How do ducks float? - Why do ducks float?

Short Answer: Ducks float in water because of buoyancy. A duck weight less than the weight of the water it displaces (buoyant force), so it floats.

Long Answer: Ducks weight less because of their feather coatings, interlocking feathers, and hollow bones.

Feather Coatings
Ducks have a uropygial gland (oil gland) near the base of the tail. It secretes an oily substance that ducks spread over the bodies using their beaks. This oily substance creates an oil coat around the feathers to repels the cold water. Without this oil coat, the ducks will get wet and thus heavier, like swimming with your clothes on. The oil coat also keeps ducks warm.

Interlocking Feathers
Ducks' feathers have tiny barbs that trap air between feathers and skin. This creates a balloon-like effect that increases ducks' volume but adds very little weight, so that ducks have larger buoyancy. Ducks can release these air bubbles, which allows them to invert their bodies and submerge their heads.

Hollow Bones
Ducks have hollow bones to lower their body weight, but their bones are not hollow like a drinking straw. Ducks' bones are filled with woven bone material that gives them maximum stability.