Revealed: why the sparrows are dying out

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Me old cock sparrer, we may have cracked it. New evidence is pointing at a solution to the greatest wildlife mystery of recent years - why the house sparrow, Britain's most familiar bird, has been vanishing from London and other towns and cities.

Many suggestions have already come and gone. Magpies, cats, pesticides, peanuts, climate change and home improvements are among the myriad causes that have been put forward for the sparrow's startling disappearance, since The Independent highlighted it in 2000 with the offer of a £5,000 prize for the first properly accepted scientific answer.

Our prize has never been claimed. But the offer still stands, and now comes research that, in the opinion of the world's leading sparrow expert, may get closer to a solution of this enigma than anything that has gone before.

It comes from Kate Vincent, a postgraduate researcher at De Montfort University, Leicester, who - for the past five years - has been closely examining house sparrow breeding success for her PhD thesis.

Her research appears to bear out experimentally one of the theories that have been put forward for the birds' decline - a similar decline in the numbers of the insects and other invertebrates that sparrow chicks need for the first few days of their lives.

Sparrows are granivorous birds - they live on grain and other seeds (and the bread that is made from grain, when we put it out for them). However, when they are very young, in their first week, the chicks need animal protein in the shape of small grubs, flies, aphids and spiders.

Over three years, Ms Vincent, 28, put up more than 600 nestboxes on houses in Leicester and the city's suburbs and her remarkable finding was that, in the summer, completely unseen by the outside world, considerable numbers of sparrow chicks were starving to death in the nest.

Furthermore, those whose diet had consisted largely of vegetable matter - seeds and scraps of bread - were much more likely to die than those whose diet had plenty of invertebrates. (With 50 per cent insects, they had an 80 per cent chance of surviving. With no insects, they had only a 30 per cent chance.)

Ms Vincent worked out the chick's diet by analysing their droppings. In an ornithological labour of Hercules, every time she weighed and measured a chick in the nest, she collected the poo it would tend to deposit in her hand; and then, under the microscope, she could identify in it the tiny remains of insects - an aphid leg here, a beetle mandible there - and estimate their abundance.

The chicks that were dying were largely in the sparrows' second brood of the year, and that provides an explanation for the population decline as a whole. As many young sparrows do not survive their first winter, every year the species needs two or three broods (of four chicks each) to keep the population at least level. If the second brood is failing, the population will start to fall.

Ms Vincent found an 80 per cent success rate in the first brood, but only a 65 per cent success rate in the second.

The strong implication is that insects and other invertebrates are becoming much scarcer in Britain in summer - which, although Ms Vincent's research does not specifically prove this, is suspected by wildlife researchers.

"Kate Vincent's evidence, especially her finding of complete broods of chicks dead in the nest, is quite compelling," said the world expert on the house sparrow, Denis Summers-Smith. "Insect decline may well be the crucial factor."

* The Independent's £5,000 prize is for a peer-reviewed paper in a scientific journal, which, in the opinion of our referees (the RSPB, the British Trust for Ornithology and Dr Denis Summers-Smith) accounts for the recent, sudden and precipitate decline of the house sparrow, Passer domesticus , in Britain, particularly in towns and cities.