Birds preen to clean and oil their feathers and remove parasites. A tiny Ruby-throated Hummingbird has about 940 feathers to keep clean and properly aligned for aerodynamic flight.

To preen, a hummer first ruffles its feathers, fluffing out and then uses its long bill to nibble along each feather. The nibbling removes oil, dirt, and parasites. As it nibbles, the bird takes tiny droplets of fresh oil from a gland at the base of its tail and works the oil into its feathers. Next, it thoroughly cleans its flight feathers, running each wing feather through its bill. This action zips or closes the places where feather barbs have separated, restoring each feather's smooth surface. Then, the hummer uses its tiny claws to scratch hard-to-reach places, such as the back of its head and neck. It also cleans its bill from base to tip, using claws and nearby branches to wipe off sticky nectar. When done, the hummer ruffles its feathers so they all fall neatly into place, stretches each wing, lowers its head, fans its tail, and flies off.