Last spring I spent some time in the US looking at birds in Washington DC and New York City. That's not such an improbable idea as it may seem, for both metropolises harbour parks with wonderful wild bird populations, especially in May, when I was there: Washington has Rock Creek Park, a 2,000-acre stretch of natural forest to the north of the city centre, while New York's Central Park is an 800-acre green glade in the forest of skyscrapers.
Both are teeming with birdlife, above all when the spring migrants arrive, the birds which winter in the Caribbean and Central America and fly up to breed in the northern US and Canada, and of these the most stunning are the warblers, the brilliantly-coloured small songbirds which have been described as "the butterflies of the bird world". I saw several of them both in Rock Creek and Central Park, and wrote about it here; but what I did not mention were the birds I saw first in both cities, which were sparrows.
There are more than 30 species of indigenous American sparrows (technically, they're buntings, but sparrows is what they're called), from the lark sparrow and the song sparrow to the white-throated sparrow, but the birds I saw in the two big cities were of solidly British origin: they were examples of the house sparrow, Passer domesticus.
If the English language is our most successful export, the house sparrow must surely run it a close second, as we have sent it, just like English itself, all over the world (birds were often taken abroad in the 19th century by British emigrants who wanted something familiar to remind them of home). Now it can be found everywhere from South America and South Africa to Australia, as well as all over the US; the first American birds were released in New York in 1850 and by 1890 the species had spanned the continent and reached California.
In downtown Washington last May, house sparrows were the first birds I saw, flocking and chirping in Lafayette Square, just up from the White House; they were the first birds I saw in New York, hopping about at the 72nd St entrance to Central Park. And in both cases I received identical, wry responses from US birders when I pointed out how common and ubiquitous they were, and how rare they have become in British cities – "Would you like ours?" By no means everyone in the US is a fan of the chirpy Cockney sparrer.
But it is remarkable, nonetheless, to see the birds so numerous in two great American conurbations when in London over the last 20 years the identical creatures have virtually vanished. What on earth has happened to them here, that has not happened to them there? We just don't know. The disappearance of the house sparrow from British towns and cities presents one of the major mysteries of our wildlife, and a mystery it has remained despite the offer of a £5,000 prize for solving it offered by The Independent nearly 11 years ago (it's still on offer).
Now the world's leading expert on sparrows has a new theory. Denis Summers-Smith, the Scottish-born engineer who has been studying sparrows since 1948, and has written five books on the birds, will be presenting it next week at an ornithological meeting in Newcastle.
For a long time, Denis held the view that the population crash was being caused by a reduction in the availability of the small insects that house sparrow chicks need in the first few days of their lives, and research has shown that this is indeed happening; but more tests showed that even when supplementary insect food was provided and the chicks managed to leave the nest, populations still did not increase.
Denis believes some extra phenomenon is making the young birds "unviable" and he thinks he knows what it may be: air pollution and, especially, particulates, the microscopic soot particles produced by diesel exhausts. World Health Organisation research has shown how children's development can be affected by particulates and Denis thinks this may apply to young birds just as well.
He doesn't have any evidence. He is merely speculating, and inviting research scientists to test out his hypothesis (and part of his presentation is a defence of the role of speculation in science). But whether his speculation proves accurate or not, he freely admits it will be his last. "I'm 90, and this will be my swansong," he said this week.
When The Independent launched its prize (Denis has been one of our referees), he did not imagine, he said, that more than 10 years on, the mystery would remain unsolved; for my part I find it awe-inspiring that he is grappling with it still, as he enters his own 10th decade.
What future for our forests?
The conditions of our sparrow prize are rigorous – entries must be a scientific paper published in a peer-reviewed journal. But the terms for another £5,000 prize are less so, and that is the essay competition on The Future of England's Forests, which we are sponsoring with the wildlife conservation charity Fauna and Flora International.
So a reminder: if you are interested, write an essay of 1,500-2,000 words on "The Future of England's Forests" and email it to email@example.com by midnight on 25 March 2011. A prize of £5,000 will be awarded to the writer of the essay which the judging panel considers the best, and the winning entry will be published in The Independent (subject to meeting editorial standards). For terms and conditions see independent. co.uk/legal.