Puffins from the North Sea's largest breeding colony venture much further afield during the winter than previously thought, a study has shown.
More than 75% of the seabirds fitted with "geolocator" tags headed for the open waters of the Atlantic Ocean, rather than staying in the North Sea.
Until now, very little was known about where puffins went during the winter as the birds spent the entire time at sea.
The findings by British researchers appear in the journal Marine Biology.
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."
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.
"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.
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.
"One of the big gaps in seabird biology is what do they do during the winter," Professor Harris explained.
"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.
"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 once these (geolocator) tags became available and were working well, the puffin was an obvious choice to use them on."
During the 2007 breeding season, the team fitted 50 birds on the Isle of May, a National Nature Reserve off the east coast of Scotland, with geolocators.
The loggers work by measuring light levels, recording when dawn and dusk occurs each day.
With this data, researchers can calculate day length, when midday occurs, and the daily longitudinal and latitudinal co-ordinates for the individual bird.
The researchers retrieved 14 devices during the following spring, and were able to download data from 13 of the tags.
Professor Harris, who has been studying puffins for 37 years, said that it was too early to suggest why the puffins were making the extended journey to the Atlantic Ocean.
"At the moment, we are trying to get more information on what they eat," he told BBC News.
"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.
"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 [food availability].
Adverse 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 2003 and 2008.
Puffin numbers on the Isle of May had been increasing for half a century, with the population reaching about 69,300 pairs in 2003.
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.
A 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.
A survey in 2008 recorded just 36,500 puffins, down from a record high of 55,674 in 2003.
Female puffins only lay one egg a year, so a high mortality rate among adults across a few years can quickly destabilise the population.
Professor Harris said that the population crash was unexpected.
"Puffins normally survive very well, and suddenly we had two years when they did not," he said.
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.
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.
"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.
This behaviour meant that the tag dried out, recording an "in flight" reading when the bird in fact was still on the water.
"So we succeeded in one of our objectives, which was to find out where the puffins were going, but we failed on the other, which was to find out when the birds were flightless."
The team plan to continue fitting geolocators on puffins over the coming years, enabling them to build a better picture of the behaviour and movements of the birds during the winter months.