For birds, preening isn’t done out of vanity. Grooming is very important to the well-being of birds and most birds do it several times a day. When preening, birds clean their feathers of dirt and parasites and they also arrange their feathers for more efficiency in flight. Most birds also spread oil from a special gland called the uropygial gland, which helps waterproof feathers and keep them flexible.
Other cleaning habits can include bathing in water or dust to clean the feathers and dislodge parasites. Birds often stretch their limbs and fluff their feathers to make sure they are all in the proper place.
These preening and cleaning times are generally great for bird photography because the birds are liable to stay in the same spot while they focus on the task. Stretching, bathing, fluffing and preening also put the birds into interesting positions and accentuate their wings and other body parts.
Here are a few examples with notes: