What keeps a duck dry even though it spends almost all its time in the water? Feathers do. A duck’s feathers overlap closely, creating a protective outer layer that sheds water instead of allowing it to reach the skin.
The feathers are also maintained through preening. Ducks spread oil from a gland near the base of the tail across their plumage, helping preserve the feather structure and improve its water-shedding ability. Tiny interlocking parts of each feather, called barbs and barbules, form much of the physical barrier.
The oil itself is not the only waterproofing mechanism, so “feathers” is the best broad answer. Ducks also have dense, soft down feathers underneath the outer layer, which trap air for insulation. This keeps body heat in while the bird swims in cold water.
A related fact is that waterfowl periodically molt and replace worn feathers. If plumage becomes damaged or heavily contaminated, a duck can lose some of its waterproofing and become vulnerable to chilling.