Preening is a bird's way of grooming its feathers to keep them in the best condition. While preening, birds remove dust, dirt, and parasites from their feathers and align each feather in the optimum position relative to adjacent feathers and body shape. Most birds will preen several times a day to keep themselves healthy.

https://www.thespruce.com/why-birds-preen-386448
https://www.2zoo.com/vb/showthread.php?t=95463
https://www.elaard.com/66579
https://books.google.com.eg/books?id=1nn7DwAAQBAJ&pg=PT46&lp...