Simon Barnes on an 'invader' from East Asia which has become a fixture in our gardens.

There's no excuse for ever doing a stroke of work at my desk. The view from the window is always so much better than the view on the screen. My own words are lame and dreary when compared to the vista of Norfolk marsh I can see by turning my head through 90 degrees.

Excuse me… the last paragraph was interrupted by a grey squirrel feasting on a shaggy inkcap five yards from the window, an impressive fungus that looked far too big for him. But he ripped bits off in strips and consumed them, in the words of the great Gerald Durrell, in the manner of a nervous person eating canapés at a cocktail party.

Deer drop in regularly to feed in front of this window, and there's no point in even pretending to work when there's a large and furry fellow-mammal almost within stroking range. Move slow, even when you're on the far side of the window: any hint of suddenness and they're off.

Mostly they're Chinese water deer, which are introduced exotics as the name suggests. They escaped and were released from collections and established a home in our low-lying wet places: the Broads, either side of the Waveney, are ideal, and so is the Fen Country. They're in trouble on their Asian home grounds and it's been estimated that 10 per cent of the world's population can be found in this country.

But this deer was different. Bright tan, black vee on his face, and small antlers: none of the homely, teddy-bear-like face of the Chinese water deer. This was a muntjac; more properly known as Reeves's muntjac. This is another introduction; its native range is also China and East Asia.

Like so many other exotic species, these muntjacs originated in the collections at Woburn Abbey. A few were released on the Norfolk-Suffolk border, and in other places in this country between 1947 and 1952. These are forest-dwellers not wet-loving animals like the Chinese water deer, and the two don't come into contact much. This was the first I had seen from my desk.

They're common enough in the woods on what we on the Broads amusingly call hills: here everything is either marsh or hill. They prefer browsing from trees to eating grass, and so they are a trial to foresters. They have adapted suburbs and fringe habitats, even golf courses.

Two species of muntjac were introduced to Britain, and it's likely like that in some places the Reeves's and the Indian muntjac hybridised, which adds yet more confusion to the ancient question of when is a species not a species. But the Reeves's is the one you find in Britain today.

John Reeves was a British civil servant and amateur naturalist who served in Macau in the 19th century. He sent the first specimens of the deer back to Britain. There is also a species of pheasant named for him, and you can see it in East Anglia.

Individuals have been released into the countryside along with all the other pheasants; we release 40 million game-birds in Britain every year. Keen birders have already seen the Reeves's and are now waiting patiently for the magic moment when it's officially added to the British List; Reeves's pheasant will then become what's known as an Armchair Tick.

But I was watching the deer. The muntjac, a handsome male, walked away and headed back up in the direction of the woodland on the hill, while out in the wetness of the marsh the Chinese water deer continued their more watery lives. And though we are supposed to hate and despise all exotic species, I've always found a heretical liking for these bold immigrants. Dumped in a strange place they have found a way to survive and to prosper.

It'd be near impossible to eradicate them now. We've lost the chance to build a wall to keep them out (and the deer will pay!) - so perhaps we'd better tolerate and admire them.