The number of puffins at the UK's largest single breeding colony has plummeted by almost a third in the past five years, according to new data.Nesting pairs on the Isle of May, in the Firth of Forth, in Scotland, have declined from 69,300 in 2003 to 41,000, despite a steady rise in the population over the past 40 years.Ornithologists had expected the number of breeding pairs to reach 100,000 this year.The exact cause of the downturn is not yet known, but researchers believe climate change could be to blame.It is thought warmer sea temperatures and intensive fishing could be affecting the development of plankton which, in turn, could be having an impact on the amount of fish available for the puffins to eat.Conservationists said the evidence that "supremely adapted" birds such as puffins were struggling was very troubling.The survey was led by Professor Mike Harris, from the Centre for Ecology and Hydrology, who has been monitoring and studying the puffin population on May for 36 years.He 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."