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The differences in plumage of a Blue Grosbeak, from top to bottom, between a breeding male (alternate plumage) a non-breeding male (basic plumage), a female and a related Indigo Bunting

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.

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 pre-alternate moult, the resulting 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.^[1] Many ducks have bright, colorful plumage, exhibiting strong sexual dimorphism to attract the females. However, they moult into a dull plumage in the non-breeding season. This drab female-like appearance is the eclipse plumage. When they shed feathers to go into eclipse, the ducks become flightless for a short period of time. Some duck species remain in eclipse for one to three months in the summer, while other would retain the cryptic plumage until the next spring when they undergo another moult to return to their fancy breeding garb.

There are hereditary as well as non-hereditary variations in plumage that are rare and termed as abnormal plumages. These include excessive dark pigmentation or melanism, lack of pigmentation ranging from leucism to albinism, presence of excessive red or yellow pigmentation leading to, respectively, erythrism or xanthochroism. There can also be polymorphism in some birds particularly the owls and cuckoos where certain colour variations are more widespread and examples of these include the hepatic forms of cuckoos.

Such abnormalities can be present throughout the bird or restricted to specific feather tracts.