Birds preen to maintain their feathers, to keep themselves warm and dry, to remove parasites, and to strengthen relationships. Birds often preen running their beaks through their feathers or scratching their heads with a toe. Some birds have a special comb-like structure on one toe for this purpose.
Preening helps to maintain birds’ feathers and skin in several ways. Birds preen to groom and repair feathers. Feathers have parallel barbs attached to the feather shaft, and many barbules like tiny hooks that latch the barbs together. When barbules detach, a split appears in the vane of the feather. Birds preen to realign the barbs and barbules: during preening, a bird carefully nibbles its way from the base of the feather toward the tip, reattaching the barbules and mending splits. Birds often preen after they’ve been dust bathing or diving, or when the weather is wet, times when feathers need grooming. As well, in hatchlings or molting birds, new feathers (pinfeathers) are wrapped in a tough sheath that must be removed by preening so the feather can open.
These activities are not random, a bird works through its feathers methodically, preening them all. Preening waterproofs and conditions feathers. Most birds have a preen gland, or uropygial gland, at the base of the tail. The gland produces uropygial oil, a waxy substance used to waterproof and condition feathers. Rubbing the preen gland with the beak, a bird picks up the oil and then distributes it onto feathers by rubbing the beak over the feathers. It’s thought that the oil from the preen gland also helps to make the feathers supple and strong and prevents them from drying out. Some species of birds, such as bustards, do not have uropygial glands— these are usually birds that live in dry habitats where protection from getting wet is not an issue.
Other birds, particularly diving birds, have large uropygial glands. American Dippers, for example, have uropygial glands ten times the average size of those of birds in their Order. Preening and distributing the oil over feathers is important for birds that spend a lot of time in the water. Some birds, particularly herons, have powder-down feathers. Powder-down feathers are distributed all over the body in some birds, while in others they occur in patches. These feathers break down into a powdery substance when a bird preens, and the powder is distributed over the rest of the feathers by beak and claw in a similar fashion to uropygial oil. Powder-down provides some water resistance.
Birds often like to take dust baths before preening, coating their feathers with dust and sometimes even rolling in dust. It’s thought that dust helps to dry wet feathers, soaks up extra uropygial oil, and inhibits parasites. Preening removes parasites. When a bird nibbles a feather, putting barbules back in place, it also removes small mites and other parasites. In addition, uropygial oil is thought to have antibacterial, antifungal, and antiparasitic properties that inhibit parasites and other harmful organisms. Many birds pick up ants and rub them through the feathers while preening, a behavior known as anting. This presumably leaves a thin coating of formic acid on the feathers. Though this behavior is not completely understood, scientists believe that the formic acid discourages parasites.
Allopreening enhances pair bonding. Birds that bond in pairs, either for a breeding season or for a longer time, frequently engage in allopreening, preening each other’s feathers. This happens particularly during the breeding season and is thought to strengthen the bond between the two birds. Provided by Rosemary Drisdelle. Visit her website at RosemaryDrisdelle.com.
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