Until I took Maureen Lahiff’s Bird Anatomy and Physiology course, I didn’t know that most birds have a preen gland. Technically, it’s the uropygial gland, and it’s located far down the spine, at the base of the tail. You can see this towhee in the video above going repeatedly to a spot low down on its back; that’s the preen gland. The gland has a pair of nipples. When the bird squeezes the nipples with its bill, a complex oil (“preen oil”) comes out, and the bird spreads this over its feathers, and sometimes over its legs and feet. What exactly the oil does varies with the bird and circumstances, but generally the oil conditions the feathers. Feathers once formed are lifeless and subject to brittleness. breakage. parasite attack, and contamination by dust, dirt, fungi, and pollutants. The preen oil is particularly important for waterbirds. It’s the reason why “water runs off a duck’s back.” In some birds, chemicals or beneficial bacteria in the preen oil may protect feathers against mites, lice, and harmful bacteria. The physical action of running feathers through the beak not only cleans and conditions the feathers but also helps to zip up the tiny hooks (barbules) that keep feathers together and provide insulation, water-resistance, and smooth air flow in flight. Because the condition of feathers is so important to a bird, it typically spends considerable time preening. At rest, they may preen at least once an hour. Depending on species and season, they may spend around 10 percent to 25 percent of their waking time preening.
In addition to preening with its bill, the towhee in the video vigorously scratches its head and neck with its feet. That’s also typical behavior. A bird can’t, after all, preen its head with its bill, anymore than we humans can sniff our own ears with our nose. Many mammals, notably monkeys, help one another maintaining these areas, but social preening in birds is uncommon. Crows are one of the species that does this; I happened to see a pair of crows preening one another in the park.