A common behavior seen in all waterfowl, including ducks, is called "preening". Preening is the act of combing the feathers with the bill to keep the outer feathers waterproof. This combing also removes parasites and "re-locks" the barbs and smaller hooks of each feather together to form a protective layer that repels water. A third function of preening is to spread an oil over the feathers which is obtained from the preen gland on the rump. The duck rubs its head over the gland to pick up the oil. It then rubs its head over the feathers to spread the oil evenly. The oil keeps the feathers bright and soft and allows water to run off in droplets - as in the phrase 'water off a duck's back'. ( http://www.lafayette.in.gov/DocumentCenter/View/1806/Pekin-Duck-PDF)
