The most important act that a bird performs is the preening of its feathers. They begin by grasping with their bill, one feather at the base and nibbling towards the tip to remove oil, dirt and parasites. Or they may just simply draw the feather through the partially clamped bill in one quick movement to smooth the feather barbs and remove dirt so they will lock together. This process also works fresh oil into the feathers from the preen gland located at the base of the tail.
Some birds help to preen each other’s heads, usually paired birds at the nest site. Mutual preening is always concentrated on the head and neck, which a bird cannot reach with its own bill. These mutual caresses are thought to remove foreign objects from feathers, as well as reinforce pair bonds and reduce aggression.
One captive Giant Cowbird at a zoo in Texas frequently offered its head to people and solicited touching. Many responded by scratching the cowbird’s head, and whenever people stopped, the bird displayed again to invite more preening. Caged parrots will often inch along the perch and bow their heads to people, an invitation to scratch their heads.
Taken from the book Canadian Feathers: A Loon-atics Guide to Anting, Mimicry and Dump-nesting, by Pat Bumstead