The Harlequin duck is a duck that lives in the sea. The female is plain, but the male has white markings. This is called disruptive coloration The male can be difficult to see against surfaces that are not a solid color, like water that ripples. The point is that it disrupts the appearance of body shape, but there are other explanations. In general, bright colours in male birds serve two main functions. One is sexual selection by the females, in which the colours act as evidence of the male's suitability. The other is territory control by the males during the mating season. Only experiments and long observation can decide which factors are in play.