Preening is a maintenance behavior found in birds that involves the use of the beak to position feathers, interlock feather barbules 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.
Importance of Preening
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. Mechanoreceptorsat 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, but studies have
found no clear correlation between the size of a bird's gland and the exposure
to water that its lifestyle dictates.
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.
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