Analysis: Cats, cars, and cleaner streets lead to the fall of a once-common bird

Scientists are beginning to explain why there are 10 million fewer house sparrows in Britain than there were 30 years ago

Cats, sparrowhawks, lead-free petrol, loft insulation and cleaner streets. All of these may be factors inthe staggering decline of the house sparrow.

But these are only guesses, not certainties. The real reasons for the fall of passer domesticus will not be established for another five years, scientists said yesterday.

After nearly two years of analysing 40 years of data about the house sparrow, a report funded by the Department for the Environment, Food and Rural Affairs (Defra), has painted the most authoritative picture so far of the bird's plight.

The study – which also examines the decline of the starling – concludes that there are now 10 million fewer house sparrows in Britain than there were 30 years ago. In the early 1970s, there were 12 to 15 million pairs. Now there are only six million.

The inquiry, led by the British Trust for Ornithology (BTO), has found that a large drop in survival rates for sparrows in their first year of life during the mid-1970s has played a vital role in their misfortune.

In the early 1960s, more than half of house sparrows lived to be two years old, but the figure subsequently slumped to 30 per cent. This led the population to plummet because they were failing to reach breeding age.

Since then, the sparrow's numbers have continued to decline. While the cause is still unknown, it may be as simple as the fondness cats have for chasing birds.

The report points out that the increasing numbers of people in Britain are keeping cats as pets. One study of a rural village in Bedfordshire showed that up to one-quarter of the breeding pairs of sparrows in the village may have been harmed by cats.

"Cat predation is also likely to account for a large proportion of the juvenile mortality in the village," the study said.

Cats are not the only problem. The sparrowhawk population, badly hit by DDT in the late 1960s and 70s, has returned strongly and colonised urban areas. One sparrowhawk nest in Kensington Gardens in London in 1996 was found to contain the remains of 38 sparrows.

In addition, an estimated 16,000 house sparrows were legally killed in 2001, mainly by farmers. However, that factor, which accounts for only 0.1 per cent of the population, is considered to be insignificant, the report suggests.

While the sparrow is disappearing in Edinburgh and Dublin, its sharpest decline is in London, where its numbers have dropped by 59 per cent between 1994 and 2000.

The capital once used to have some of the highest levels of sparrows in the country – Tony Blair once said the bird was more cockney than the Cockneys – but St James's Park, which once thronged with sparrows, lost the last of its birds in 1999. Of the royal parks in central London, only Kensington Gardens continues to harbour small numbers of the bird.

The picture, however, is not uniform throughout Britain. There has been no reported decline in Manchester, and urban and suburban populations in Wales and in rural parts of Scotland appear to be rising. The report said the reasons for such marked differences in population trends were unclear.

The report says that modern buildings may provide fewer nest sites for birds. House repairs, including blocking off gaps, may also have had an effect.

A survey in Bristol found that there was a correlation between house sparrow numbers and the extent to which loft insulation has been installed into homes.

"This may result in fewer nesting opportunities and there may also be a possibility of adverse respiratory effects from airborne fibreglass on breeding birds or their chicks," the report said.

Increasing urban sprawl may have also reduced the amount of food available. House sparrows tend to avoid travelling long distances. Adults in flocks fly less than two miles to find food in the autumn.

Cleaner streets and building on waste ground, where weed seeds once threw up a rich source of food for birds, may have made foraging more difficult, and cereal fields may also now be out of range.

The success of house sparrows in urban areas is crucial to their survival. About half live in towns and cities, with the largest numbers in the more urbanised central-eastern and southern regions of England.

A separate study has also begun looking into the potential impact of lead-free petrol, the use of which has increased dramatically over the past 15 years. The research will look into whether the volatile organic compounds that it contains as a substitute for lead might affect the abundance of green fly that house sparrows feed to their chicks in the first few days after hatching.

Yet some parts of England do not seem to have been affected, while species such as tits, which also feed on aphids, have not seen similar declines.

The Defra study has shown that more birds are flocking to gardens, particularly in winter, to feed from bird tables. The researchers suggest that feeders and tables that are not cleaned properly could lead to increasingly widespread illness. "It remains a possibility that house sparrows are susceptible to some sort of disease transmission," it said.

The author of the study, Dr Humphrey Crick, a senior ecologist at the BTO, said new foods, such as black sunflower seeds, were becoming popular. It was possible, he said, that such foods did not suit the house sparrow.

Researchers believe they have already solved the mystery of the decline of the house sparrow in the countryside, where grain barns have been sealed off and more efficient harvesting has driven the sparrows to rural gardens for food. They are now appearing earlier in wintertime in the gardens, which appear to be the most favoured habitat for the species.

The study found, however, that breeding performances in the towns compares badly with the countryside.

Dr Crick said: "I am not totally surprised that we haven't actually pinpointed the exact cause in towns. I am quite pleased we have got a pretty good idea of what happened in the country. People have ignored towns as a habitat for so long. People are only just waking up to the fact that towns are an important place for wildlife as well as people.

"We should now be able to find the exact causes. I would have thought we could be cracking it within five years and we should be able to put into progress things which will reverse the decline."

The BTO is launching an appeal for £100,000 today that will pay for a nationwide survey to try to look more closely at the differences between regions.

Donations can be sent to House Sparrow Appeal, BTO, The Nunnery, Thetford, Norfolk, IP24 2PU. Cheques should be payable to BTO House Sparrow Appeal.