By Mark Kinver
Science and environment reporter, BBC News
The annual 40,000-mile trip by a species of sea bird around the Pacific Ocean is the longest migration recorded by electronic tracking, scientists say.
The sooty shearwaters' journey took them from breeding colonies in New Zealand to winter feeding sites in Japan, Alaska or California.
Their migration path covered the whole of the Pacific region, taking about 200 days to complete, researchers found.
The study appears in the Proceedings of the National Academy of Sciences.
Between January and March 2005, 33 birds at two breeding colonies in New Zealand were fitted with tags weighing 6g, allowing researchers to track their journey.
Species name: Puffinus griseus
Length: 40-51 cm
Wingspan: 94-109 cm
Weight: 400-800 g
Diet: fish, squid, krill
Global population: 20 million
Red list status: Near Threatened
(Sources: UCSC, RSPB, IUCN)
In the autumn of that year, 20 of the tags were recovered when the birds returned to their burrows at the breeding grounds; 19 of the devices had successfully recorded the birds' movements.
Data showed that some birds travelled up to 910km in a day, and dived to depths of 68m in their search for food.
Scott Shaffer, a research biologist at the University of California, Santa Cruz (UCSC), and the paper's lead author, said the data presented some surprising findings.
"The fact that the birds went to specific places in the northern Pacific and stayed there for the remainder of the migration, and then came back to New Zealand did surprise us.
"Previous hypotheses had suggested the birds did a sweep of the North Pacific before heading back south," Dr Shaffer told BBC News.
The team found that the birds made a prolonged stopover at just one location in either Japan, Alaska or California.
The data also confirmed the birds' migration path covered the whole of the Pacific region in a massive figure-of-eight pattern.
The researchers said this was likely to be a result of the birds using the global wind system and being influenced by the Coriolis Effect, the apparent deflection of objects as they move over the rotating Earth.
Indicators of change
Although the sooty shearwater global population is an estimated 20 million, Dr Shaffer says any marked decline in their number could prove to be a useful indicator of impacts of climate change or overfishing.
The tags have given a new insight to the birds' migration pattern
"If you are travelling all that way, and if you get there and there is no food it is going to be very tough to recover and get back."
Previous studies had shown that the population of sooty shearwaters off the coast of California had declined dramatically.
This was attributed to warming oceans which in turn had led to a fall in the amount of food for the birds.
These declines were also seen at the breeding colonies, Dr Shaffer added.
"A bird that travels all that way is doing so with the idea that it knows that when it gets to those sites in the north there is going to be food.
"If the climate is changing and food resources are falling, we could see the birds changing locations or see fewer and fewer birds returning because of the changing oceanic conditions," he suggested.
Future studies will hope to build up a better picture of the sea birds' migratory path, including whether individual birds are returning to the same winter location or choose a different site every year.
The research is part of a project called Tagging of Pacific Pelagics (TOPP) that is applying tags to 23 species of top predator in the Northern Pacific Ocean.