Country Matters: Wren the architect, cuckoo the intruder

WHAT does the hen buzzard think about as she sits brooding her eggs, 75 feet above ground in the upper branches of a beech tree? Not much, I guess; I imagine her metabolism, like that of other birds, slows down while she is incubating, and that the long days and nights pass in a kind of torpor.

Seen from below, her nest is an untidy great bundle of sticks. Without a set of climbing irons - promised me, but never sent, by a man who ascends telegraph poles for a living - I shall not get a glimpse of the inside, for the trunk of the tree rises smoothly for 60 feet without a branch; but I know from past years that the centre of the nest will be a deep cup lined with grass, leaves and moss. In it will be lying two or three eggs, almost round, with reddish-brown splotches scattered on a white background.

I envy the buzzards the site that they have chosen - or rather, the one to which they have returned. Their tree stands in a wood on a very steep hillside, and from their vantage point they look down, through boughs still mostly bare, on to a plunging, brilliant green carpet of wild garlic, tinged here and there by patches of emerging bluebells.

During the next couple of weeks, as the leaves come out and the canopy closes, their world will become ever more secret. Yet they will always have some noise for company: in windy weather the tree-tops roar like the sea, and on still days the air is filled by the rushing murmur of a stream at the foot of the slope. The only creatures that pass frequently beneath their tree are foxes, badgers and roe deer.

Among all the birds breeding in our area, the buzzards are the most spectacular - the highest fliers and the highest nesters. At the other end of the scale are the moorhens, tucked down into tufts of rushes around the lake in the valley, as low as they can go. Between these extremes, dozens of other species have pitched into the struggle to bring up young, and the annual battle for survival is on.

For children especially, there is no more fascinating rural hobby than looking for birds' nests, and no thrill greater than that of finding tiny, highly coloured eggs cradled in a cup woven from grass and wool. Wildlife films, brilliant though they are at probing nature's secrets, cannot replace the excitement of live discovery.

During a school visit to a farm the other day, a teacher remarked that children, conditioned by the hectic pace of television programmes, expect events in the country to unfold at breakneck speed. As she put it, 'If they look down into a molehill, they think they're going to see that fat fellow with a beard come along the tunnel, spouting about conservation.'

A good hunt for birds' nests, in my experience, brings budding naturalists back to earth: it takes time and perseverance, for the rewards are by no means immediate.

The mysteries to be explored are endless. Why does the colour of eggs vary so widely from one species to another? Why should the eggs of a hedge-sparrow be plain bright turquoise, while those of a house-sparrow are dull white, spotted with grey? Why should those of a thrush be blue flecked with black, and those of a blackbird (a closely related species) mottled green and brown? Above all, if some eggs, such as those of the woodcock, are camouflaged so that they merge into the background of dead leaves and forest litter on which they are laid, why are those of many songbirds so garish as positively to attract the attention of predators?

The textures and shapes of nests are no less diverse. Blackbirds furnish their maternity homes with soft linings of hair, wool and grass, whereas thrushes make cups of hard mud. Magpies also build mud cups, but the design of their nests is more elaborate, and includes a roof or dome made of sticks. The most intricate structures are those made by tits and wrens: complete spheres or egg- shapes of soft material, each with a little round entrance hole in the side. Wood pigeons, in contrast, take little trouble, fecklessly laying a few sticks on top of each other to form flat, unlined platforms, off which their plain white eggs frequently roll.

At this time of year no birds are more destructive than carrion crows, which seek out the eggs and fledglings of other species with relentless persistence. Every time I go down to the lake, there lurks a crow, black as night, sitting low in one of the poplars, watching and waiting. When disturbed, it flops away over the field, but comes craftily round in a circle to resume its vigil. I do not doubt that it has already found and eaten many of the moorhens' eggs, and I fear it may also have raided the nest of the greylag goose.

A celebrated photograph, taken on Salisbury Plain during an experiment run by the Game Conservancy, showed more than 50 pheasant and partridge eggshells lying on the ground beneath a single crow's nest. Moreover, these 50-odd eggs were only the ones that the pair of crows had brought home; nobody could tell how many they had eaten elsewhere.

Ordinary observation suggests that birds must have extremely sharp eyes, and it is no surprise to learn that their colour vision is superior to that of humans. Little therefore escapes the beady gaze of a crow, or a magpie, which is almost as deadly a predator.

It seems extraordinary that ground-nesting birds such as pheasants and partridges, which have to contend with foxes, cats, rats and stoats as well as airborne vermin, ever hatch any eggs at all.

Pheasants, in particular, have a fatal propensity for nesting beside tracks and lanes, along which dogs are likely to pass; moreover, they often start laying in scanty undergrowth, instinctively certain that nettles and other weeds will grow up to give them better concealment. Their best protection from foxes seems to lie in the fact that, when they incubate, their scent dwindles almost to nothing.

The safest birds are those that take up residence in holes, such as owls and nuthatches. These last - sleek and sensible little creatures - reduce the size of the entrances to their nests with a lining of mud, thereby cutting the chance of a raid by that other voracious egg-eater, the grey squirrel.

As if all these native killers were not enough, this week there arrived on the scene yet another menace to songbirds: the cuckoo, which evolution has fashioned as a super-efficient parasite.

Not only does a female lay up to 25 eggs in a season - three or four times as many as most small birds - she dumps them one by one in the nests of unsuspecting foster species, such as robins and warblers, and by some miracle modifies their colours so that they resemble the victim's own eggs. Moreover, cuckoo eggs hatch in a very short time (12 1/2 days), thus ensuring that each chick arrives on the scene early enough to evict other eggs, or rival chicks, over the side of the nest, one by one.

The mellifluous double call that heralds the arrival of summer is relatively harmless, for it comes from the male cuckoo.

It is the thick, throaty, bubbling chuckle which signals trouble, for that is the call of the female, staking out her territory and watching for the giveaway movements of potential victims.