“THE story of migrating birds is the story of promise — a promise to return.”

Those are the opening lines of the celebrated 2001 French documentary “Le Peuple Migrateur,” or “Winged Migration.”

Birds rule the sky, and since the earliest days of humans, man has been fascinated by creatures that fly and live over long distances. Some of their journeys are epics hard to imagine.

Starting in late summer and early fall, as weather starts to cool in the Northern Hemisphere, many bird species from as far north as Siberia begin their annual flight to the Southern Hemisphere, where spring and summer are dawning. Six months hence, the process is reversed.

The birds tend to migrate a lot faster in the spring than in the autumn. In spring, nature tells them it’s time to breed, nest and raise their young. Often they fly nonstop for several days.

Tang Sixian, associate professor at the School of Life Science at East China Normal University, cites the example of a tagged godwit that flew from Australia nonstop for four days to reach Shanghai in 1990. It was flying at a speed of about 75 kilometers an hour.

“Often when birds fly north, they are in a hurry to breed, while in the autumn the strategy is to make more stops along the way for replenishment,” Tang said.

An autumn migration typically takes two months, while a spring might be completed in 40 to 50 days.

Another migratory bird called the great knot, which usually weighs 250 grams, can lose half of its body weight by the time it arrives in Chongming.

Migrators don’t always arrive. Some die along their long journeys. When many dead birds were found near the cross-sea bridge in Ningbo, ornithologists guessed that they were hypodynamic. In other words, they just didn’t have the strength to continue.

Tracking the birds

The migration to the south this time of year includes both adult birds and juveniles born this summer. To ensure survival of the species, the fledglings sometimes don’t travel with their parents.

“They do that to prevent too many birds arriving in one place at the same time, causing food shortage,” Tang said. “So they migrate in different batches.”

There are three key elements for successful migration — colonization, punctual timing and fixed routes.

“It’s almost impossible for a single bird to fly from Australia to Siberia,” Tang said. “So they usually travel in large groups.”

Wild geese fly in different formations to reduce air resistance.

For some species, like the waders, adult birds make the trip first, while young birds practice their flying skills before taking off.

But the question arises: How do young birds who don’t travel with parents know the right route to take?

“Some scholars believe the ability to find the right route is genetic, and some say that a few adult birds always remain behind to lead the youngsters on the route,” Tang explained. “Different birds have different migration strategies.”

In the past, research largely depended on identification rings attached to birds’ feet to plot the course of migratory routes, but that required a bird to be caught and inspected. Modern technologies are aiding ornithologists.

“We put little flags with different colors on the birds’ feet, each representing a different location,” Tang said. “For example, black and white means the bird comes from Chongming, and you can learn this information by just watching a bird.”

Even more advanced are small satellite transmitters that send signals hourly or daily along the migration route. A transmitter can work up to three years.

“With different research technologies, we know some young birds and adult birds take separate trips, and by using marking technology, we know who leaves first,” Tang said.

Shanghai — the essential station

Southward bound, millions of birds will pass over Shanghai this autumn. Siberian birds left a little earlier than usual, in July this year.

The migratory birds spotted in Shanghai are mostly passing through. Other birds that winter over here usually arrive in November.

Shanghai is in the middle of the great migration routes. Two-thirds of the 445 species recorded in Shanghai are migratory birds, passing through the city twice a year.

The most common species of migrating birds that either pass through or stay in Shanghai include waterfowls, raptors, shorebirds and passerines.

“You can see large numbers of wintering-over birds in conservation areas like Chongming Dongtan, Nanhui Dongtan and Dianshan Lake.” Tang said. “Sometimes you can see them in large, green fields.”

Urban expansion doesn’t sit well with nature. Migratory birds favor large, open spaces, like the greenbelts near airports. That, however, poses risks for planes.

Major metropolises all face the problem of shrinking habitats for wildlife.

“It’s true that bird habitats are getting smaller,” Tang said. “Shanghai has spent years addressing this situation by build conservation areas and passing wildlife protection laws. But it’s inevitable that economic growth collides with protecting nature.”

Wildlife inside designated conservation areas is well protected. In China, anyone found hunting or harming wild animals is punishable under law.

“Shanghai is restoring natural habitats and providing space for birds, but their numbers are down from what they were two to three decades ago.”

The public needs to be educated about the stake it has in preserving nature.

“The bird is a barometer of sorts,” Tang said. “After 30 years of research in ornithology, it is obvious that birds are sensitive ecological indicators. If birds cannot survive in a place, nor can humans in the end. Protecting birds is everyone’s responsibility. By protecting them, we are protecting ourselves.”

Where to watch migratory birds

• Chongming Island is the best bird watching spot in Shanghai. The Dongtan conservation area is the only place in Shanghai where hooded cranes come to winter over. The birds are species under first-level national protection in China.

• Hengsha Island is also an ideal site for spotting wild birds, but you need a car to get there.

• Nanhui Dongtan is a wetland to the east of Dishui Lake. Its about 80 kilometers from Lujiazui.

• Fengxian Century Forest Park is an artificially planted urban ecological forest and a common gathering place for birds.

MIGRATORY birds in Shanghai

MANY waders pass through Shanghai twice a year. Some come to spend the winter here. These medium to large sized shorebirds are found worldwide and inhabit coastal areas. Here are some common species of waders that can be found in Shanghai.

Great knot 大滨鹬

Scientific name: Calidris tenuirostris

The great knot is a relatively large wader with a length of approximately 27 centimeters.

The birds are long-billed with greyish plumage. The face, throat and breast plumage is spotted black, and there are white feathers on the wings. Their bills are black and their feet a greenish grey.

The great knot’s calls sound like chucker-chucker-chucker or nyut-nyut. The birds feed on mollusks and insects.

They breed in northeastern Siberia and winter in Australia.

Bar-tailed godwit 斑尾塍鹬

Scientific name: Limosa lapponica

These large waders at about 40cm in length and their long bills are slightly upturned.

During breeding season, the plumage of bar-tailed godwit is brown, with red on the neck, breast and belly. In winter, the red feathers are white. They feed mostly on insects and crustaceans.

The birds breed in northern Europe and Siberia, and fly to Australia and New Zealand for winter.

The bar-tailed godwit holds the record for the longest nonstop flight for a land bird. The birds’ nonstop journeys are also the longest without pausing to feed.

Sharp-tailed sandpiper 黑腹滨鹬

Scientific name: Calidris alpina

A small species of shorebird, the sharp-tailed sandpiper is about 19cm in length and has a medium-sized bill that is slightly downturned.

The tail is black in the middle, flanked by white. The birds’ calls in flight sound like dwee.

The sharp-tailed sandpipers often feed in grasslands and mudflats, favoring insects, worms and fish.

They breed in northeastern Asia and winter in Southeast Asia and southern China.

Spoon-billed sandpiper 勺嘴鹬

Scientific name: Calidris pygmaea

A common small wader with short legs, the spoon-billed sandpiper is named because its bill tip is flat.

About 15cm in length, the birds have a brownish-red head, neck and breast, with black streaks appearing in breeding season. In winter, the reddish color turns brownish grey.

These waders prefer sandbanks and eat with the bill almost straight down, with a side-to-side movement.

The spoon-billed sandpiper breeds in northern Europe and Asia, and winters over in Southeast Asia. The bird is on the list of critically endangered species.

According to the IUCN Red List, the breeding population in 2009/2010 was estimated at 120 to 200 pairs, roughly equivalent to 240 to 400 mature individuals and 360 to 600 individuals in total, although this is thought to be an optimistic estimate.