A wild duck has returned to Hong Kong after an epic 12,000-kilometre (7,500-mile) round-trip to the Arctic which conservation experts say has provided new information about bird migration.

Environmental group WWF said the female northern pintail duck, which was fitted with a transmitter in December last year, returned to Hong Kong's Mai Po Nature Reserve at Christmas.

The bird was the only one of 23 from Hong Kong tagged with a miniature solar-powered transmitter to have returned to the Chinese territory, the group said in a press release.

The tracker shows the bird left Hong Kong on February 25 and reached the Arctic Circle in mid-June.

It stopped in east and northeast China and the Yellow Sea off South Korea before reaching Siberia, where it stayed for three months presumably for breeding before heading south in late September.

Flying at an average speed of 50 kilometres (31 miles) an hour, the duck travelled 1,700 kilometres (1,060 miles) in three days, stopping in Russia and Japan before reaching the southern Chinese province of Guangdong on December 18.

It eventually returned to the Hong Kong wetland nature reserve around Christmas after a round-trip totalling about 12,000 kilometres.

The BBC said WWF used Google Earth to locate the duck's feeding areas and route back to Hong Kong.

Katherine Leung, an expert with WWF Hong Kong, told the South China Morning Post the tagging project provided important information on bird migration.

"During migration, ducks face many threats, like natural predators, hunters and diseases.

"Another worrying trend is development projects, including (land) reclamation, which results possibly in habitat loss for them and other waterbirds," she said.

"Their migration route will help us protect them better in the future."

Only two other transmitters of the 23 fitted to the ducks are still working. Others likely fell off, were not transmitting or the ducks had been hunted.

The BBC said on its website one of the birds was shot dead over Russia and its transmitter was tracked to what was believed to be the hunter's home.

Another duck, a Eurasian wigeon, appeared to be staying in North Korea having spent more than a month there, the WWF said.

The project was carried out by WWF Hong Kong in partnership with the University of Hong Kong's microbiology department, Asia Ecological Consultants and the US Geological Survey to study wild duck migration and the role of migratory birds in avian influenza.

Hong Kong was the site of the world's first major outbreak of bird flu among humans in 1997, when six people died of a mutation of the virus, which is normally confined to poultry.