A dramatic drop in the population of puffins at their main North Sea breeding site has alarmed scientists.
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After almost 40 years of breeding success puffin numbers on the Isle of May have plummeted by 30 per cent.
It is not known whether the sudden decline is merely a blip or whether the tiny iconic bird has joined the list of sea birds in long term decline in the North Sea.
The sudden drop in numbers was revealed in a survey carried out every five years on the island off Scotland's east coast by scientists from the Centre for Ecology & Hydrology.
The Isle of May is home to the largest colony of puffins in the North Sea and has been the centre of the UK science community's research into the bird for over three decades.
Numbers have increased dramatically from a handful of pairs 50 years ago to more than 69,000 pairs at the time of the last count in 2003.
Scientists had expected numbers to soar to more than 100,000 pairs this year and are baffled by the loss of almost one in three birds.
Professor Mike Harris, Emeritus Research Fellow at the Centre for Ecology & Hydrology, who has studied puffins for 36 year, said: "Something worrying appears to have happened over last winter and probably the one before.
"Puffins appear to be joining the ranks of other seabirds in the North Sea that are suffering reduced breeding success and decline in numbers."
The puffin (Fratercula arctica) is instantly recognisable by its bright red and black eye markings and vivid orange legs and is known as the clown of seabirds. But its comical looks and endearing traits has made it one of the world's favourite birds.
Adults arrive back at their breeding colonies in the Shetland and Orkney Islands, Northumberland, Anglesey and the Isle of May in March and April and they leave again in mid-August. They nest in burrows on the cliff tops and rely mainly on sand eels to feed their young.
The disappearance of the sand eel due mainly to industrial fishing by factory ships in the North Sea is believed to be one of the main factors in the puffin's decline on May.
Bird numbers are assessed by carefully examining burrows for signs of occupation in late April after the birds have cleaned them out ready for breeding. In past surveys the occupancy rate was nearly 100 per cent, but this year it was down to only 70 per cent.
Scientists also noticed fewer birds than usual had returned to the island and those that did were underweight compared to previous years suggesting they may have had a difficult winter.
Unusually high numbers of puffins, including some ringed on the island in previous years, were washed ashore dead during the last two winters.
Professor Harris said: "We need to repeat the survey next year to check the unlikely possibility that a large numbers of puffins took a summer off from visiting the Island. We also need to widen the survey to include other colonies in the North Sea to measure to what extent the puffin population is declining in the area."