A combination of design and preening oils repel water from water bird plumage. Birds like geese and ducks are constantly maintaining their feathers ability to repel water. This is achieved by grooming the feathers with a waxy oil from a preening gland, the uropygial gland, on the tail. This improves the water repellent properties of the feathers because the surface tension of water is stronger than that of oil. At the same time the feathers are cleaned of dust, dirt, and parasites, aligned for optimal flight and waterproofing performance and the Velcro-like filaments (barbules) on the barbs are zipped up to produce a smooth surface. Some families of birds do not possess a uropygial gland but use a white powder from down feathers to do a similar job. The photograph covers a small area of the wing where the primary coverts overlay the primaries