Suddenly, from the hanging wood above them, burst a harsh and alien sound - a single bark or scream, followed by another and another and another, each a few seconds apart.
I, being familiar with the noise, knew what it was; but the sheep, which had never heard it before, immediately took fright. As one, they ran, instinctively streaming together for protection from all corners of the field and charging downhill towards the safety of the farmyard.
Considering that they pay no attention to the barking and yelping of foxes, which make winter nights hideous, their reaction was surprisingly swift and extravagant. But for once their instinct misled them, for the animal making the noise was no predator - only a muntjac, or barking deer.
Nevertheless, this was the first time - so far as I know - that one of these curious creatures had entered our valley. I have seen them not far away, to north and south, but never so close to home, and their arrival in west Gloucestershire is further evidence of their spread throughout the south of England.
It has been said by many authors (myself among them) that our present large population of muntjac derives entirely from animals imported to Woburn Park by the 11th Duke of Bedford around the turn of the century. Now detailed research by Norma Chapman and others, published in the latest issue of Deer, the journal of the British Deer Society, has shown that in fact there were earlier importations, beginning in 1820, and that later, in this century, there were several deliberate releases in different parts of the country.
Woburn was indeed the main centre of activity, with major importations between 1893 and 1903, by which time 125 muntjac had been brought in. Yet although some animals escaped from the park and spread outwards, as has always been claimed, others were given flying starts by being set down in Oxfordshire, Northamptonshire, Warwickshire and Kent - and this certainly helps to account for the speed with which they have colonised much of the south of England.
Ms Chapman and her colleagues have also settled another question which has vexed specialists for years - that of whether the animals in question are Indian or Reeves's muntjac, or hybrids of the two. It has now been established that hybrid offspring are sterile and that the larger, more aggressive Indian strain has died out, so what we are left with is Reeves's, originally a native of China.
I doubt if anyone is bothering to introduce muntjac now. On the contrary, the talk is all about how best to control the ever-expanding population, for the little deer, which stand about 18in high at the shoulder, are an increasing menace to foresters - and woe betide your roses or camellias if they invade your garden.
That evening, in our own valley, the barks continued for more than a minute before at last the animal fell silent. In my experience muntjac call mainly for two reasons - either to proclaim territorial rights or in alarm. Being largely nocturnal in their habits, they seem to be myopic in full daylight, and take fright at phantoms, whereupon they launch long-drawn-out verbal offensives to assert and reassure themselves.
As to those seasonal mysteries on which I had been pondering: one concerned our broad beans, which miraculously escaped blackfly and produced a splendid crop. Yet during the crucial months of June and July, thistles in the fields above the garden became smothered by the wretched pests, every stalk thickly coated in black. It was as if the insects - which apparently pass over in terrific swarms at high altitude - had descended to a certain level, but not, thank goodness, the extra 100ft which would have taken them into our garden.
Another recent puzzle was that of the upwardly mobile bread crust. One afternoon my wife was walking along a path in the wood when Zephyr, our younger labrador, suddenly took off, bristling, straight up the steep hill, on what was evidently the hot scent of a fox. A few minutes later she reappeared, carrying in her mouth half the end-crust from a loaf of bread.
This, as it happened, was easily recognisable, for my wife bakes all our own bread, and the crust was a superannuated one which she had thrown out for the chickens barely half an hour earlier. Holmes or Poirot, called in to reconstruct events, could only have concluded that a fox must have taken it off the muck heap and carried it away along the hedge for about 500 yards, only to drop it when surprised by a stout labrador huffing on its trail.
A third curiosity is that my wife's horse Storm has developed a phobia about one very small area of land where a track comes up out of the wood beside a field to join a main road. Normally the steadiest of creatures, he is not easily spooked; but something about that particular spot gets him.
Oddly enough, the place has strong Roman associations. Up that track, it is said, slaves used to carry water from a spring in the valley to the temple of Mercury which archaeologists have excavated in the field above the wood; and one feature of the temple was a well, into which people threw curses inscribed on little scrolls of lead when they wanted to bring ruin on their enemies.
Can it be that Storm is picking up some emanation from the distant past which eludes human sensibilities?
Such little conundrums add spice to country life. And now we have another in the reaction of the sheep to the muntjac bark. Instinct is normally pinpoint accurate. Deer, for instance, are terrified by the smell of lion dung, even though there have been no lions in Britain for thousands of years. Countless generations after the big cats' disappearance, the genes of modern ungulates still carry a warning triggered by that powerful scent.
I find it strange, therefore, that the sheep misinterpreted the call of the barking deer. Yet now it looks very much as if muntjac have come to stay - and the sheep will no doubt soon realise that they are not flesh-eating Chinese dragons, but cloven-footed herbivores like themselves.