The Song Thrush is already a relatively rare sight

They also say it is possible that crop spraying and pesticides may be killing off young Blackbirds.

The estimated population of Blackbirds in Britain in the period between 1988 and 1991 was 4,400,000 pairs, but this may have declined by more than 1,000,000 pairs.

Blackbirds nevertheless outnumber the Song Thrush by about five to one, according to estimates, and their song is the most obvious feature of the dawn chorus in urban and suburban areas.

Jeremy Greenwood, Director of the British Trust for Ornithology, said: "It is very sad for us to have found this evidence that one of Britain's favourite birds, the Blackbird, is in trouble.

"It is still very common and one of the top birds in our extensive survey of gardens but these results from ringers all over the country show they are yet another species in serious decline."