England's biggest colony of puffins has seen the birds' numbers fall by a third in just five years, a survey shows. Experts had expected to see a slight increase in the population on the Farne Islands, owned by the National Trust.
The Trust says the size of the decline is unprecedented, adding that it will carry out another survey in 2008 in order to monitor the situation.
One theory is that many of the birds are dying from starvation during the eight months they spend at sea.
"We were expecting a slight increase because since the last survey in 2003, we had a number of good years for puffins," explained David Steel, the Trust's head warden on the islands.
"There were plenty of chicks fledging from the nests, so we were not only getting enough to maintain the population, but increase it.
"But something is going badly wrong somewhere."
The three-month survey, carried out on eight of the islands, recorded 36,500 breeding pairs. The previous survey, which was conducted in 2003, counted 55,674 pairs.
Mr Steel told BBC News that there was no visible explanation for the decline on the islands.
"There was no evidence that they were having difficulties.
"The birds had been bringing in good quantities of food, and there was not a predator problem on the islands."
One suggestion for the downturn is the survival rate over the winter months, when the puffins spend eight months at sea.
"The dramatic decline on the Farnes leaves no doubt that the North Sea has lost a substantial number of its puffins," explained Professor Mike Harris, emeritus fellow at the Centre for Ecology and Hydrology.
"With poor survival of adult birds a likely factor in the decline, we urgently need to know more about puffins during the eight months of the year that they spend in the open sea."
The results reflect the findings of an earlier survey on the Isle of May, in the Firth of Forth - the UK's largest puffin colony. Published in June, it also showed a decline of about 30%.
David Steel said this year's results had left his team feeling anxious about the puffin population on the Farne Islands.
"This colony is the fourth largest in Britain, so to see it experience such a decline is a worry."
In order to monitor the situation, Mr Steel said the Trust would carry out another survey next year.
"We are going to target the islands that saw the biggest decreases," he explained.
Puffins nest in burrows, so their numbers are assessed by counting the number of occupied burrows, after the birds have cleaned out their nests and before vegetation begins to grow over.
Mr Steel explained: "We can then compare them with this year's results because we actually have 20 square metres on each island mapped, so we can go back to a particular spot and see what the impact is next year.
"Hopefully, we can now judge it on an annual basis and keep a much closer eye on things."