The females of many breeds of domestic ducks are unreliable at sitting their eggs and raising their young. Exceptions include the Rouen duck and especially the Muscovy duck. It has been a custom on farms for centuries to put duck eggs under broody hens for hatching; nowadays this role is often played by an . However, young ducklings rely on their mothers for a supply of preen oil to make them waterproof; a chicken hen does not make as much preen oil as a female duck, and an incubator makes none. Once the duckling grows its own feathers, it produces preen oil from the sebaceous gland near the base of its tail.