Good question. Ducks have a spot on their rump called the preen gland. From this gland, an oily substance is produced and the bird will roll his head around on it. The bird then rubs the body feathers to spread the oil evenly. Doing this keeps the feathers bright and supple, and creates a waterproof barrier that allows the water to run off. This is where the saying “water off a duck’s back” comes from.
It’s also why our oil-only products are called “FiberDuck®.”