Final answer:
Birds like ducks can sit on water due to their buoyant and water-repellent feathers, which combined with air pockets in their bones, ensure their average density is less than water. Coating of oils from the preen gland on their feathers helps them remain on the surface. Soapy water, which reduces surface tension, can cause issues for their buoyancy.
Step-by-step explanation:
The ability of birds such as ducks, geese, and swans to sit on the surface of the water despite having greater densities than water is due to their buoyancy and the water-repellent nature of their feathers. These birds have specialized feathers that are coated with oils from their preen gland, which makes them water-repellent; this prevents water from saturating the feathers and adds to their buoyancy. When water does not wet their feathers, the birds' entire volume (including air trapped in their feathers) is taken into account when calculating their overall density, and since air is much less dense than water, this helps make the average density of the bird less than that of water, allowing it to float. On the other hand, soapy water reduces surface tension and can penetrate their feathers, making the birds less buoyant and unable to sit on the surface as easily.
Furthermore, bird bones have air pockets which reduce their weight and give them an overall density that is generally lower than many other animals. These adaptations are critical for their survival and enable them to utilize water surfaces for rest, feeding, and other behaviors.