Hundreds of puffins have washed up dead on beaches along the UK coast following the severe weather conditions.
It is thought the birds may have died of starvation after being unable to feed following storms in the North Sea.
Hundreds of puffins have already washed up along the UK's east coast, stretching from Aberdeenshire to Northumberland, but experts believe "perhaps thousands" could die in the "Big Freeze".
The death of such large numbers of seabirds in one incident is known as a "wreck".
And experts at the Centre for Ecology and Hydrology said it is the worst "wreck" in more than 60 years.
Professor Mike Harris, from the Centre for Ecology & Hydrology, told the Daily Record: "There is currently a major wreck under way which is the largest in the North Sea for at least 60 years.
"This was first noticed at the end of last week by people who regularly check the beaches of north-east England for dead birds. Then over the weekend reports started to come in of dead and dying puffins in beaches all the way north to Aberdeenshire.
"Most birds were emaciated and had obviously died of starvation.
"To date, I have heard of maybe 400 dead puffins and there will undoubtedly be many more, perhaps thousands, and this compares with just a handful over a whole typical winter. This will certainly be the largest wreck of puffins in the North Sea for over 60 years."
Dr Barnaby Smith, also from the Centre for Ecology & Hydrology, which is part of the Natural Environment Research Council, said the birds might have been unable to find enough food to survive after becoming exhausted using up all their resources to fight against the unseasonably cold temperatures and strong easterly winds.
RSPB Scotland said it had received a number of calls about puffins, as well as razorbills and guillemots, washing up on beaches.
A spokesman told the BBC: "We are fast approaching the start of the seabird breeding season, where tens of thousands of seabirds return to their colonies to raise their young.
"The recent events could have an impact on the success of this year's puffin breeding season, a species already suffering population declines.
"This may be the worst puffin wreck we have seen for almost half a century.
"We are in close contact with experts from the Centre for Ecology & Hydrology to learn more about what is happening but it appears that the prolonged and unprecedented weather is making life extremely difficult for this species."
Tom Brock from the Scottish Seabird Centre, said the discovery was "extremely distressing".
He told the BBC: "We will continue to monitor the current situation closely as it develops, but clearly the significant loss of seabirds just ahead of the crucial breeding season is a major cause for concern - not least given that Scotland is of international importance for its seabirds and is home to over 45% of Europe's seabirds."
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