SOS: save our sparrows

A team set up nets in a park to catch house sparrows. They were weighed and had their wings and legs measured and identification rings fixed to their legs before being released.

Experts from the RSPB hope the data will provide clues to the cause of the birds' disappearance. For example, low weights could indicate a shortage of food. Each bird was also checked for signs of diseases which could be wiping out the population.

House sparrows were once as ubiquitous as pigeons in London but numbers have fallen by 71 per cent in 10 years. Populations are in decline all over Europe but the problem is worst in the capital, where there are now fewer than a million breeding pairs.

Scientists disagree about the cause. Dr John Mallord, research biologist for the London House Sparrow Project, said: "We haven't figured it out yet. There are a number of theories.

"Lack of food is limiting the number of young but even when we feed them and we get more young, they don't necessarily survive into adulthood. There must be other factors.

"What we do know is that if the decline continues at this disturbing rate there will be no house sparrows left in London within 10 years. It's not a case of waiting for numbers to drop to zero. When they reach a certain level every colony collapses but you don't know what that level is until it happens.

"We need to find the cause early enough to do something about it."

He and colleague Chris Orsman spent five hours yesterday ringing 17 sparrows which flew into the 9ft nets in Laycock Park, Islington.

It is one of the few places in London which still has a healthy population of sparrows, along with Regent's Park and Coin Street on the South Bank. The last count in Kensington Gardens found only eight. The RSPB team transferred-the trapped sparrows into calico bags to keep them calm. They checked the health of each and recorded their vital statistics: a heavy bird with long legs is more likely to survive. The average weight was 24g.

Ringing the birds will let conservationists keep track of them without having to catch them again.

Mr Orsman said: "The rings are loose enough not to hurt and tight enough not to slip over their feet."

LIKELY CAUSES

Starvation: a study in Leicester found baby sparrows were dying in their nests, suggesting their parents could not find enough food. The RSPB is running a feeding experiment using 66 colonies in London.

Loss of habitat: Londoners have paved over 12 square miles of front gardens, reducing food sources. Loft conversions destroy the holes sparrows like to nest in and new buildings are often designed without nesting cavities.

Pollution: Poor air quality may affect life expectancy and breeding potential.

Predators: Cats, crows and sparrowhawks have increased in London and have been blamed for killing sparrows. But they have also increased in places where numbers of the small birds remain steady.

Isolation: Sparrows live in colonies and find mates from other colonies nearby. As numbers decline there is less chance of colonies being close enough for them to meet.