self-grooming, as the action of a bird in preening its feathers, or mutual grooming as part of species behaviour, as among monkeys and other mammalian groups. Mutual grooming, which is often derived from display behaviour, cements social bonds between individuals of a group or colony. The term preening is usually used to describe cleaning behaviour in birds. In some birds, oil from the preen gland, picked up from the feathers after exposure to sunlight, is a major source of vitamin D. A form of cleaning behaviour called cleaning symbiosis occurs between certain fishes or between certain shrimps and fishes. The cleaner is allowed by the recipient fish to clean the latter of external parasites, which the cleaner eats. Both cleaner and cleaned thereby benefit.