In order to fly at peak performance, birds must carefully maintain their feathers. This means removing dirt, excess oil, and parasites. Birds do this by running their bill from the base of a feather to the tip, a process known as preening. In addition to removing debris, preening also realigns feathers, improving their aerodynamic performance.
See this preening behavior demonstrated by a blue, gray, yellow, and orange stork-billed kingfisher (Pelargopsis capensis). It’s found “in the tropical Indian subcontinent and Southeast Asia, from India to Indonesia.” Naturalist Carl Linnaeus was the first to formally describe it in 1766.