To the south, the sky was dark with grouse, which were pouring in by the thousand

The start of another season has emphasised once again the extraordinarily enigmatic nature of the red grouse. Some moors are alive with strong coveys; others, in spite of confident predictions, have practically none, and owners from the Duke of Westminster downwards, have had to cancel projected shooting days left and right. Even after years of scientific research and on-the-hill observation, our understanding of this elusive game bird is still far from perfect.

It is all too well known that grouse populations explode and collapse with bewildering rapidity, and that a bumper year is often followed by a crash. In one area monitored by the Game Conservancy Trust, for instance, the population fell from 650 one August to 19 the next. Equally, it is common knowledge that grouse are strongly territorial in spring, and that individual pairs take pains to stake out their nesting grounds; but then, in late autumn, and particularly in wild weather, the birds congregate into enormous packs many hundred strong.

Why do they do this? And who can explain stories like the one told by the veteran sporting artist, Raoul Millais?

In his youth his great-uncle used to rent Fealar, the high-lying deer forest in Perthshire. There were practically no grouse on the ground, and, as Millais remembers, "you could walk all day for a brace and a half". But as he was returning to the lodge one evening in the autumn of 1919 his companion suddenly exclaimed, "My God - look at that!" To the south, the sky was dark with grouse, pouring in by the thousand to settle on the flats.

Normally, at that time of year, the use of shotguns was banned at Fealar, in case the noise should disturb the deer. But at dinner that night the young men in the party persuaded their host to let them hold a special shoot in the morning. Out went four of them, and got 110 brace. Next day the whole vast swarm of grouse had vanished to the north.

In the 1950s Millais heard of another such mass-migration, this time from the the head keeper at Dunrobin, on the east coast of Sutherland. There, one bitter day in January, when snow was lying and a westerly gale was blowing, so many thousand grouse streamed in to settle in the shelter that they turned the lee flanks of the hills black. Then, to his horror, the keeper saw the whole mass suddenly lift off: the gale caught them and whirled them out over the North Sea, surely to their deaths. Whether the birds were driven by lack of food, or were responding like lemmings to some migratory instinct, he could not tell.

One fact which nobody disputes is that grouse are good for the environment. Because they live mainly on heather shoots, they promote active management of moorland: for grouse to thrive, heather must be burnt regularly, and bracken suppressed, with the result that moors remain in better shape than they would if owners had no special incentive to spend money on maintainance. The control of predators such as foxes and crows also benefits other ground- nesting species such as curlews, larks and pipits.

Another certain fact is that grouse are extremely valuable. This year the going rate for shooting driven birds is pounds 110 per brace, so that a 150-brace day earns more than pounds 15,000. The problem is that, unlike pheasants, partridges or duck, grouse cannot be artificially reared in large numbers, and moor-owners can only aid and abet nature in its annual production.

Gamekeepers do this in various ways. One is to suppress predators, another to keep the heather in good condition. A third is to dose the birds against parasitic threadworms, either by catching them at night and squirting medicine down their throats, or by dotting the moor with heaps of medicated grit, which the grouse eat so that it grinds up the heather shoots in their gizzard. Yet another beneficial move is to reduce infestation by ticks, which are carried by sheep and deer, and breed in bracken. All these measures help. Yet there are other factors which nobody can control, principally the weather. If spring and early summer are cold and wet, breeding success is drastically reduced. One key fact revealed by radio tracking is that, in their first weeks of life, the chicks need a high- protein diet of insects, which their mother furnishes by leading them to boggy patches on the moor. Unless they find plenty of insects, they die.

If global warming means that Scotland and the north of England are going to have better summers, the outlook for grouse will improve. But if, as some people predict, another ice-age is already setting in, Lagaopus scoticus faces a tough future.