Preening

Image: Hariadhi

The old stone bird-bath is much weathered now, cracked by past frosts, and in places its square, layered pedestal has become encrusted with grey and yellow lichen. It stands at a point in the garden a fair distance from the house, in the middle of a quartet of rose-beds virtually surrounded by yew hedges. On three sides there are trees of varying sorts and ages, some quite large.

Human disturbance is minimal; there is no dog in the household, and the only cats are occasional and unwelcome visitors from neighbouring gardens, so the place is a haven for birds. Kept topped up daily with the watering-can, a bird-bath provides a focal point for the garden and an endless source of movement and interest.

The most compulsive bathers are starlings, which create a tremendous amount of spray as they attack the water with their wings. Just now, in mid July, the greyish-brown juvenile starlings are especially numerous, and crowds of them are all trying to get into the water at once, leaving the less pushy dunnocks and song thrushes waiting on the grass nearby.

From time to time, when the starlings permit, almost all the birds in the garden will come down to drink or bathe, and this is often the best chance to get a view of shy or otherwise hard-to-see species. This afternoon a blackcap has been down, as well as a willow warbler and, earlier, a great spotted woodpecker.

They expose themselves to view because they must. Bathing for these birds is essential to maintaining the plumage in good order. Its function is not primarily one of cleansing, but of preparing the feathers for what comes next. Indeed, most birds are careful to avoid getting their plumage drenched, because this eventually renders the feathers brittle and, more importantly, robs the bird of its power of flight.

The water-bath is a carefully controlled wetting, spreading an even layer of moisture over the feather surfaces. Having repeatedly ducked its head into the water and flicked spray over its back, the bird performs special shaking movements to rid itself of surplus water. The exact pattern of these movements varies from group to group; some birds, such as gulls, can shake their feathers even while flying.

A bird like the blue tit is typical of the garden bathers. Immediately it has finished at the water, it retires to a less exposed position in order to preen in safety.

The great majority of birds have a special gland just above the root of the tail. The gland secretes oil which keeps the feathers supple and waterproof. Preen oil has another function too. Birds, like humans, cannot synthesize vitamin D inside their bodies. We manufacture it in our skin, provided we are exposed to the sun. Vitamin D synthesis likewise takes place in the oil; the vitamin is then either absorbed through the skin or ingested by the bird while preening.

The first act of preening is to stimulate the oil gland with the bill; oil is then quickly transferred to the plumage with quivering and stroking movements. The bird must work fairly rapidly because the oil hardens on exposure to air. The fact that the feathers are damp delays the hardening and enables the oil to be spread more evenly.

Having made a quick distribution of the oil, our blue tit, as soon as it has the opportunity, moves on to the next, longer and more leisurely, phase of preening. The outer feathers are ruffled up, to make them easier to get at, and each area receives its share of meticulous attention. A bird’s plumage is one of its most important assets, and much time each day is devoted to its care, both in these special sessions and during any odd moment.

There are two main types of preening movement. With the bill closed the bird sleeks down disarranged feathers and flicks away foreign particles. With it open, individual feathers – especially those of the wings and tail – are lightly combed.

The structure of a feather is a miracle of design. A large flight feather consists of a central shaft fringed on each side with about a hundred filaments, each of which, in turn, is fringed with smaller filaments or barbules. Each barbule has several hundred minute hooks which interlock with neighbouring barbules; there may be a million such hooks in a single feather. The combing action helps to restore the interlocking pattern and produce once more a smooth and continuous vane for flight.

Besides preening, birds in the garden may be seen maintaining their feathers in other ways. A favourite pursuit, especially of thrushes and blackbirds, is sunning. The bird, feathers ruffled and wings drooped, squats with its back to the sun, apparently just enjoying the heat: but the light helps to disturb parasites among the feathers, and makes them come out into the open where the bird can preen them away. A more extreme sunning posture has the bird sprawled with one wing fanned out towards the sun, its body tilted sideways and the tail swung round to the same side.

Dust-bathing is also indulged in by some birds, including sparrows and wrens. Its purpose is not clear, but may also have something to do with discouraging skin parasites. The same is true of the strangest feather-maintenance practice of all, anting.

In this the bird puts ants among its feathers, or simply squats over an ants’ nest, allowing the ants to crawl into its plumage. It is thought that the exudations of the ants, like preen oil, help to keep the feathers in good condition. The type of ant usually chosen secretes formic acid when it is angry, which certainly acts as an insecticide, and the ants themselves may attack any parasites they come across. Hundreds of species of perching birds have been observed anting, but starlings, crows, and especially jays, are particularly addicted to it.

In fact there are starlings on the lawn at this moment, taking ants from a nest halfway down the path.

This is the kind of ornithology anyone can study, whether from a window or a deckchair. All you need is a sharp pair of eyes – and a bird-bath. It need not be elaborate; an old dustbin lid will do, as long as it provides a shallow gradient and its site a measure of seclusion.

(Introduction to these pieces; see all)