The main reason is a small deer, shaped like a pig, with antlers like salad tools. This is the muntjac. Often known as the barking deer because of the loud, cough-like roars it makes at any time of the year, day or night, the muntjac is Britain's fastest-spreading wild animal. Descended from a few animals that escaped from the park at Woburn Abbey in Bedfordshire a hundred years ago, the muntjac is now the second commonest deer over much of southern England. A recent census by the Mammal Society estimated that Britain is now home to at least 100,000 muntjac, three times as many as 10 years ago.
This has been a quiet explosion. Muntjac are secretive animals that are hard to spot. But their effect on our natural broad-leafed woodlands is all too obvious. Arnie Cooke has studied them at Monks Wood National Nature Reserve, in Cambridgeshire, where they first became a problem in the mid-1980s. Before the muntjac, he remembers, the wood was full of primroses and natural regeneration was booming. Today, however, it has become a bleaker place with few flowers and no regrowth. Monks Wood is now dominated by plants that muntjac reject, such as grasses, sedges and aspen. And in place of the former shrub layer, there is a muntjac-high browse-line from one end of the wood to the other.
"The muntjac is different to other British deer in that it has a very simple gut," says Cooke. "It's a selective feeder that nips off easily digestible items, such as flowers and fresh leaves. Some of them have even developed a taste for bluebell leaves." To the rage of flower-lovers it enjoys nibbling the buds of orchids. And among its other favourites are primroses and that East Anglian speciality, the oxlip, arguably Britain's most beautiful woodland wild flower.
But it is the muntjac's taste for the juicy young shoots of coppiced trees that is spelling doom for the nightingale. At Monks Wood, which used to be home to some 20 pairs, it has virtually gone. At Bradfield Wood, in Suffolk, the songbirds now nest only inside areas fenced off to keep the deer out. Rob Fuller, who is studying the effects of deer on breeding birds there, is in no doubt why Bradfield Wood's nightingale chorus has all but ceased. Boom time for the muntjac in the early 1990s produced a sudden nightingale slump.
"Nightingales like to nest in thickets close to the ground," Fuller says. "When deer numbers increase, this shrub layer, where many birds nest and feed, disappears. The nightingale is the one we are most worried about, but deer-browsing is affecting other birds too, including blackcap, garden warbler, willow warbler and even dunnock." Heavy browsing by deer is not only hoovering up the wood's carpet of flowers, it is also reducing the best singers in the woodland chorus.
Unfortunately, deer-fencing is not the perfect answer. To begin with, the muntjac is small enough to creep through holes in all but the best-maintained fences (and once inside they may not be able to get out). And some deer-browsing can be positively beneficial, reducing the dominance of brambles and creating patches of bare ground where nightingales like to feed. The ideal is not a fenced wood but a wood with a small population of deer - the sort of woods that we had in southern England up until the 1980s.
According to English Nature, deer-browsing has reduced the wildlife value of a full quarter of woodland sites of special scientific interest (SSSIs) in England. Some 8,000 hectares of our very best woods have been "severely impacted" by deer. As the most rapidly expanding deer species in numbers terms, the muntjac is heavily implicated in this loss. But they first escaped into the wild more than a century ago, so why has it suddenly become such a problem?
Cooke points out that the modern lowland landscape of small woods surrounded by fields of cereals and oil-seed rape is a highly muntjac-friendly environment. "The deer have food and cover in the woods and can sneak out at dusk to feed on farmland." And as the woods fill up with deer, more and more of them are wandering into gardens to help themselves to flowers and vegetables, such as Cooke's courgettes. In many small towns it has become a common, if rather startling, sight to watch a deer the shape of a pig, with swept-back antlers and nasty-looking tusks, slip across the road and disappear behind a garage.
Climate change also favours the muntjac. Unlike other deer, muntjac do not have a breeding season. Does can come into season at any time of the year. They can also conceive again within days of giving birth. Moreover, the youngsters become sexually mature at eight months and can live for up to 20 years. Population dynamics is on their side. The main thing that held the muntjac in check was its sensitivity to the cold. Now that winters are growing milder, life is getting better and better for the barking deer.
Is the rise and rise of the muntjac unstoppable? Cooke thinks not. Although eradicating the animal would be impossible, as well as probably unpopular, it would be possible to manage the problem. At the moment, the main way of controlling it is by shooting, organised by local deer-management groups. But however many are killed, others soon move in to replace them. "We need intelligent control on a landscape scale," says Cooke. "It will need better co-ordination effort and better incentives, but it is possible."
One such incentive would be a bigger public appetite for muntjac meat. "There's no money in muntjac at the moment," says Cooke. One way of learning to live with the muntjac might be to promote delicious meaty recipes devised by the likes of Nigella Lawson. We can all do our bit to help the nightingale. Muntjac pie, anyone?
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