This behaviour meant that the tag dried out, recording an 'in flight' reading when the bird in fact was still on the water. 'What we didn't realise then but now know is that when puffins sleep they often tuck their feet into their plumage,' Professor Harris revealed. Researchers are not sure how long puffins are left flightless, so the CEH team had hoped that a device on the tag that measured when the birds' feet were in seawater would provide an insight. Over the winter, the birds undergo their main moult in which they lose their wing feathers, making them flightless and vulnerable to adverse conditions, such as storms or poor food supplies. 'Puffins normally survive very well, and suddenly we had two years when they did not,' he said. Professor Harris said that the population crash was unexpected. Geolocators have offered biologists an insight to where puffins go in the winter Yet a survey in 2008 recorded 41,000 pairs, less than half of the 100,000 pairs that would be expected if the previous rate of increase had continued.ДЄ similar decline in puffin numbers has also been recorded on the Farne Islands off the Northumberland coast, England's largest breeding colony of the seabirds.ДЄ survey in 2008 recorded just 36,500 puffins, down from a record high of 55,674 in 2003.ДЇemale puffins only lay one egg a year, so a high mortality rate among adults across a few years can quickly destabilise the population. Puffin numbers on the Isle of May had been increasing for half a century, with the population reaching about 69,300 pairs in 2003. ДЄdverse conditions that limited the birds' access to food was one hypothesis for why there was a dramatic fall in the population of North Sea puffins between 20. 'The problem has been that until we know where they go, we cannot know what they are eating and whether there has been a change in. 'We do not really know what species of fish or crustacea they eat during the winter the suggestion is that they eat less fish and more plankton. 'At the moment, we are trying to get more information on what they eat,' he told BBC News. 'So once these (geolocator) tags became available and were working well, the puffin was an obvious choice to use them on.'

'Up until now, the devices that have been available to fit on birds have been too heavy for puffins, which only weigh about 400g. 'So, it's fantastic when all of these problems that you thought you would never solve, but then a technology appears that allows us to get somewhere at last. 'One of the big gaps in seabird biology is what do they do during the winter,' Professor Harris explained. He said the development of geolocators - small location loggers that weigh 1.5g, which are fitted to birds' legs - allowed the team to track the puffins' movements for the first time.

'What we were completely unprepared for was that they made a one-to-three-month trip into the Atlantic and then came back to the North Sea,' said lead author Mike Harris.

They added it challenged the previous view that puffin populations on the east and west of Britain remained separate from each other, during both the breeding season and during the winter. Writing in the journal, a team of researchers from the Centre for Ecology and Hydrology said: 'The finding that more than three-quarters of the birds made a major excursion into the Atlantic was entirely unexpected.'

Puffin eggs