Preen: verb; (of birds) to maintain (feathers) in a healthy condition by arrangement, cleaning and other contact with the bill (Collins Concise Dictionary).
Interestingly, one dictionary said it also related to animals tidying and cleaning their fur with their tongue, but I’ve never heard the word used that way. Information as to the word’s origins varies according to which dictionary you consult – the Collins says it first appeared around the 14th century and probably comes from prunen, thence preinen, meaning to stab, pierce or prick, referring to the action of the bird’s bill when preening.

Preening is an extremely important action for birds, to keep their feathers in the correct position for flight and for the preservation of body heat; to clear away dirt and parasites; to assist with the process of moulting; and, in those birds that have a uropygial gland, to maintain their waterproofing by spreading oil from the gland over and through their feathers.