It is one of five British birds classified as “red listed” — because of their dramatic decline since 1974 — that have had a big increase in numbers in the past decade.

“Numbers are now significantly higher in England, Scotland and Wales than in 1994. It is good news for the song thrush,” said Graham Appleton, of the British Trust for Ornithology.

The song thrush’s reversal in fortune is charted in the trust’s annual report on birds that breed in Britain.

Worrying declines in numbers began in the 1960s and the population plunged even farther in the 1970s and 1980s, when intensive farming and loss of woodland areas are thought to have devastated their survival chances.

The reasons for their revival are still unclear to ornithologists but gardeners and more ecofriendly farming are believed to be behind it. “The fact that numbers are going up is very welcome, but we don’t know why they’re going up,” Mr Appleton said.

“We believe it’s probably down to more environmentally sensitive farming and because gardeners are using less in the way of pesticides.

“It could well be because we are all gardening now and are making a difference for the song thrush.”

The number of house sparrows, which have suffered a mystifying population drop in England over the past 30 years, appears to have stabilised in Britain.

In England there was a 9 per cent decrease from 1994-2005 but in the last year there have been signs of a halt to the decline, with a 1 per cent rise from 2004-05.

Elsewhere in Britain, the house sparrow has done far better, with observers reporting a 47 per cent increase since 1994 in Scotland and a 107 per cent rise in Wales. Mr Appleton said: “It looks as if the house sparrow decline has stabilised. That’s better than still going down.

“It could be that they are suffering from having fewer insects available in the summer and less food overall at all times in the year.”

Biologists are still trying to find out why the house sparrow population has fluctuated so wildly but one suggestion is that modern houses are too well-maintained. A study that was carried out in Bristol found that the bird did far better in areas where householders were struggling to pay for improvements to their homes.

The trust’s survey found that 22 of the 221 species of breeding bird declined severely from 1994-2005 but that 51 increased their numbers significantly. Willow tits and wood warblers headed the list of birds in decline, with both species slumping by more than half over the 11-year period of the survey.

Of those on the red list of birds under threat, having suffered a 50 per cent fall from 1974-1999, nine continued to plummet in numbers from 1994-2005. They include the willow tit, grey partridge, turtle dove, starling, skylark and linnet.

Five red-listed birds, including the song thrush, experienced a significant rise in numbers between 1994 and 2005. The other four are the grasshopper warbler, marsh tit, reed bunting and tree sparrow.

The most populous bird, according to the report, was the wood pigeon, followed by starlings, blackbirds and rooks.

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