Since the chickens couldn't have made it across the open Pacific on their own, they must have come along for the ride with Polynesian sailors, the researchers say.
The findings will appear tomorrow in the journal Proceedings of the National Academies of Science.
Polynesians are generally believed to have been the world's best sailors at the time when they colonized the Pacific islands, Storey said.
Their ancestors began these long journeys more than 3,000 years ago. The last of the Pacific islands to be colonized were the Chatham Islands, east of New Zealand, in about A.D. 1300.
But no one had found any concrete evidence of South American settlement by Polynesians. Many scholars thought that chickens first reached the New World on European ships.
Then archaeologist and study co-author Daniel Quiroz, of the Dirección de Bibliotecas, Archivos y Museos in Santiago, dug up 50 chicken bones from at least five individual birds at the Chilean site called El Arenal.
(See a map of Chile.)
The researchers used carbon dating on the same bone used for DNA testing. The analysis suggested the bone was buried between A.D. 1320 and 1410.
These dates, which fit well with those of other artifacts found at the same site, were determined with a dating technique called thermoluminescence.
The dates of the Chilean chicken bones also roughly fit with when Polynesians would be expected to have reached the Americas, since they probably traveled eastward from Easter Island, which was first settled as late as A.D. 1200.
The new chicken data are the first confirmation that Polynesians made it to the continent, said study co-author Atholl Anderson of the Australian National University in Canberra.
Other evidence has suggested Polynesians made it to Chile, where the Mapuche people live today.
"There are many words in Mapuche language which are close to or identical with Polynesian, [and] the Mapuche war club is very similar to a distinctive Polynesian form," Anderson said.
But the evidence was "circumstantial," he said.
Other studies have suggested that Polynesians got crops from the Americas. The strongest case is the sweet potato, which originated in the Americas.
Polynesians were growing this crop on the Cook Islands as early as a thousand years ago.
"The most plausible explanation for this transfer was seafaring Polynesians making a round-trip voyage," said Patrick Kirch of the University of California, Berkeley, who led the sweet potato discoveries but was not involved with the Polynesian study.
"However, it was not possible to completely rule out the ... theory of 'drifting' South Americans on a raft loaded with sweet potatoes," he added.
This might not be as improbable as it sounds.
In 2005 a crew of a tiny Mexican fishing boat ran out of fuel and drifted for nine months, winding up near the Marshall Islands, some 8,000 miles (5,500 kilometers) away. Three of the five on board survived the trip.
But with the new findings of Polynesian chickens, Kirch said, "what we now have is hard, empirical evidence that these contacts actually took place."
The next step is to try to find the earliest signs of Polynesians' animals in the Americas. For that, lead author Storey figures the best bet is looking for remains of rats.
(See related: "Rat DNA Offers Clues to Pacific Colonization, Study Says" [June 9, 2004].)
"None of the dominant species, such as pigs and dogs, made it to every Pacific Island," Storey said. "[But] the rats did."
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