By Mark Kinver
Science and environment reporter, BBC News
The data offered an insight in to unknown behaviour of Manx shearwaters
Electronic tags have offered an insight into the mysteries of the 20,000km migration of Manx shearwaters.
A team of UK scientists found that the birds made regular "stopovers" lasting up to two weeks, probably to feed and replenish their energy reserves.
The data was recovered from logging tags fitted to six breeding pairs of Puffinus puffinus from Skomer Island, off the coast of Wales.
The findings appear in the journal Proceedings of the Royal Society B.
"Every one of the 12 birds made at least one stop during its migration in one place for up to two weeks," observed co-author Tim Guilford of the Animal Behaviour Research Group at the University of Oxford.
"We have interpreted this as being stopover behaviour because this is common in terrestrial migrant birds; essentially, they stop to refuel," he told BBC News.
But, he added, sea birds that migrated over open seas did not normally display this behaviour because, unlike terrestrial species, they were not able to return to the same feeding spot each year.
However, in the case of the tagged Manx shearwaters, a small bird weighing about 400g, they appeared to have adopted the same behaviour as it offered the "optimal migration strategy".
Professor Guilford suggested that the birds were more likely to survive if they made a series of regular stops rather than flying directly to South America.
"If they flew directly, they would have to have a larger fat reserve in order to make the journey," he explained.
"They could do that, but on the other hand that would mean the bird would be flying the first part of the migration weighing more than it needed to.
"It is a complex trade-off between the aerodynamics of long distance flight and the risks and time constraints of having to stop and refuel.
Professor Guilford said the data did not throw up too many surprises as far as the birds' migratory route was concerned.
"The route that they took was very broadly consistent with what people using more traditional methods, such as ringing, thought they had taken.
All 12 birds fitted with the tags returned to the UK breeding site
"It is gratifying that these techniques, which have been the mainstay of our understanding of avian migration for so long, turn out to be broadly correct."
However, he added that the data from the tags did reveal a few differences.
"They go a little bit further south than we expected, but that was probably the result that human settlements were much more sparse, where there were fewer people to recover the rings."
The team used two different kinds of logging tags, both of which were designed and made by the British Antarctic Survey.
"They are very similar except that one is a slightly more recent version, which records contact with salt water," Professor Guilford explained.
"This allowed us to know whether the bird was sat on the water or diving, or whether it was flying."
By combining the two data sets (location and flight/stationary), the researchers were able to work out the birds' migration pattern and behaviour en route.
Seven of the tags could record when the bird was in salt water
Rather than using a satellite tag, which beams the data back almost straight away, the team opted for a "logging device" that stored the information until it was physically retrieved from the bird.
"This is the slightly nerve-wracking side of things," he admitted. "The primary benefit is that they are much smaller than devices that transmit."
All of the tagged birds returned to the UK breeding site, which suggested that the tags did not inhibit them during their seven-month migration.
"We have gone to great lengths to try and minimise the impact of our devices," said Professor Guilford.
Although the Manx shearwater, as a species, is not under threat, he added that the findings helped improve our understanding of what was happening in the world's seas and oceans.
"Although they are doing very well, they are still limited to big but compact populations on islands, which are very vulnerable to predators etc," he explained.
"So it is nice to say that we can now begin to understand what these birds depend upon in terms of resources on land, and now at sea.
"We won't be stopping with Manx shearwaters, we are beginning to put them on puffins as well."