By Mark Kinver
Science and environment reporter, BBC News
The number of North Sea puffins had fallen dramatically between 2003 and 2008
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."
Tagging puffins is tricky - Richard Bevan shows the BBC's Rebecca Morelle that you have to dodge tern attacks and puffin pecks to fit the devices
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.
PUFFINS IN DETAIL
Lifespan: more than 30 years
Females lay just one egg a year
Puffins live in metre-long burrows
Both parents incubate the egg
Chicks are mainly fed sandeels
They reach sexual maturity in one year, but tend not to breed until year five or six
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