The total of feathers, the outstanding character distinguishing the Class of birds (Aves) from all others. Beside the great differences in plumage between species, there are differences related to age, sex and season, and even individual variation sometimes occurs. The plumage of individuals is periodically renewed (moult). This often is the case after the breeding season, giving the bird its 'non-breeding plumage', which regularly differs from the 'breeding plumage'. The plumage functions as a mechanical and thermal protective covering of the body and is an essential component of the flight organs (wings, tail), also streamlining the body. As it is the most important factor in the appearance of a bird, it plays a big role in recognition of the mate (or other members of the same species) and in the manifestation of (secondary) sexual characters in many species (male<->female dimorphism).
See picture for components and introductory chapter on feathers:

bird topography

Alternative form for plumage : plumages.