A live decoy is by far the most effective

Ever been bitten by a magpie? I don't advise it, for the sturdy black beak with a slight hook on the end packs a tremendous nip. I know, because I have spent the past week taking live magpies out of my newly- borrowed Larsen trap and knocking them on the head.

If that sounds heartless, consider how many eggs and fledgelings of songbirds I must have saved. My bag of predators to date is nine, and over the next few weeks they would have been up and down the hedgerows raiding the nests of lesser species. Once you have heard a blackbird screeching as it powerlessly watches a magpie devour its babies, you do not feel very charitable towards the black-and-white robbers.

In recent years magpies have increased prodigiously, and even the Royal Society for the Protection of Birds acknowledges that they are now considered by many to be a pest species. Their success is ascribed partly to the fact that there are fewer gamekeepers to persecute them, partly to the ban on organochlorine pesticides, which means a better supply of grassland insects - a staple item in magpies' diet. Another factor is surely the increase in road traffic, and the consequent massacre of small animals and gamebirds on the road - for magpies are great opportunists and eaters of carrion.

An RSPCA leaflet gives many interesting facts about them - for instance that between 25 and 60 per cent of any local population does not breed, but remains in flocks throughout the year. Magpies live for between three and 15 years; they generally pair for life, but if one is killed, a new mate will appear within 48 hours. When food is abundant, they hoard any surplus by digging small holes in the ground with their beaks, placing their titbits in them, and covering the caches with grass, stones or leaves.

In spring the birds develop a strong territorial interest, and it is this that makes catching them easy. They cannot stick having a stranger on their patch; if they see one a combination of curiosity and aggression forces them to investigate.

This compulsion to intervene makes the Larsen trap extraordinarily effective. It was devised by a Danish gamekeeper in the 1950s and, like many good ideas, it is very simple. It consists merely of a cage made of wire netting and wood, divided into two compartments, each with a separate hinged lid.

You put a live call-bird in one side, with food, water and a perch, and jam the lid of the other compartment open with a collapsible rod, made of two accurately-cut pieces of dowelling set end-to-end and held in position by a spring. When an incoming bird lands on what looks like a solid perch, the rod drops away in two halves, and lets the lid snap shut.

Although the trap will work when baited with eggs or meat, a live decoy is by far the most effective, since it draws attention to itself by hopping around. One of the device's advantages is that you can quickly release any bird of the wrong species and let it go, none the worse.

The friend from whom I borrowed my trap caught 37 magpies in his garden last year, and noticed a pronounced increase in songbirds as a result. This spring he set the ball rolling by staffing the trap with one of last year's bag, fresh from the deep-freeze. This, though immobile, did the trick, and he caught 16 more.

When I brought the trap home with the latest decoy inside, its efficacy was such that I caught my first victim within five minutes. Since then I have caught an average of one a day, and the thick, fruity chattering that used to wake us up in the mornings has become a thing of the past.

Opinions vary about how long one should keep the same decoy: some people say you should despatch it every time you make a fresh capture, and use the newcomer as the call-bird instead. All I can report is that my fellow has been in captivity for a month and remains a star performer: he (if it is a he) eats well, looks well, has plenty of energy, and does not panic when humans come near.

His staple diet is a patent dogfood mix containing dried meat; he also eats corn, but I think it is important that he has some meat as well, since carrion would form part of his wild diet.

The Larsen trap is equally good for catching crows - but first you need a captive crow, and for the moment I am concentrating on maggies.

It is unpleasant to keep any wild creature in gaol, and I do not enjoy making one captive suffer for the unsociable habits of its kind. But the thought that I am giving hundreds of songbirds a better chance of life is enough to harden my heart.