Here is a tibit of info on preening
Preening waterproofs and conditions feathers
Most birds have a preen gland, or uropygial gland, at the base of the tail. The gland produces uropygial oil, a waxy substance used to waterproof and condition feathers. Rubbing the preen gland with the beak, a bird picks up the oil and then distributes it onto feathers by rubbing the beak over the feathers. Its thought that the oil from the preen gland also helps to make the feathers supple and strong and prevents them from drying out.
Some species of birds, such as bustards, do not have uropygial glandsthese are usually birds that live in dry habitats where protection from getting wet is not an issue. Other birds, particularly diving birds, have large uropygial glands. American Dippers, for example, have uropygial glands ten times the average size of those of birds in their Order. Preening and distributing the oil over feathers is important for birds that spend a lot of time in the water.
Some birds, particularly herons, have powder-down feathers. Powder-down feathers are distributed all over the body in some birds, while in others they occur in patches. These feathers break down into a powdery substance when a bird preens and the powder is distributed over the rest of the feathers by beak and claw in a similar fashion to uropygial oil. Powder-down provides some water resistance.
Birds often like to take dust baths before preening, coating their feathers with dust and sometimes even rolling in dust. Its thought that dust helps to dry wet feathers, soaks up extra uropygial oil, and inhibits parasites.
Read more at Suite101: Why Do Birds Preen?: Preening With Beaks and Claws is Important for Feathers and Skin | Suite101.com http://www.suite101.com/content/why-do-birds-preen-a18521#ixzz1PNppETV7