We hear the hunter before we see him. Being dead quiet is not how this bird catcher operates.

As we wade through the salt marsh, the plaintive bubbling cry of what sounds like a whimbrel wavers in the wind, gusting from the estuary of the Yangtze river.

Then we spot Jin Weiguo in camouflage, sitting low in the tall grass with a large yellow parasol for shelter from the elements. He is blowing a stubby bamboo whistle, the source of the whimbrel's song. 

Using this unremarkable-looking whistle, Jin can mimic the calls of dozens of shorebird species.

It is a skill he learned from his father almost 50 years ago. For decades, the Jin family hunted shorebirds or waders to sell for food at local markets.

Today, Jin is trapping birds in the middle of Chongming Dongtan National Nature Reserve, a protected area of coastal wetlands. But he is not trying to kill them. Instead, he is deploying his traditional bird hunting skills in the cause of conservation science.

For three hours, Jin has been mimicking whimbrels to lure them to a shallow pool, around which he has placed bird decoys and a net trap.

This protected area is one of the few safe havens in China for shorebirds migrating along the East Asian Australasian Flyway

Jin sits about 100ft (30m) away and whistles. If he tricks a passing whimbrel to drop down to socialise with the dummy waders, he will tug on a rope leading into the reeds. The net will then drop over the duped bird.

So far this morning, he has not had much success, but that is to be expected. "Whimbrels are quite smart," says Wu Wei, one of the reserve's scientists.

Chongming Dongtan's research unit has employed Jin for ten years, to help with its research on migratory birds. They offered him the job after bird hunting on his old patch was banned by the authorities, and the area converted into a national reserve.

The Reserve's extensive tidal mudflats and salt marshes cover 93 square miles (240 sq km) of the seaward tip of Chongming island, a great tongue of alluvial sediment in the mouth of the Yangtze. Downtown Shanghai is about 37 miles (60km) to the south.

The Yellow Sea has been blessed with great expanses of mud

This protected area is one of the few safe havens in China for shorebirds migrating along the East Asian Australasian Flyway: one of the planet's eight great bird migration routes.

50 million birds, from cranes to songbirds, journey along the Flyway twice every year. It stretches from breeding grounds in the far north to wintering grounds in Southeast Asia, South Asia and Australasia. 8 million of them are shorebirds or waders. 

The midway hub of the Flyway is the Yellow Sea, which lies between northern China and the Korean Peninsula. Millions of shorebirds come here for the rich invertebrate life in the vast tidal mudflats around the coasts.

In the spring, they spend about one month, and in the autumn three months, gorging on marine worms and molluscs, before migrating onwards. Many of these shorebirds will fly non-stop for as many as seven days to their next destination.

The fuel stations of the East Asian Australasian Flyway have been disappearing at an accelerating rate

The Yellow Sea has been blessed with great expanses of mud, thanks to the hundreds of millions of tonnes of sediment delivered to it each year by the Yellow and Yangtze Rivers.

Like other areas of Yellow Sea intertidal mudflats, Wu says that Chongming Dongtan is "like a fuel station on the highway". Its salt marshes are crawling with small crabs and other prey, on which birds like whimbrel can fatten up before their onward flights.

But the shorebirds that find their way to Chongming Dongtan are the lucky ones. Elsewhere along the Yellow Sea coast, the fuel stations of the East Asian Australasian Flyway have been disappearing at an accelerating rate.

From Shanghai to the North Korean border, vast stretches have been enclosed by sea walls and drained. The new land can then be used for farming, aquacultures, industrial and port developments, and golf courses.

Of the total area of mudflat that existed 50 years ago, less than one-third remains. For the birds, the cost has been severe.

The Flyway's largest shorebird, the Far Eastern curlew, has declined in number by more than 80%.

The populations of three other threatened waders – bar-tailed godwits, great knots and red knots – have almost halved in the last five years, according to research published in February 2016.

The diminutive spoon-billed sandpiper, the world's smallest and most endangered shorebird, is poised for global extinction. There are only an estimated 200 pairs left in the world.

The few protected reserves like Chongming Dongtan on the Yellow Sea are vital for the Flyway's survival.

By mid-morning, Jin's trickery is bringing in the birds. Two whimbrels fall for it. Opportunistically, he changes tune when some Terek sandpipers appear over the marsh. His imitation of their piping calls fools two of these smaller waders into his trap.

Now I realise they can come from far as away as Australia or New Zealand

The trapped birds are put into baskets and taken to a wooden hut. Here we learn that a second traditional trapper employed by the reserve has also caught two whimbrel plus a grey plover.

Wu and his colleagues attach metal identification rings to each bird's legs. If they are re-captured in migration surveys elsewhere along the Flyway, they will be identifiable as birds from Chongming Dongtan.

One leg is also fitted with a coloured plastic flag with a unique combination of engraved letters and digits. These can be read by Flyway surveyors looking at birds through telescopes.

The birds are also weighed and have other vital statistics recorded. Examination of their flight feathers reveals each bird's minimum age. A couple of body feathers are also taken for DNA analysis.

Before Jin started trapping for the scientists, he had no idea that the birds came to Chongming Island from such distant places.

In the old days there were as many as 40 hunters whistling for waders on the marshes of Chongming Island

"Now I realise they can come from far as away as Australia or New Zealand," he says. "With our work, we know where they go to breed and how old they are. (We are learning) many things."

Jin says he has lost count of the number of birds he has trapped as a reserve employee, but it must be many thousands. "There was one day when I caught more than 200 birds," he says. "They didn't have enough people to band and flag them."

Wu estimates that about 50,000 migratory birds have been trapped and ID-tagged at Chongming Dongtan Nature Reserve since 2002. Normally, birds are caught for this purpose in mist nets, which conservation scientists deploy at night. But in this reserve, Wu says that most of their birds have been tagged, thanks to the traditional Chongming trapping method.

However, Jin's technique is beginning to die out.

According to Jin, in the old days there were as many as 40 hunters whistling for waders on the marshes of Chongming Island. Then the authorities started to actively protect the birds and make greater efforts to prevent hunting in the area.

There was one day when I caught more than 200 birds

That was good news for the birds. These days, hunting for food or sport is no longer a major problem for migratory shorebirds in this part of China – although it remains a serious threat to species on the Flyway in southern China, parts of Southeast Asia and also north-east Russia.

"Young people stopped learning how to do it," says Jin, who is 56 years old. "I am one of the young ones who can catch birds now."

Andrew Luck-Baker is a producer in the BBC Radio Science Unit.

Life on the East Asian Flyway is a four-part series on the BBC World Service.

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