please answer it today
Water bird bones are hollow for flight and for swimming as that would reduce the amount of weight to be pushed through the water by their feet. The oil in their feathers waterproofs the feathers so they don't get bogged down every time they swim. They get their waterproofing oil in a special gland with a nipple at the base of their tails. Most birds have this gland. The air sacs are connected to the lungs and are full of air. Combine all of these items and you have birds that are capable of sitting on top of the water. Not all water birds have the waterproof feathers though. Cormorants have to sit with their wings stretched after swimming because they need to dry out their wings before they can fly again. Geese and Ducks don't have to do this.
Birds have very light and hollow bones plus water birds have feathers that repel water rather than soaking it up. Furthermore, I would imagine that water birds have more fat in their breast/abdomen to make them more buoyant in water.
Ducks and geese have an oil gland on their tail. This oil is used to make the feathers repel water. If you ever observe these birds in the wild sitting on the water, you will see them nibble at the gland on their tail and then distribute the oil throughout their feathery covering with their beak. Most people who don't know this probably think that they are just nibbling at annoying bugs and lice, but they are oiling and grooming themselves.
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