Ducks have a special gland positioned near their tails, called the preen gland. This gland produces an oil, which ducks rub over their feathers with their beaks to maintain their waterproof effect. This oil creates a protective barrier that stops feathers becoming waterlogged. These waterproof feathers insulate ducks from water and cold temperatures. Beneath the waterproof outer feathers is a fluffy, soft layer of down feathers, which keep ducks warm. This protective barrier of waterproof feathers is a very effective system, but it requires constant maintenance, therefore wildfowl spend a lot of time preening and washing.
Answered by Dave Paynter, reserve manager, WWT Slimbridge.