Preening is a found in birds that involves the use of the beak to position feathers, interlock feather that have become separated, clean plumage, and keep ectoparasites in check. Feathers contribute significantly to a bird's insulation, waterproofing and aerodynamic flight, and so are vital to its survival. Because of this, birds spend considerable time each day maintaining their feathers, primarily through preening. Several actions make up preening behaviour. Birds fluff up and shake their feathers, which helps to "rezip" feather barbules that have become unhooked. Using their beaks, they gather preen oil from a gland at the base of their tail and distribute this oil through their feathers. They draw each contour feather through their bill, nibbling it from base to tip.
Over time, some elements of preening have evolved to have secondary functions. Ritualised preening has become a part of some courtship displays, for example. It is also a displacement activity that can occur when birds are subjected to two conflicting drives. Though primarily an individual function, preening can be a social activity involving two or more birds – a behaviour known as allopreening. In general, allopreening occurs either between two members of a mated pair or between flock members in a social species. Such behaviour may assist in effective grooming, in the recognition of individuals (mates or potential sexual partners), or in reducing or redirecting potential aggressive tendencies in social species. Most allopreening is confined to the head and neck, smaller efforts being directed towards other parts of the body.
Ingestion of pollutants or disease-causing organisms during preening can lead to problems ranging from liver and kidney damage to pneumonia and disease transmission. Injury and infection can cause overpreening in caged birds, as can confining a bird with a dominant or aggressive cage mate.
Etymology The use of the word preen to mean the tidying of a bird's feathers dates from Late Middle English. It appears to be a variant of the word prune; one now-obsolete definition of prune meant "anoint", based on the Latin ungere, which had the same meaning. This usage was combined with the Scottish and northern English dialect preen meaning "pierce" or "pin", due to the "pricking" action of the bird's beak during preening.
Importance
Preening is a maintenance behaviour used by all birds to care for their feathers. It is an innate behaviour; birds are born knowing the basics, but there is a learned component. Birds that are hand-reared without access to a role model have abnormalities in their preening behaviours. Despite spending considerable time in their efforts, they do not use proper techniques to groom effectively and may do a poor job overall as a result. Displaced feathers can cause birds considerable trouble; such feathers might become damaged, could interrupt the smooth flow of air over a flying bird, or might allow the bird's body heat to escape. Preening allows a bird to reposition such displaced feathers. There is evidence that filoplumes, specialised feathers buried under a bird's outer covering of contour feathers, help to signal when contour feathers have been displaced. Mechanoreceptors at the base of the filoplumes only fire when contour feathers are displaced or the filoplume moves. Preening enables birds to remove dirt and parasites from their plumage, and assists in the waterproofing of feathers. During moult, birds remove the sheaths from around their emerging pin feathers while preening.
Because feathers are critical to a bird's survival – contributing to insulation, waterproofing and aerodynamic flight – birds spend a great deal of time maintaining them. When resting, birds may preen at least once an hour. Studies on multiple species have shown that they spend an average of more than 9% of each day on maintenance behaviours, preening occupying over 92% of that time, though this figure can be significantly higher. Studies found that some gull species spent 15% of daylight hours during the breeding season preening, while another showed that common loons spent upwards of 25% of their day preening. In most of the studied species where the bird's sex could be determined in the field, males spent more time preening than females, though this was reversed in ducks. Some ratites, which are not dependent on their feathers for flight, spend far less time on maintenance behaviours. One study of ostriches found that they spent less than 1% of their time engaged in such behaviours.
Preen oil
Fully grown feathers are essentially dead structures, so it is vital that birds have some way to protect and lubricate them. Otherwise, age and exposure cause them to become brittle. To facilitate that care, many bird species have a preen or uropygial gland, which opens above the base of the tail feathers and secretes a substance containing fatty acids, water, and waxes. The bird gathers this substance on its bill and applies it to its feathers. The gland is generally larger (in relation to body size) in waterbirds, including terns, grebes and petrels. However, studies have found no clear correlation between the size of the gland and the amount of time a species spends in the water; it is not consistently largest in those species that spend the most time in the water.
Preen oil plays a role in reducing the presence of parasitic organisms, such as feather-degrading bacteria, lice and fungi, on a bird's feathers. One study of Eurasian hoopoes showed that the presence of symbiotic bacteria (Enterococcus faecalis) in their preen oil inhibited the growth of the feather-degrading bacteria Bacillus licheniformis. Enterococcus faecalis did this by releasing a bacteriocin. Female hoopoes transfer preen oil onto their brood patches and eggs, which results in the transfer of bacteria as well. Preen oil and bacteria are rubbed into microscopic pits on the surface of the eggs during incubation. This alters the colour of the eggs (darkening them) but there is also evidence that the bacteria may help to protect the developing chicks. Other studies have shown that removing or restricting access to the uropygial gland typically results in a higher bacterial parasite load on the plumage, though not necessarily of feather-degrading bacterial species. Preen oil may play a part in protecting at least some species from some internal parasites; a study of the incidence of avian malaria in house sparrows found that uninfected birds had larger uropygial glands and higher antimicrobial activity in those glands than infected birds did. There is even evidence that the foul-smelling preen oil of hoopoes and wood hoopoes may help to repel mammalian predators.
Preen oil helps to maintain the waterproofing of a bird's plumage. Though the oil does not provide any direct waterproofing agent, it helps to extend the life of the feather – including the microscopic structures (the barbs and barbules) which interlock to create the waterproof barrier.
While most species have a preen gland, the organ is missing in the ratites (emu, ostriches, cassowaries, rheas and kiwis) and some neognath birds, including bustards, woodpeckers, a few parrots and pigeons. Some species that lack a preen gland instead have powder down feathers which continually break down into a fine dust that the birds apply to their contour feathers while preening. These powder down feathers may be scattered throughout the bird's plumage or concentrated into dense patches. As well as helping to waterproof and preserve the bird's feathers, powder down can give a metallic sheen to the plumage.
Preening action
A bird's plumage is primarily made up of two feather types: firm vaned or pennaceous feathers on the surface, with softer down feathers underneath. Both feather types have a central shaft with narrower branching from that shaft. Pennaceous feathers also have much smaller barbules branching from the entire length of each barb; these barbules have tiny hooks along their length, which interlock with the hooks of neighbouring barbules. Barbules can become unhooked as a result of a bird's daily activities – dislodged when the bird brushes up against vegetation, for instance, or when it interacts with another bird during fighting or mating. Preening may involve two kinds of bill actions: nibbling (or mandibulating) while working the feather from base to tip, or stroking with the bill either open or closed. The nibbling action is the one used most often; it is more effective than stroking for applying preen oil, removing ectoparasites, rejoining unzipped barbules, and rearranging feathers. The stroking action is typically done in the direction the feathers lie, with the bill either opened or closed. Stroking is used to apply preen oil, as well as to dry and smooth plumage. Grebes stroke more vigorously with an open bill – a behaviour known as "stropping". Penguins use their whole heads to stroke, in a motion referred to as "wiping". Birds regularly fluff up their plumage and repeatedly shake their bodies while preening. Experiments have shown that the shaking action can "rezip" a majority of split feather barbules.
Birds cannot use their beaks to apply preen oil to their own heads. Instead, many use their feet in an action called scratch-preening. Once they have gathered preen oil on their beak, they scrape a foot across their bill to transfer the oil, and then scratch the oil into the feathers on their head. Longer-necked birds may rub their head directly on their uropygial gland. Some species (including nightjars, herons, frigatebirds, owls and pratincoles) have comb-like serrations on the claw (a pectinate claw) of the middle toe which may aid in scratch-preening. Some species stretch their leg over their lowered wing to reach their head (known as "indirect" scratching), while others extend their leg between their wing and their body (known as "direct" scratching). There is some evidence that the method used by a species may be related to its ecology. For instance, New World warblers that are primarily arboreal tend to be overwing scratchers, while those that spend significant time on the ground are typically underwing scratchers. In general, preening takes place while the bird is perched, on the ground, or swimming, but some of the more aerial species (including swifts, swallows, terns and albatrosses) preen while flying. Many birds have a slight overhang at the tip of their upper mandible. Experiments suggest that this allows birds to apply shearing forces that kill the flat-bodied feather lice; the removal of the bill tip caused an increase in feather lice due to ineffective preening.
Preening is often done in association with other maintenance behaviours, including bathing, dusting, sunning, oiling or anting, and can either precede or follow these other behaviours. All birds typically preen after bathing. Groups of birds sometimes all groom individually at the same time. This has been seen in species ranging from herons to blackbirds.
Secondary functions