Birds: Feather Maintenance
Proper maintenance ensures that the feathers are kept in good condition between moults. Preening is the most important form of maintenance and a bird will spend quite a large proportion of its day pulling feathers through the tip of its bill to clean them and zip up any loose barbs. It will also peck at its feathers to remove foreign matter including feather parasites. The bird will work through the various feather tracts, lifting its wings to peck at the primaries and turning its head through all sorts of angles to get at the rump and tail feathers.
It is impossible for a bird to preen its own head so it resorts to scratching it with one foot while balancing on the other. Some fish-eating birds, such as Herons, Cormorants and Gannets have a comb on the inside edge of a hind toe, this is known as a pectinated claw and is helpful in removing fish slime from feathers. The insect-eating Nightjar also has a pectinated claw.
When there is water available, bathing may often precede the daily routine of feather maintenance. Everyone who has a bird-bath or pond in their garden can watch Blackbirds and other birds regularly having a splash. The point of bathing is not to wash the feathers so much as to dampen them so that the oiling which follows next is more effective: the washing of feathers and skin seems to be of only secondary importance. A Blackbird, when it bathes, stands mostly in shallow water often with its feathers fluffed up. It dips its head under the water, in such a way that the water is scooped up and falls over its back, or else it will bend forward and put its bill in the water and with its feathers fluffed, splash water with its wings. A moment or two later it will sit back in the water with its tail low and by flicking the tail upwards will splash water over its body.
Other species have very different methods of bathing. Swallows and terns bathe in flight by dashing themselves into the water and sending up a shower of spray, then flying on with hardly a hesitation. Kingfishers dive repeatedly into the water and then preen themselves on a nearby perch, while ducks use the safety of the water where they can dive or swim away from any danger. Some species such as pigeons and Starlings will bathe in the rain with their wings spread out.
Birds only need to dampen their feathers; if they get them too wet this will affect their ability to fly and is therefore dangerous. Many birds have a ‘rain-posture’ during which they stand upright with their feathers close to the body so that the rain runs off the plumage quite rapidly.
The reason the Cormorant and to a lesser extent the Shag hang out their wings in this way is still not clear; whether it is to dry out the feathers is still disputed by ornithologists.
The most common method of removing the moisture from the feathers is to raise and then depress the contour feathers and at the same time relax and jerk the body forward. Waterbirds, which dry themselves while resting on the water, raise and flap their wings in addition to this movement. Cormorants and Shags leave the water to preen their feathers and can be regularly seen standing on rocks and posts with their wings outstretched; why they should do this is still not clear. Ducks and some gulls also perform special movements in flight to shake water off their feathers; gulls can often be seen shaking their wings and bodies in flight after bathing.
After bathing and drying a bird may smear oil on to the feathers from the oil gland which is situated just above the rump. The oil gland is largest in some of the seabirds and waterbirds and may be partly related to their aquatic life and help with waterproofing, but there is still debate as to the true function of this gland which is missing from Woodpeckers and some other species. The oil is an acid fatty secretion and in some species, such as the female and young of Hoopoes, smells offensively. It has also been suggested that it helps to preserve the horny covering of the bill. Furthermore there is some experimental evidence that the secretion applied to the plumage may enable vitamin D to be synthesized under the influence of sunlight and then presumably absorbed either through the skin or ingested in subsequent preening.
Several species of birds can be found deliberately exposing their bodies to the sun using a special posture. As with other feather-maintenance activities there is some controversy as to the cause and function of this behaviour. Some people consider that the posture in which the bird lies on its belly with wings and tail spread widely, is a simple response to temperature. Sometimes, the wings may be raised and feathers twisted round – a position which may vary according to the species. However, this may not be the whole story because, while young birds react as though it were a simple temperature response, adults may react to a image of the sun. Even on cool days exposure to the sun changes the chemical quantity of oil from the preen gland, and maybe it is the light rather than the warmth which is biologically important. Blackbirds which sunbathe regularly also seem to have a regular place to do it. However I have not seen this recorded for other species.
Some species of bird, particularly game birds and others which may have originated from arid areas, dust themselves by driving sand and other fine particles amongst their feathers. They will find a dusty hollow and drive the dust into the feathers either by shuffling themselves or by scratching the dust with their feet, sometimes both the wings and feet are used. House Sparrows, as many keen gardeners know, frequently dust themselves. Why this is done is not fully understood but it may have something to do with dislodging or discouraging the feather and skin parasites.
In addition to all these methods of feather maintenance some passerines anoint themselves with the body fluid of ants. The bird which I have seen doing this most often is the Starling. The important part of the fluid is formic acid which is produced by some worker ants but fluids from other groups of ants are also used. Once again nobody is certain as to the function of this behaviour. However, it probably assists in the care of the feathers as formic acid is known to be an insecticide and may kill or discourage the ectoparasites. Anting is actually carried out by the bird smearing ants with its bill directly on to the feathers or by letting the ants swarm over it and penetrate between the feathers while ejecting their formic acid.