Media playback is unsupported on your device Media caption The tag was recovered from a male red-necked phalarope in Shetland

A tracking device which weighs less than a paperclip has helped scientists uncover what they say is one of the world's great bird migrations.

It was attached to a red-necked phalarope from Scotland that migrated thousands of miles west across the Atlantic to the Pacific Ocean.

The journey has never before been recorded for a European breeding bird.

The red-necked phalarope is one of the UK's rarest birds, and is only found in Shetland and the Western Isles.

We are freezing up here in Shetland and it's quite nice to think of our red necked phalaropes bobbing about in the warm tropical waters of the Pacific Malcie Smith, RSPB

The RSPB, working alongside the Swiss Ornithological Institute and Dave Okill of the Shetland Ringing Group, fitted individual geolocators to 10 red-necked phalaropes nesting on the island of Fetlar in Shetland in 2012.

Each geolocator weighed 0.6g and was fitted to the bird with harnesses made from tubing.

It was hoped the trackers would shed light on where the birds, which are smaller than starlings, spend the winter.

After successfully recapturing one of the tagged birds when it returned to Fetlar last spring, experts discovered it had made an epic 16,000-mile round trip during its annual migration.

It had flown from Shetland across the Atlantic via Iceland and Greenland, south down the eastern seaboard of the US, across the Caribbean and Mexico, ending up off the coast of Ecuador and Peru.

After wintering in the Pacific, it returned to Fetlar, following a similar route.

Gender roles

Prior to this, many experts had assumed that Scottish breeding phalaropes joined the Scandinavian population at their wintering grounds, thought to be in the Arabian Sea.

Although long, the phalarope migration is beaten by some distance by Arctic terns, which make a return trip of about 24,000 miles between the North and South poles each year.

Image copyright Andreas Trepte/www.photo-natur.de Image caption It had previously been thought the birds wintered in the Arabian Sea

However, the phalarope is the only known westward migration into the Pacific. This westward movement in late summer and autumn is into the prevailing weather and in virtually the opposite direction to all other migrants leaving the UK.

Numbers of red-necked phalarope in Scotland fluctuate between just 15 and 50 nesting males.

Malcie Smith of the RSPB told BBC Scotland he had almost fallen out of his chair when the tracking results showed where the birds had gone.

He added: "We are freezing up here in Shetland and it's quite nice to think of our red necked phalaropes bobbing about in the warm tropical waters of the Pacific.

"What it tells us is that this bird isn't part of an offshoot population from Scandinavia. Our Shetland population is actually an offshoot of a North American population.

"It means that what we thought was a kind of medium-distance migrant is actually a long-distance migrant. It is one of the world's great migrations.

"We've known for some time that birds undergo big migrations. We all know about Arctic terns and swallows and this is pretty much in the same ballpark."

Scotland marks the southern limit of the breeding range of the small, colourful waders, with the species far more abundant further north.

The birds are perhaps best known for turning the tables on traditional gender roles, with male phalaropes incubating eggs and raising young in the summer, while the female uses her brightly coloured plumage to attract new partners.

In winter, phalaropes congregate in large flocks at sea in regions where currents create cold, nutrient-rich water and support blooms of plankton on which the birds feed.

By continuing the project and retrieving more tags from phalaropes after their winter migration next year, experts hope to learn the extent to which the Scottish population may be impacted by future changes at sea.

They also hope to learn how the species might respond to any change, and whether any negative impacts in these wintering areas can be mitigated by conservation management in Scotland.