It was once a common or garden bird. Now it's not common or in your garden. Why?

It might at first seem inconsequential; but remember the miner's canary.

It might at first seem inconsequential; but remember the miner's canary.

The sudden and dramatic disappearance of the house sparrow from many of Britain's towns and cities, after centuries of untroubled urban survival, is a sure indication that something has gone seriously wrong in the sparrow's ecosystem - and maybe in ours too. The problem is, nobody knows what.

Some think it is just the lesser availability of seed, now horses have disappeared from towns. Some think it is the greater prevalence in urban areas of magpies and sparrowhawks as predators. Some are starting to think it is a mysterious disease. And some think it is a profounder cause, such as the mass but hitherto unnoticed disappearance of the insects the birds need to feed to their very young chicks, which would imply a serious pollution threat to us all.

But although research is going on into house sparrow numbers and their decline, little scientific work is being carried out into the cause. So today The Independent offers a prize of £5,000 for the first convincing scientific evidence of why this bird, which for so long has stood in our culture for all that is humble but hardy, is suddenly in terrible trouble.

It should not be happening. House sparrows are the ultimate survivors; they have been found breeding 14,000ft up in the Himalayas and nearly 2,000ft down in a Yorkshire coal mine. The bird occurs naturally all across Eurasia, and flourishes where it has been introduced in Southern Africa, the Americas and Australasia; but for some reason, no longer in Britain.

The prize is for a person or persons who can find out why. It is offered for a peer-reviewed paper in a scientific journal which, in the opinion of our own referees - the Royal Society for the Protection of Birds (RSPB), the British Trust for Ornithology (BTO), and the sparrow expert Denis Summers-Smith - explains the house sparrow's recent precipitous decline, especially in towns and cities.

As its purpose is to stimulate interest and observation by the public as well as scientific research, non-scientists will be considered for inclusion if their particular observation or theory proves to be the starting point for the final scientific explanation. As scientists from the RSPB and the BTO themselves are among Britain's leading ornithological researchers, they will not be barred from entering.

Interest in the house sparrow's decline has shot up since The Independent pointed out in March that while Paris was still full of the birds, London was empty. Kensington Gardens, where 2,603 sparrows were counted by the ornithologist Max Nicholson on one day in 1925, may now have none. St James's Park and Buckingham Palace gardens, teeming havens for other wildlife, certainly have none; you will look in vain for a sparrow among the thousands of pigeons in Trafalgar Square.

Then in April, we showed in a detailed survey of 12 major cities outside London that the bird's disappearance from urban centres was widespread. House sparrows have now virtually gone from Glasgow, Liverpool, Sheffield and many other cities and towns.

Yesterday, as the RSPB began its own nationwide survey of house sparrow numbers in conjunction with Children's BBC, the charity's head of research, David Gibbons, welcomed the offer of the prize from The Independent.

"We think it's an excellent idea," Dr Gibbons said. "We have been aware of the bird's decline in the countryside, but less so of its decline in cities. But we think it's happening, we're very worried about it, and we need to engage with it."

Jeremy Greenwood, director of the BTO, also welcomed the prize. "The house sparrow's decline is a very significant event," Dr Greenwood said. "It has gone down on farmland, like other farmland birds such as the yellowhammer and the grey partridge, and we understand the reasons for that, which are to do with intensive farming. But the loss of sparrows in urban habitats is a different matter and it's fair to say the cause is not known."

Dr Summers-Smith, the world expert on the house sparrow, said he would be "delighted" to help referee the prize. Dr Summers-Smith, of Guisborough, Cleveland, is the author of the standard monograph on Passer domesticus, published in the celebrated Collins New Naturalists series in 1963. He has written three other books on the bird, and is continuing his research.

And his latest work does not make for encouraging reading. The decline of the house sparrow across the nation as a whole, he says, is about 65 per cent since 1970, a figure that agrees with the RSPB's estimate. But his most recent studies indicate the decline over the same period in towns and cities alone is a staggering 92 per cent.

"It's a quite extraordinary figure, considering how the birds were once so common that people simply did not notice them," he said. " The Independent's prize is a very good and timely idea because it will get people thinking about what I consider to be one of the most remarkable wildlife mysteries of the last 50 years."