The UK's bird population has plummeted by 44 million in four decades, according to a study carried out with the help of volunteer ornithologists.
The dramatic decline in numbers - equivalent to one for every person in England and Wales - has been caused by changes in farming methods and the weather, conservation groups say.
Figures collated by organisations including the Royal Society for the Protection of Birds and the British Trust for Ornithology found that the number of nesting birds has slumped from 210 million in 1966 to 166 million.
In their report, The State of the UK's Birds, they warn that while numbers for some species, such as the house sparrow, show signs of recovery, further declines are inevitable as birds cope with changes to the environment and conservationists battle for investment in wildlife-friendly farming to be maintained. There is particular concern that spending on wildlife protection in Britain will be disproportionately slashed as a result of cuts in the EU budget this week, with the monitoring of the health of the bird population dependent on continued support from volunteer birdwatchers.
"The State of the UK's Birds report is a great example of 'citizen science' in action," said Dr Tim Hill, Natural England's Chief Scientist. "Most of the information upon which the report is based is derived from the efforts of the nation's network of skilled volunteer ornithologists who contribute to national monitoring schemes."
Dr Andy Musgrove, of the BTO, said: "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."
The 21 per cent drop in Britain's birds is equivalent to the loss of a single breeding pair every minute for the past 46 years, with species including the house sparrow, turtle doves and the wren suffering steep declines.
The differing fates of two dove species encapsulated the shifting fortunes of the UK's birds. There are estimated to be only 14,000 breeding pairs of turtle doves, a significant drop from their 140,000 peak in 1966. Despite coming to Britain only in 1955, there are now one million collared doves.
"Our bird population has been on a rollercoaster ride and it will continue," said Grahame Madge, of the RSPB.
The damage to habitats from change of land use and management of the countryside is thought to be behind much of the decline. Cold winters have also had a dramatic effect.
The bulk of UK spending on conservation linked to farmland, about £243m a year, comes via the Common Agricultural Policy. But up to 20 per cent could be cut from the EU's rural development budget, which conservationists say would be disastrous.
Flying high? population change, 1970-2010
Corn bunting: Known as the fat bird of the barley, a change in cropping patterns and practices have severely affected its numbers. Down 90 per cent to 11, 000 pairs.
Lesser spotted woodpecker: About the size of a sparrow, it requires extensive high quality woodlands and they are now few and far between. Down 70 per cent to 1,500 pairs.
House sparrow: The iconic and enigmatic sparrow continues to be overall decline. A shortage of food seems to be part of the problem. Down 64 per cent to 5,300,000 pairs.
Lapwing: Has suffered historically through the loss of suitable habitats, with the drainage and conversion of marginal land into farmed land. Down 56 per cent to 140,000 pairs.
Kestrel: One of our most familiar birds of prey. Its decline may be linked to the loss of marginal habitat and the associated decline in the numbers of small mammals. Down 44 per cent to 46,000 pairs.
Greater spotted woodpecker: This highly adaptable and robust woodpecker is doing well, taking advantage of increasing bird feeding in gardens and, perhaps, increasing amounts of woodland. Up 368 per cent to 140,000 pairs.
Collared dove: First bred in the UK in around 1955. Its spectacular rise across Europe remains something of a mystery, but seems to be related to its ability to adapt and live with humans. Up 333 per cent to 990,000 pairs.
Red kite: Once abundant across much of the UK, this spectacular bird of prey is making its way back thanks to a series of highly successful reintroduction projects. Up 575 per cent to 1,600 pairs (1995-2010 figures)
Blackcap: A small warbler whose adaptable nature has allowed it to buck the trend in recent times compared with close relatives, with an increase in numbers. Most warblers migrate to Africa and many of these are in decline, but the clever blackcap is increasingly staying closer to home in Europe. Up 222 per cent to 1,200,000 pairs.
Nuthatch: Like the greater spotted woodpecker, the nuthatch may be benefiting from an increasing amount of woodland in the environment and perhaps from climate change. Up 232 per cent to 220,000 pairs.