patches. As well as helping to waterproof and preserve the bird's feathers, powder down can give a metallic sheen to the plumage. Preening action A bird's plumage is primarily made up of two feather types: firm vaned or pennaceous feathers on the surface, with softer down feathers underneath. Both feather types have a central shaft with narrower branching from that shaft. Pennaceous feathers also have much smaller barbules branching from the entire length of each barb; these barbules have tiny hooks along their length, which interlock with the hooks of neighbouring barbules. Barbules can become unhooked as a result of a bird's daily activities – dislodged when the bird brushes up against vegetation, for instance, or when it interacts with another bird during fighting or mating. Preening may involve two kinds of bill actions: nibbling (or mandibulating) while working the feather from base to tip, or stroking with the bill either open or closed. The nibbling action is the one used most often; it is more effective than stroking for applying preen oil, removing ectoparasites, rejoining unzipped barbules, and rearranging feathers. The stroking action is typically done in the direction the feathers lie, with the bill either opened or closed. Stroking is used to apply preen oil, as well as to dry and smooth plumage. Grebes stroke more vigorously with an open bill – a behaviour known as "stropping". Penguins use their whole heads to stroke, in a motion referred to as "wiping". Birds regularly fluff up their plumage and repeatedly shake their bodies while preening. Experiments have shown that the shaking action can "rezip" a majority of split feather barbules. Birds cannot use their beaks to apply preen oil to their own heads. Instead, many use their feet in an action called scratch-preening. Once they have gathered preen oil on their beak, they scrape a foot across their bill to transfer the oil, and then scratch the oil into the feathers on their head. Longer-necked birds may rub their head directly on their uropygial gland. Some species (including nightjars, herons, frigatebirds, owls and pratincoles) have comb-like serrations on the claw (a pectinate claw) of the middle toe which may aid in scratch-preening. Some species stretch their leg over their lowered wing to reach their head (known as "indirect" scratching), while others extend their leg between their wing and their body (known as "direct" scratching). There is some evidence that