Cleaning, rearranging, and general maintainence of feathers. Birds often preen to rid the feathers of parasites or to keep flight feathers in optimal condition. While preening, many species of aquatic birds apply oil from a gland at the base of the tail to their feathers to keep themselves waterproof - or at least mostly dry.

Feather maintenance behavior where a bird uses its beak to grab a feather and "nibble" along it. This cleans and smoothes the feather. Many birds use oil from an oil gland above their tails ( on the rump ), to spread on the feathers while they preen.

Using the beak to smooth and restore to position flight feathers that have become separated.