Cars and alien shrubs are being blamed for the mysterious disappearance of house sparrows from British towns and cities.

A lack of insects for chicks and fledgelings to eat has been identified as the most likely reason for the bird's vanishing act, which has mystified ornithologists for more than 20 years.

The food shortage has been caused by car pollution, the concreting over of front gardens and the replacement of native plants with alien ornamental and evergreen varieties.

The house sparrow population has been in freefall all over Europe since the 1980s. In Britain it has fallen by 68per cent since 1977. In London numbers fell 60 per cent between 1994 and 2004, and the bird has almost vanished from the centre of the capital.

“This is one of the most mysterious and complex declines of a species in recent years,” said Kate Vincent, who took part in research into the phenomenon at De Montfort University in Leicester. “The study highlights that sparrow chicks are hatching but they aren't surviving. This is partly down to the loss of green spaces through development, tree removal and the conversion of front gardens for parking.

“The loss of deciduous greenery within urban areas may have made life much more difficult for birds like house sparrows that need large numbers of insects to feed their young.”

A pair of adults need to raise at least five chicks to maintain the existing population, but they are failing to do so because so many chicks die in the first week of life. Many fledgelings are undernourished when they fly the nest and die soon afterwards.

House sparrows in Leicester and surrounding villages were observed from 2001 to 2003 as part of the research project. Numbers fell by 28 per cent during that time.

Traffic levels, which have risen 82 per cent since 1982 in Britain, were highlighted as one of the most likely reasons for the shortage of insects, with a direct correlation identified between the size and weight of house sparrows and the amount of nitrogen dioxide, a pollutant from vehicles, in the air.

The researchers, whose findings are published in the journal Animal Conservation, also pointed out that many adult sparrows are killed by cars. They said that people could help by encouraging insects into their gardens.

Phil Grice, senior ornithologist at Natural England, said: “This study highlights the importance of using native varieties of plants in our urban green spaces.”

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