An unexpected decline in puffin numbers on one of their most successful breeding colonies has raised fears that the birds are the victims of a North Sea fish famine.

To the puzzlement of ornithologists, thousands of puffins have vanished from the Isle of May, in the Firth of Forth, in Scotland.

The population on May had been rising steadily for the past 50 years and ornithologists had expected that the island would host 100,000 pairs this year. But when they carried out five-yearly count in April they discovered that the population had dropped from 69,300 pairs in 2003 to 41,000 this year.

Mike Harris, of the Centre for Ecology and Hydrology (CEH), said that the most likely explanation for the drop was that the adult birds were starving to death during the winter.

After the breeding season ends puffins disperse and spend winters at sea, but for the past two winters unusually large numbers of emaciated puffin bodies have washed up on the Orkney and Shetland islands.

Professor Harris said that many of the birds that returned to May this year appeared to be underweight. “Our guess is there’s something happening at sea during the winter. For puffins to suffer it would have to be something over a big scale, something over a North Sea scale.

“There are two possibilities. One is food and the other is weather, and there’s no suggestion the weather has changed that dramatically.

“There are big changes in the North Sea. Some are natural, some are induced by climate change or man. There’s been some change in the marine environment, which is having an adverse effect on puffins over a quite wide area. It’s not just a little local phenomenon.”

Puffins tend to live for 25 to 30 years, and until now, the survival rate for adult birds in the North Sea was 95 per cent year on year.

On May, it was calculated that on average 85 per cent of the breeding puffins returned, but this year the proportion has plummeted to 55 per cent. Evidence from Norway, where one of the most successful puffin breeding colonies has suffered a 10 per cent fall in the population, suggests that the slump is more widespread.

Scientists from the CEH have decided to carry out another count on May next year - instead of waiting until 2013, as had been planned - to keep a close eye on the birds.

In the past decade, shifts in the location of sand eels, a source of high protein, have affected puffins during the breeding season. However, this is not a factor for the rest of the year when the birds’ diet is not range-restricted by the need to fly back to their young.

Times Recommends

• Donor organ allowed child’s own heart to heal
• Michael Jackson: Not the final chapter
• Brüno toned down for 15 rating