Climate change is to blame for a drop in the number of some birds that visit Britain each winter, the RSPB says.
Seven species of regular visitors appear to be in decline
The charity said many wildfowl no longer needed to migrate as far as the UK from places like Greenland and Siberia because of warmer winters.
Numbers of seven regular visitors, including the shelduck, mallard and turnstone, are declining, it warned.
But the overall number of waterbirds wintering in the UK has doubled since the late 1970s, a report adds.
The State of the UK's Birds 2006 report, says in particular the number of wading birds including the black-tailed godwit and the avocet, had increased markedly, mainly due to action by conservationists.
But as winters become milder both in the UK and elsewhere, it appeared that some birds were not forced to fly as far as the UK to find suitable conditions, the RSPB said.
Bird experts said the precise reasons for the decline of the seven species varied, but a common theme appeared to be climate change.
Dr Mark Avery, the RSPB's conservation director, said: "The UK has had both the perfect climate and perfect habitats for these birds, but the evidence is growing that climate change impacts are starting to bite.
"Sea level rise and warmer winters are reducing their numbers, undermining our importance for birds."
'Urgent action needed'
Conservationists say that every winter the UK is home to about five million ducks, geese, swans and wading birds.
They come from the icy lands of northern Europe, Greenland, Siberian and Arctic Canada and are attracted by the relatively milder winters and availability of food.
Dr Baz Hughes, head of species conservation at the Wildfowl and Wetlands Trust, said: "Every winter, the UK's estuaries, marshes and wetlands throng with the arrival of ducks, geese, swans and wading birds, making the UK one of the most important countries in the world for some of these birds.
"However, we are becoming increasingly concerned about the declining numbers of some populations.
"While some are simply taking advantage of milder winters by staying on the continent, others like the Greenland white-fronted goose, are in real and rapid decline.
"Conservation action is needed urgently to reverse these declines."
Food shortage fears
Natural England's chief scientist Tom Tew said: "Sympathetic and effective management of the UK's world-renowned network of internationally important waterbird sites and the surrounding countryside is vital if we are to minimise the adverse impacts of climate change on our overwintering birds.
"Agri-environment schemes have a crucial role to play by encouraging land management practices that benefit birds and other wildlife."
The report said climate change's affect on migration patterns had already been "particularly noted" in Northern Ireland with declines of pochard and Bewick's swan.
It is feared the areas used for winter nesting and feeding instead do not have the same food availability and may affect bird populations in the long term.