'Was I witnessing a high-level wife-swap?'

This is the season when birds and animals stake out breeding territories - and none of our resident species make more noise about it than the buzzards.

The other evening, whistles burst out from both sides of the valley. The first calls rang from the wood on the hill to the south, my right, instantly answered by others from my left. Seconds later a big, dark shape floated over from the south, black against the sky. Then a second appeared from the north. A pair coming together? Apparently not: like ships in the night, the two passed each other and carried on without the least deviation. Was I witnessing a high-level wife-swap?

Then two birds soared out over the southern skyline, wheeling in circles. Suddenly another pair appeared low over my head. Cries blasted off from every direction. A fifth bird started calling from somewhere behind me. The squeaks of little owls were drowned out by the piercing volleys from overhead. Not until darkness fell did the big hawks at last settle and fall silent. Almost as vociferous are the carrion crows, the most voracious predators of other birds' eggs and fledgelings. Every morning crows take up vantage points and proclaim their local supremacy with loud, harsh calls.

On land patrolled by gamekeepers, the habit is often their undoing, for it enables the keeper to pin-point their nesting sites. One man I know keeps the stuffed skin of a fox especially for this purpose: set out in a field a short distance from the edge of a wood, the bright russet decoy proves an irresistible lure. Cruising crows swoop down to mob it, and the keeper, lurking in a hide with his shotgun, picks them off one by one.

So effective is the fox, in fact, that helpers frequently borrow it, and it whizzes about the country with a mobility it never achieved in life. By such means, keepers can ensure that their own ground will be clear for the critical months in which game birds breed, because at a certain point, all surviving crows settle down to nest in whatever territories they occupy and no more cross-country movement takes place.

It is not only the predators which move around. Even humble rabbits seem to migrate about now, moving down out of the woods, in which they have spent the winter, to breed in the hedgerow burrows which form their summer homes.

The only creatures which seem hell-bent on staying put are the greylag geese on our neighbour's farm. In past years one or two pairs have arrived in February and bred on the lake in the valley: then, come autumn, they have taken off for wintering grounds elsewhere.

Last summer, however, they seemed to find conditions so congenial that they never left. Three pairs of parents raised a total of 13 goslings, so that by August there were 19 hefty birds devouring the grass and messing up the fields with their slimy droppings. The longer they stayed, the more irritated the farmer became, but being fond of all wildlife, he could not bring himself to shoot them.

There they remained throughout the winter, and now, as the mating urge comes on them, chaos reigns in the flock. The ganders are constantly demonstrating - hissing, thrashing their wings, shaking their heads and extending their necks in menacing fashion - and it is hard to see how so many geese are going to settle down in one relatively small area: there are not enough individual territories to go round.

No doubt nature will sort things out somehow. It may be that, if the birds are all closely related anyway, this year's eggs will prove infertile and produce no offspring. Should that happen, migratory instincts will probably reassert themselves, and autumn will once again see the geese on their way.