They used to be ubiquitous in our towns and cities, flitting, pecking and chirping noisily.

But now they have all but gone, and the streets and gardens have fallen silent.

House sparrows - the brown, boisterous opportunists that were Britain's best-known and most-loved urban birds - have virtually disappeared in our built up areas. Scientists say their numbers have plummeted by up to 90 per cent since the 1970s.

No-one is exactly sure why, though the loss of hedges, trees, insects and flowers have all been blamed. Predation by domestic cats and other animals may also be implicated, as well as pollution.

But now conservationists, communities and schools have launched campaigns to try and bring back house sparrows. One of the pioneering projects is in Glasgow, where the birds have suffered a very steep decline.

The ornithologist, JD Summers-Smith, recorded a massive 98-99 per cent decline in sparrows in suburban Glasgow between 1959 and 1997. The birds are now recognised as one of the most endangered across the UK, and are on the "red list" of the most vulnerable species.

"Despite a large amount of research into the subject, we still don't conclusively know what is causing the long-term decline in house sparrows," said Toby Wilson, a senior conservation officer with the Royal Society for the Protection of Birds (RSBP) in Scotland.

"Studies have shown that large numbers of house sparrow chicks are starving in their nests as a result of a lack of insect food, which could be limited by pollution and a reduction in greenery in urban areas."

Predation by cats could put extra stress on the birds, which aren't always quick enough to escape. "A reduction in dense hedges and other forms of cover for house sparrows may leave them more exposed to predators," added Wilson. "It may well be a combination of factors and vary in different areas."

RSPB recommends that cat owners put a collar and bell on their pets, and try to keep them indoors an hour after sunrise and an hour before sunset, when birds are most active. It also encourages the planting of dense, high hedges in which sparrows can take cover.

In Scotland, RSPB has teamed up with Glasgow City Council and the University of Glasgow to try and identify where there are surviving sparrows. By studying them, researchers hope to learn more about how to boost their numbers.

As part of a project "giving nature a home in Glasgow", Govan High School has been appointed as an "ambassador school" for house sparrows. It has identified a leafy, muddy part of its playground as a good habitat for house sparrows and has planted flowers to help provide sparrows with food. The school is also planning to put up nesting boxes.

"We were shocked to discover the number of house sparrows living in cities like Glasgow had dropped," said Stephen Lowrie, a 15-year-old pupil at the school.

"Our Eco Committee wanted to prevent this decline and with the help of the RSPB we are going to try and boost the number house sparrows living in our local area, starting with our school playground."

Dr Ross MacLeod, an expert on birds from University of Glasgow, described the demise of house sparrows as "catastrophic". Conservationists were struggling to find ways to reverse the declines and aid population recovery, he suggested.

Volunteer surveyors had been recruited in Glasgow to help pinpoint where the birds remain and where they manage to breed successfully. "What we are starting to find is that house sparrows prefer gardens that have both bird feeders and nice thick and high hedges or bushes that are not trimmed too closely as these make great places to keep safe from predators," he said.

"The other factor that seems likely to be important is the availability of nest sites so we're investigating if nest boxes will help house sparrows return to gardens they have disappeared from," he added.

"After such large declines house sparrow recovery will be a long process. But if we in Glasgow can come together to help identify the causes and come up with sparrow-friendly gardening suggestions that will help people improve the environment, then the project will have been a great success."