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The Deer Initiative is using high-tech thermal imaging equipment to monitor deer populations. investigates

Under the cover of darkness, the pickup truck nosed down the farm track, a suspicious-looking character perched high on the back and two more figures crouched in the cab, with no illumination save the vehicle's dim sidelights. Thieves on the lookout? Poachers on the prowl? No, nothing to worry about here unless you happen to be a deer.

It was midsummer, and I was spending the short hours of darkness with David Jam, regional liaison officer for the Deer Initiative, on the edge of the Lincolnshire Wolds. In the company of Malcolm Armstrong, forestry ranger and head of field operations for the Lincolnshire Deer Group, we were out deer spotting. If you're looking for fallow, roe and muntjac, night-time is definitely the right time, but you will need technical backup. In David's case this comes in the form of a 40,000 night-vision camera. "Actually, this is not the very latest," admitted David, as he got the Flir Milcam thermal imaging camera out of its box. "It is pretty advanced military spec, though, and we still needed a special import licence to get it out of the US. On a clear night, with a good line of sight, I can see deer up to 5km away and I can count them accurately at 2km." Our objective was, of course, to count deer, not shoot them.

The Deer Initiative is mounting its operation in support of the local landowners and stalkers in order to assess minimum densities and distribution of the different deer species in the district. There have always been deer in this area, mostly fallow, which are associated with the local ancient deer parks. In recent years, however, increasing road casualties and complaints of crop damage from farmers especially to high value vegetable crops on the fen edge have suggested that numbers are on the up. Local estates and stalkers have attempted to monitor the population, but with limited success.

"No-one has an accurate handle on the numbers," says David. "So we have provided support to the landowners and deer managers by bringing in this night-vision equipment." Fitted with a 50mm to 250mm zoom lens, the camera turns darkness into day, picking out birds, animals and vegetation as though they were lit up by a searchlight. Because no illumination is involved, the deer remain undisturbed and continue about their nocturnal business. David linked the camera by cable to a digital video recorder and monitor, which I carried on my lap in the passenger seat. By this means, Malcolm and I had the same view as David. With a radio link from the viewing platform to the driver's seat, we were in business.

Operations started as soon as it became dark enough for the deer to venture out of the woods and on to the fields. We had been creeping down a track between two fields on the edge of the wood for less than five minutes when David's voice crackled over the radio. There was a group of fallow to our left. They shone out as white dots on the screen, so, using the zoom lens, David went in for a closer look. There were six of them, all does, approximately 400m away and unconcerned by our presence. With the sighting confirmed and the identification of species and sex agreed between us, the record was logged on a map by Malcolm.

In due course, David would be able to study at leisure the video and compare it with the GPS data and map in order to build up an accurate picture of the local deer population. No sooner had we moved on than two more deer appeared on the screen. At first they looked like roe, but a closer examination showed them to be more fallow, this time warier. After watching us for a few moments, they trotted back into the wood.

Slowly but surely I got used to identifying the different species. Antlers can be difficult to see on a thermal imaging screen unless they are in velvet, when the warmth of the protective network of blood vessels makes them stand out brightly.

It's not just deer that are evident: a flock of sheep gave us an interesting few minutes, as did a herd of reclining cattle. We spotted a couple of foxes and it was amazing to see how many hares there were out on the fields. Inanimate objects also provide clues to the night-time movement of wildlife. Coming along a hedgerow, David trained his lens on the woodland edge a field away and we quickly picked out deer. Also evident were a series of bright patches along the headland where the crop gave way to tree cover. These were the couches where deer had laid up, the difference in soil temperature caused by their body heat showing up clearly on the video screen. Two muntjac quickly gave themselves away yards from the edge of the ride. Equally intriguing were other creatures lying on the woodland floor until we realised they were piles of chipped brushwood that were heating up as they decomposed.

A thermal imaging camera is an amazing instrument, but it does not allow you to look through solid objects. While you catch a glimpse of a deer's flanks through the trees, once it moves off into the undergrowth, it is gone. Likewise in a field of oilseed rape David caught a glimpse of brightness five yards from the hedge along which we were driving. Malcolm stopped the vehicle and the creature moved behind the forest of rape stems. Only when it crossed a tramline into the body of the crop did we clock it as a fallow doe.

By 2am, our tour was complete, and with four or so hours of recorded video, we had seen 61 fallow and five muntjac, all of them logged and recorded. "I was quite surprised that we saw as many as we did, with it being so breezy," said David. "This is also the worst time of the year for counting in terms of the amount of cover, the height of the crops and the social activity of the deer. In February, you can see herds of 60 to 100 and last year, near Salisbury, I saw a group of 190 walking across the road in front of me, with two independent witnesses to prove it. That night we saw more than 500 fallow."

We didn't see any big herds, but we spotted plenty of small groups of three or four animals and, of course, we saw only a fraction of what was actually there. "In this sort of country, at this time of year, my margin of error is about one-third, so there are probably 180 to 200 fallow in this area. In open country we would be seeing around three-quarters of all the deer present." This, however, is not the precise census figure that the Deer Initiative is looking for. Aided by the GPS trace, David will drive exactly the same route in the winter and at this time again next year, so that a pattern should emerge. This can be used by landowners and local stalkers to inform their cull.

In the countryside of lowland England, thermal imaging is an ingenious and practical way to count deer and, with two cameras in its armoury, the Deer Initiative is prepared to use them to support deer management groups or estates wherever they combine in a co-operative manner to manage deer.

From the likes of David Jam, there's no hiding place for deer even at night.