Plumage refers both to the layer of feathers that cover a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage vary between species and subspecies and can also vary between different age classes, sexes, and season. Within species there can also be a number of different colour morphs. Differences in plumage are used by ornithologists and birdwatchers in order to distinguish between species and collect other species specific information.
Basic and alternate plumage
Almost all species of birds moult at least annually, usually after the breeding season. This resulting covering of feathers, which will last either until the next breeding season or until the next annual moult, is known as the basic plumage. Many species undertake another moult prior to the breeding season known as the prealternate moult, the resilting breeding plumage being known as the alternate plumage. The alternate plumage is often brighter than the basic plumage, for the purposes of sexual display, but may also be cryptic in order to hide incubating birds that might be vulnerable on the nest.