Bathing and preening are both aimed at looking after the bird's feathers, because if they become damaged the bird cannot fly and so can neither escape from predators or search for food. The bird also becomes susceptible to extremes in temperature, especially the cold weather, because it cannot fluff up its feathers to keep warm.
You can encourage birds to bathe in your garden by providing water in bird baths.
Nearly all birds bathe frequently in water to keep their feathers in tip-top condition, though Starlings, Blackbirds and House Sparrows are the ones we are most likely to see. During the summer, bathing in water also helps the bird to keep cool.
Once in the water, the bird fluffs its feathers to expose the skin, submerges its belly and breast in the water, rolls back and forth by dipping its head into the water and creates a shower by flicking its wings. When finished, the bird shakes off the excess water and then flies off somewhere to dry and preen.
Some birds, for example House Sparrows, take dust baths (often after a water bath) by rolling about in dry dust or soil. The dust is thought to absorb excess preen oil and remove dry skin, lice, etc. Among your flowerbeds and planters you may find tell-tale bowl-shaped hollows where they have been bathing.
Blackbirds and other thrushes often sunbathe, laid down and with wings outstretched. The sun is thought to straighten the birds feathers and help the preen oil to spread through the feathers. Some ornithologists have suggested that it may also draw parasites to the surface where the bird can remove them or that the ultraviolet light in the sunlight converts chemicals in the preen oil into Vitamin D. However, one could be forgiven for thinking that they simply enjoy it.
Additionally, some birds like Blackbirds, Starlings and Jays will adopt a sunbathing posture on an ants' nest, or even pick up ants in their bill and rub them on their feathers. Ornithologists believe the formic acid that the ants release may kill feather lice.
Preening is a seemingly more careful process than bathing and often follows bathing.
The bird gently strokes or nibbles along the barbs of each feather, starting at the quill and working towards the tip, so that they are properly arranged.
In addition, most birds have an oil-secreting gland, called the preen gland (or uropygial gland) underneath their tail - this is the Parson's nose on a chicken or goose. The bird rubs its bill against the gland and then spreads the oil over the surface of the feathers. This oil keeps the feathers flexible and aids waterproofing but also kills bacteria and fungi.
For some species of birds, for example wildfowl and seabirds, such as Mallards and Black-headed Gulls, oiling the feathers is particularly important to that the feathers do not become water-logged while in the water.
If water is not available naturally in your garden, such as in a stream or pond, then you may provide a bird bath to help them and you will be able to watch their entertaining antics.
Birds wanting to bathe and drink will use anything that will hold water. Unfortunately, if the container has steep sides, such as a bucket or water barrel, the birds could drown. Ideally, the birdbath should have shallow sloping sides and a rough surface so that the birds (and other animals) can climb in and out or paddle around the edge without the risk of slipping or drowning. The bath should also have an area deep enough for them to bathe, say 5 cm (2 inches) deep.
The bird bath can be constructed simply from a large dish, or an upside-down dustbin lid that has either been sunk into the ground or supported on three or four bricks. Place some gravel or several stones in the centre so the birds can get in and out of the water easily.
Alternatively, bird baths of various designs can be purchased from bird food suppliers and some garden centres.
While bathing, the birds will be vulnerable to predators, so place the birdbath close to bushes that can provide cover for the birds, but ensure that cats and Sparrowhawks cannot use the same bushes for a surprise attack on the bathers.
For whatever reason, birds in some parts of the country seem unwilling to use hanging or pedestal-based bird baths, if you have one of these and birds do not use it, try placing the bath, if you can, on the ground or in different areas of the garden.
Hygiene is important with bird baths because there are many diseases that birds carry and transmit to each other and to people, and the water will become dirty quickly with dead leaves, feathers, bird droppings, food and algae.
Unlike a pond or stream, the water in the bath will not be freshened by flora and fauna and so must be kept clean and fresh by changing the water regularly. Clean the bird bath regularly using a scrubbing brush and a cleanser designed for pet bowls, etc., and swill the bath afterwards with boiling water, taking care not to scold your self.
Keeping the bath water ice-free can be difficult.
The simplest way is to pour hot water on to the ice to melt it, but if the weather is very cold it will freeze quickly again.
You could try sitting the bath on bricks and having a lit tea-light underneath, placing the bath in a sheltered position, or using a thermostatically controlled immersion heater.
Never use chemical anti-freezes, such as automotive anti-freeze, salt or alcohol to defrost a bird bath or any water used by wildlife.