Climate change is affecting both bird numbers in the UK and where they live, according to a new report.
The farmland bird the corn bunting is in rapid decline
Experts say new figures - like those which show wading birds are moving eastwards - reveal the true impact of changes such as milder winters.
Ornithologists predict warmer weather could threaten some species, while also attracting new ones to Britain.
The study is by a range of groups including the RSPB, English Nature and the Wildfowl and Wetlands Trust.
The State of UK Birds 2004 study found there has been a 6% rise in the combined populations of 111 widespread bird species since 1970.
But for every 10 pairs of birds living in farmland in 1970, less than six remain today.
The report highlighted a rapid decline in numbers of farm birds, like the corn bunting, whereas a number of "generalist" species, such as the wood pigeon, were increasing.
The survey said 2004 was the worst seabird breeding season on record.
This is blamed on a shortage of sand eels, which the birds feed on, and which the report said was more likely to be related to climate change than other factors such as industrial fishing.
The population of wintering ducks, geese, swans and wading birds has dropped to its lowest level for 10 years, while the distribution of seven of the nine common species of wading birds has shifted from the warm west of Britain to the colder east since the mid-1980s.
But milder winters were also cited as a possible cause for an increase in some farmland birds, like the song thrush and tree sparrow.
The study also highlighted a general trend for birds to nest earlier and for migrating species to arrive earlier.
RSPB conservation director Mark Avery said: "The impacts of climate change are already evident on the UK's bird populations and the RSPB remains concerned that within our lifetimes these changes are likely to become ever more marked.
Woodland birds, like the lesser spotted woodpecker and willow tit, are continuing to decline, especially in southern England
The breeding ranges of the Dartford warbler, woodlark and little ringed plover have all spread northwards
The has been a reduction in the number of birds living in high altitudes, such as the snow bunting, and high UK latitudes, such as the greenshank
"Migratory birds are no respecters of international boundaries and their future is linked to concerted global actions to tackle climate change."
The report predicted the arrival of new species to the UK if warmer summer temperatures become established.
It said potential colonists could include the black kite, the cattle egret, the great reed warbler and the zitting cisticola.
Richard Hearn of the Wildfowl and Wetlands Trust said: "The latest trends for wintering wildfowl show worrying declines over the past 10 years for a number of species that previously have not been of concern."
He added: "Redistribution of these birds in response to changes in winter climate may be masking our ability to determine whether declines in the UK are representative of the whole population."