When, recently, I described the success of my Larsen trap at catching magpies, I expected a hail of abuse. On the contrary, readers wrote from all quarters asking urgently how they could acquire a trap, or plans of one. Clearly, magpies are widely hated.

The trap is a fairly simple device, made of wire netting and wood, and divided into two compartments. In one, bait, or (far better) a live call- bird is placed, complete with food and water. The other compartment is set up to trap incoming birds.

Be warned. If you start using a live decoy, you inevitably form some relationship with the bird, and have to come to terms with the fact that you are holding it prisoner.

Opinions vary about the most humane way of treating call-birds. Some people say that whenever you catch a new victim, you should adopt that as your decoy and knock the old one on the head - for some captives cannot stand the strain of incarceration, and keel over in a day or two. Other practitioners reckon that if a particular magpie settles down, you should keep it indefinitely.

The bird I borrowed off a friend was definitely a stayer. When I took him over, he had already done three weeks inside, and showed no sign of deterioration. At first we called him Hess, in reference to long service; but then we decided that a more appropriate name was Judas.

As I reported earlier, he caught his first victim on my ground within 10 minutes, and over the next two weeks he averaged one a day. I twice moved the trap to new positions, a couple of hundred yards apart, and I think this helped bamboozle the resident magpies, who, when they found an intruder on their ground, could not forbear to challenge him.

Two of the sites were within binocular range of the garden, and, by watching from a distance, I saw how fascinated wild birds were by the captive. They would land on top of the trap, hop off on to the ground, hop back on, fly up into the hedge, chatter furiously, make close passes over the cage, land again, strut about, and then fly off some distance, only to return in a few minutes.

In the end, inevitably, one would descend into the open side of the trap, spring the dummy perch and be caught. But what became perfectly clear was that the call-bird could not communicate any form of warning: for all the chatter that went on, the wild bird never took off in fright.

With the score at 15 magpies and one carrion crow, Judas had effectively emptied our end of the valley. Besides, the ripples of his good work have spread far afield: one of his victims had gone alive to a new trap down the valley, and another to an SAS training area in Wales.

At home, however, his usefulness seemed to have ended. The question was, what to do with him? At one stage my wife advocated setting him free, as a reward for good service. But, when we thought about it, this seemed ridiculous: as the aim of the whole exercise was to reduce the magpie population, it would be pointless to increase it by one.

For a few days I dithered. I continued to feed and water the bird in the hope that he still might bring off more captures. But then there was a sinister new development: he began to come under fox-attack at night.

At that stage I had the trap on an old concrete footing, so that I could not peg in the small electric fence with which I had been protecting it earlier. Instead, I relied on a chemical barrier of Reynardine, the foul- smelling fox repellent.

This seemed to work for one night, but lost its potency the next. Three times, in the morning, I found evidence of a ferocious struggle - moss scrabbled back all round the trap, the perch dragged out sideways, the baler twine securing it bitten through. On the final morning the open side of the trap had been sprung, and the front half of a rabbit, which I had put in there as an extra lure, had disappeared.

This showed that the fox had been down into the trap and set it off, but somehow escaped. I have heard of a cat being caught in a Larsen trap, but a fox would surely have set a record. In any event, Judas appeared unmoved: he must have had nerves of steel to have survived that upheaval right beside him.

By then I had grown rather fond of him, and did not look forward to putting him out of his misery. But I forced myself to do the deed, and now he is in the deep-freeze, ready to come out fresh and treacherous as ever in the spring of 1997.

Larsen traps are available from the Game Conservancy, Fordingbridge, Hants.