They say the journey made by the red-necked phalarope has never before been recorded for any other European breeding bird and they have described it as one of the world's great bird migrations.
The discovery has been made after a tracking device, which weighs less than a paperclip, was fitted to 10 red-necked phalaropes nesting on the island of Fetlar, in Shetland.
The geolocators were attached to the birds in 2012 by the RSPB, working with the Swiss Ornithological Institute and Dave Okill of the Shetland Ringing Group, in the hope of learning where the birds spend the winter.
After successfully recapturing one of the tagged birds when it returned to Fetlar last spring, scientists discovered it had made a 16,000-mile round trip during its annual migration - flying from Shetland across the Atlantic, south down the eastern seaboard of America, across the Caribbean, and Mexico, ending up off the coast of Peru. After wintering in the Pacific, it returned to Fetlar, following a similar route.
Ornithologists had assumed Scottish breeding phalaropes joined the Scandinavian population at their wintering grounds, thought to be in the Arabian Sea.
The bird left Shetland on August 1 and arrived in its Pacific wintering area in mid-October - a journey of 10 weeks, although the RSPB said there was some hint in the tracking data that the bird may have been delayed by bad weather a couple of times as it moved south down the east coast of the US.
Malcie Smith, of the RSPB, said: "To think this bird, which is smaller than a starling, can undertake such an arduous journey and return safely to Shetland is truly extraordinary.
"This tiny tracker has provided a valuable piece of the puzzle when building a picture of where phalaropes go when they leave our shores.
"We hadn't realised that some Scottish birds were travelling thousands of miles to join other wintering populations in the Pacific Ocean."
The red-necked phalarope is one of the UK's rarest breeding birds. It is now only found in Shetland and the Western Isles, and numbers fluctuate between just 15 and 50 nesting males.
Scotland marks the southern limit of its breeding range, which stretches across the Arctic regions of North America, Europe and Asia, with the species occupying the wetlands around the northern hemisphere.
It is famed for reversing traditional gender roles. In summer, male birds can be found incubating eggs and raising young, whilst the female uses her brightly coloured plumage to attract new partners. In winter, phalaropes congregate in large flocks at sea in regions where there are currents of cold, nutrient-rich water and blooms of plankton on which the birds feed.
Work will continue to retrieve tags from phalaropes after the next winter migration but Mr Smith said the discovery already had further implications for those working to help protect the species in future. He added: "If the usual wintering area of Scottish red-necked phalaropes is indeed in the eastern Pacific, then this Scottish breeding bird may be directly affected by periodic El Nino events when these Pacific waters become warmer and the supply of plankton is greatly reduced."