The image above by Johnny Wee shows the Little Heron (Butorides striatus) exposing its pair of preen glands located on its rump at the base of the tail. These glands, also known as uropygial glands, exude a viscous liquid of fatty acids, waxes and fats. They are found in most birds except Ostrich (Struthio camelus), emus, crassowaries and some pigeons, parrots and woodpeckers.
This is a rare image where the glands are totally exposed. Usually when a bird is preening, it pushes its bill between the tail feathers to reach the the glands. Once the bill is smeared with oil from the gland, the bird spreads it over the feathers.
Ornithologists believe that the oil has antifungal and antibacterial properties. It also preserves feather moistness and flexibility as well as provide an insulating and waterproofing layer.