An estimated 44 million nesting birds have been lost from the UK since 1966, according to a new report.
Experts estimate that breeding birds have vanished from the British countryside at an average rate of one pair every minute.
There are now thought to be 166 million nesting birds in the UK compared with 210 million in the 1960s.
The house sparrow is one of the biggest casualties, say researchers. Since 1966, its population has halved from 20 million to 10 million, despite a rise over the last decade.
Dr Mark Eaton, a Royal Society for the Protection of Birds scientist who worked on the State of the UK's Birds 2012 report, said: "It is shocking to think that we've lost one in five of the individual birds that we had in the 1960s, especially when you think that the 44 million birds we have lost since 1966 is equivalent to the current adult human population of England and Wales."
Changes in land use and the management of the countryside and coastal waters are believed to have contributed to the losses. In some cases, birds have found it difficult to locate suitable places to nest, or to forage for food in the summer or winter.
There were both losers and winners among different individual species, said the scientists. The trend was highlighted by the fortunes of two related species, the turtle and collared dove.
In 1966, the turtle dove was widespread with around 140,000 breeding pairs. Collared dove numbers, on the other hand, were very low as the species only started nesting in the UK in 1955.
Now there are thought to be no more than about 14,000 nesting pairs of turtle doves in the UK, while the collared dove population has exploded to around a million pairs.
Report author Dr Andy Musgrove, from the British Trust for Ornithology, said: "Amongst individual species, whilst there have been some winners, the number of losers is greater and the long-term picture is sobering. There is still more to learn though, and we need the continuing support of ever greater numbers of volunteer birdwatchers, on whose efforts all of these numbers are based."