He has trapped a pair of these furtive, nocturnal, ferret-like animals, and fitted them with collars carrying tiny radio transmitters. To find out where they hunt, what they eat and how they live, he goes on polecat patrol with a radio receiver and directional aerial to pick up the signals from his elusive quarry.
'I don't like leaving my wife in the middle of the night,' he says. 'But the polecat is such a secretive creature and little is known about its numbers or movements. If it is to make a successful comeback, the information I'm gathering is vital.'
Despite its name, the polecat is not a cat but a member of the weasel family. For centuries the creatures were persecuted by farmers and gamekeepers, their numbers hit hard by indiscriminate trapping, especially by the cruel gin trap before it was declared illegal in 1958.
The polecats retreated to their last remaining refuge in Britain, a small corner in the hills of west Wales. But now they are advancing again at a steady one mile a year, and have colonised most of Wales, crossed Hereford and Worcester and reached as far as Warwickshire. Dr Birks' beat is the west of the Malvern Hills. Just how numerous polecats are no one knows, but there are signs that they are increasing their numbers and expanding their territory.
As well as collating scientific information about their habitat and lifestyle, Dr Birks also acts as a sort of public relations officer to improve the polecat's image. He gives lectures and slide shows, chats to farmers and landowners, and extols the animal's virtues to anyone who will listen.
'It's important to change the public's perception of the polecat,' he says. 'It's got a bad reputation as a vicious killer of poultry and game. Of course, it kills to eat; it is a hunter. But I want people to see it in the same light as a bird of prey. We admire predators such as the peregrine falcon for the skilful way it swoops on and dispatches its quarry. The polecat is just as graceful and skilful, but does most of its killing underground.'
Another factor that adds to the polecat's unsavoury reputation is its smell. The expression 'stinks like a polecat' is apt. When alarmed or angry, it has the ability - like its distant cousin, the skunk - to exude a powerful odour as a defence mechanism.
Dr Birks says: 'When I trap them and bring them home in the car to be fitted with transmitters, they do leave a pong. My wife doesn't like it one bit. The air freshener is a vital piece of equipment. But polecats smell only when they are alarmed.'
At the moment he has two collared polecats, a female, Meg, and a male, Bracken, named after Kyran Bracken, the new England rugby scrum half. They range over an area of 10 square kilometres, hunting by night and sleeping, usually in rabbit burrows, during the day.
Sadly, one way of gauging numbers is by counting corpses of animals that have been run over. Dr Birks urges anyone who finds one to take it to the local museum, which will pass on details to him.
As well as logging their movements, he also collects their droppings for analysis to see what they are eating - usually rats or rabbits. 'They are doing an important job keeping down vermin. Farmers should appreciate this.'
Dr Birks was employed by English Nature but is on loan to the Vincent Wildlife Trust, which is funding the polecat project. He has also done studies on two other shy animals - the otter and the mink. 'To be actively involved in the recolonisation of an animal that was persecuted almost to extinction is a privilege,' he says.
But he has a recurring nightmarish thought as he drives home to his warm bed and his wife, Helen, after his nocturnal wanderings. 'I often think how terrible it would be if I ran over one of my own polecats.'
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