumage also occurs in some tropical land birds.