Pollution, disease and the dinner plate: the risks faced by long-distance ducks
They are nature's long-haul travellers, flying thousands of kilometres a year as they seek warmer climes for winter, and then making the flight back for the northern summer.
Their trips are fraught with danger - the weather, disease, predators and pollution.
Sometimes they can end up as someone's dinner.
The latter is believed to have happened to two wild ducks that wintered in Hong Kong in 2008. The ducks were among 23 captured at the Mai Po Nature Reserve in December that year for a tracking study.
The Eurasian wigeons and northern pintails, two common species, were weighed, tagged, inspected, photographed and fitted with tiny radio transmitters before being released back into the reserve. All were free of avian diseases.
In the months that followed, trackers monitored the birds via satellite as they journeyed across the world.
The study was funded by the United Nations' Food and Agriculture Organisation, with help from the US Geological Survey, the University of Hong Kong's department of microbiology, WWF Hong Kong and Asia Ecological Consultants, the city's largest independent ecological consultancy.
It was launched to better understand the migration patterns of wild birds, in particular implications on the spread of avian diseases such as the deadly H5N1 bird flu. Similar surveys have been carried out in other parts of the world.
One of the ducks, a female northern pintail less than a year old and known as 44697, left Hong Kong in the week of April 8 and reached central Siberia by early June. She was then tracked on June 10.
'The last signal transmitted came from the back yard of a residence in the town of Tympy in Russia. We suspect she was shot and eaten,' said Katherine Leung Kar-sin, a Mai Po reserve officer responsible for the logistics of the tracking.
Tracking records show the duck stopped in eastern Russia on June 3, after a trip of more than 3,000km from Hong Kong. It remained near a lake in Tympy until June 5. Duck shooting is legal in Russia.
Another adult northern pintail, number 44815, and named Oeing, was believed to have been shot on November 29 on the Russian island of Iturup, in the Kuril chain.
Bena Smith, manager of the reserve, said the study offered valuable information on the timing, duration and routes taken by thousands of ducks that visit Mai Po for the winter every year.
'They spend four to five months here but we don't know where they go afterwards.
'We have limited knowledge about them and there are huge information gaps. But we now have fantastic first-hand information about the birds,' he said.
Smith said tracking data could help fill in the blanks on where - outside Hong Kong - the birds stopped, where they bred and spent the summer, and the challenges, such as habitat loss and pollution, they faced.
The ducks, usually moving in flocks, begin to leave Hong Kong in mid-February, with the last leaving in late April. The distances and speeds at which the ducks travelled were amazing. One pintail covered 6,500km to the Sea of Okhotsk near Alaska. A male wigeon made a non-stop 2,000km flight for 59 hours to North Korea - with an average speed of 34km/h.
While tracking information showed the ducks headed to a wide range of places, from Inner Mongolia to Siberia, more than 90 per cent used the Yellow Sea as a staging area for food before moving on. The Chongming Dongtan and Jiuduansha nature reserves in Shanghai appeared to be the first stop after they left Hong Kong, while inter-tidal zones in coastal areas in China and the Korean peninsula were popular breeding grounds. These are zones where between high tide and low tide an abundance of marine life thrives.
Smith said the information gathered could shed light on where the resources should be put to protect the species and how habitat loss and pollution threaten migratory water birds. For instance, a huge reclamation project in Saemangeum in South Korea, where a 33km seawall dammed two river estuaries, could have a disastrous impact on migratory birds, he said.
Smith said another 23 ducks would be tracked this year. The tracking device will include GPS locators, costing HK$32,000 each, which would offer more detailed travel data.
Hong Kong lies at the heart of what is known as the East Asia-Australasian Flyway, used by more than 50 million migratory water birds from 250 different species.
In 2008-09, up to 87,600 water birds were recorded during the winter period in the Deep Bay area including Mai Po. It was the second highest count since 1992-93.