Plumage refers to the layer of feathers that covers a bird and includes the pattern, color, and arrangement of those feathers. The colors and patterns of plumage vary between species and subspecies and may also vary with age classes. Plumage provides protection, insulation, and adornment to birds and helps streamline and soften body contours, reducing friction in air and water. Feathers are normally lost and replaced at least once a year through molt and regrowth. Sexual differences are common, with the plumage of the male characteristically showing more brilliance and pattern than that of the female.