The ascent is vertical. Up, up and into the jet stream. If the conditions are not right up there it will come back down and wait. But if there is a good tailwind in the right direction it will begin an epic journey that will take it around the curvature of the Earth; from the Arctic Circle to the southern hemisphere.

Using the sun and stars as a compass, and navigating by the Earth’s magnetic field, recognising landmarks, the far eastern curlew will fly nonstop to the Yellow Sea, where it fuels up on the mudflats of north-east China.

Facebook Twitter Pinterest The Milky Way, with the Pointer stars below the Southern Cross. The eastern curlew uses the night sky to navigate. Photograph: VW Pics/Universal Images Group via Getty Images

Then it will fly through days and nights across oceans to reach the Australian coastline, an exhausted wreck of a bird that is down to skin and bones. An endurance athlete of the animal kingdom, an aerodynamic wonder, a bird that flies 30,000km a year, flapping its wings all the way because it can’t or soar or glide, the eastern curlew is a phenomenon of the natural world. It is also critically endangered. It has lost 80% of its population in the past 30 years.

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We don’t always see what is right in front of us. Tidal mudflats seem featureless, aesthetically disappointing, empty space, devoid of glossy breaking waves. But hidden beneath the mud they are fully functioning ecosystems of hundreds of species of invertebrates, crabs, shellfish and worms. Were it not for its preposterously long curving protuberance of a beak, you might not notice the brown speckled bird with that beak deep in the mud, feeding. The eastern curlew is the largest of more than 100 shorebirds that traverse the globe every year. They travel on one of nine invisible flyways that converge in the Arctic region. It is sometimes called the moonbird because if it lasts its 20-year lifespan it would have flown the distance between the Earth and the moon.

Facebook Twitter Pinterest Earmarked for development: the Toondah mudflats in Moreton Bay. Photograph: Susan Chenery

Now, if a proposed development goes ahead on the mudflats at Toondah Harbour in Brisbane, the eastern curlew will, after flying 12,000km, arrive to find part of its habitat for six months each year taken over by a $1.4bn, 42-hectare marina and 3,500 apartment complex.

Toondah is part of Moreton Bay, which is listed under the Ramsar convention on wetlands – an international treaty that protects important wetlands around the world. As well as 50,000 waterbirds, including 30 migratory species, it is the habitat of dugongs and turtles.

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For tens of thousands of years the eastern curlew has travelled on the east Asian-Australasian flyway to get here. Along with 55 other migratory species it passes through 22 countries along the way. The flyways – super highways – take birds to Africa, Europe, the Americas. Says Randall Wood, a director who is travelling and researching Flyways, a film about globally migrating shorebirds: “The birds link all those countries. It is a beautiful story of resilience, endurance and versatility.”

The world record for marathon flying is now held by the smaller bar-tailed godwit, which has been tracked travelling 11,000km from Alaska to New Zealand at an average of 50km/h, in a big arc and without stopping, in only eight days.

Facebook Twitter Pinterest Bar-tailed godwits, marathon flyers who can travel 11,000km in eight days. Photograph: Dan Kitwood/Getty Images

“Fuel is stored in the body as fat,” Wood says. “They use a combination of fat and animo acid drawn from muscle depletion, so they are literally digesting their muscles on route. What [scientists] are not sure about is how they are able to fly as far as they do with the fuel that they have got. The math doesn’t add up.

It is known that the eastern curlew shrivels organs like its reproductive system, which it doesn’t use while flying, to almost nothing to save a few grams of weight. Harry Saddler, who wrote the book The Eastern Curlew, says: “Before they leave they will feed frantically and put on 80% of their bodyweight again in fat. When they are getting ready to fly they are like balloons that are bursting and you wonder how they can get off the ground.”

Migrating birds’ ability to predict long-term weather forecasts, says Wood, “is better than the very, very best computer models we have got. They can tell months out when a storm is going to come. They will actually time their departures to literally slingshot themselves out to sea on the tail of a storm and propel themselves across oceans.”

Facebook Twitter Pinterest Film director Randall Wood in Wyoming

A whimbrel, says Amanda Lilleyman, a research associate at Charles Darwin University, was “tracked moving 200km out of the way of the eye of the cyclone before the cyclone hit”.

As eastern curlews get ready to leave for their breeding grounds in Siberia in April, their plumage change. “They get these fresh flight feathers to take them back up north,” says Lilleyman. They will also be growing brighter feathers to attract mates. Having gone to all the trouble of the energy-intensive migration to Siberia, says Saddler, “They get straight down to business because they are only there for six weeks. The male tries to impress the female by doing 15-metre-high flights while calling the whole time.” The chicks feed themselves from the moment they are hatched and are start their first migration at six weeks.

Facebook Twitter Pinterest An eastern curlew at Shoalhaven Heads on the New South Wales south coast. Photograph: Duade Paton/AAP

It is what they face on their migration that gives us the bird’s eye view of the world we have created. “Many scientists call the eastern curlew a sentinel species,” Wood says. “They are telling us about the state of the world globally in ways that many other species just aren’t.

“Shorebirds face a perfect storm. They are facing compounding impacts from climate change, extensive coastal development, extreme habitat loss, the rise of megacities in Asia, light pollution, wind turbines, pesticide use on broad-acre farms, hunters who shoot them, and competing with farmers for water.”

The climate crisis has taken its toll on another shorebird, the great knot, whose numbers have been getting smaller and smaller. “Because,” says Saddler, “the Arctic summer begins three weeks earlier than it used to. They are still flying to their traditional timetable but, by the time they get up there, the insect boom has been and gone. The chicks hatch and there is not enough food so they are malnourished, they don’t grow as large, the bills don’t grow as long.

Facebook Twitter Pinterest Two hundred breeding pairs left: a spoon-billed sandpiper. Photograph: John Harrison/Wikipedia

“So when they eventually manage to migrate and get to the mudflat to refuel and feed, they can’t reach the depth of the clams any more, so they feed on roots and plants which are much less nutritious. It just has this cascading effect down the line.”

The most endangered bird of all is the tiny spoon-billed sandpiper, which travels down from Russia to Thailand, Vietnam and Cambodia. It is thought that there are just 200 breeding pairs left. One weather event on its way and it will be gone forever. “These birds aren’t going to be saved by captivity,” Saddler says, “because they are hardwired to fly 10,000km a year.”

The flyways are only as strong as their weakest link. “Every country on the migration route is in the chain,” Saddler sats. “It doesn’t matter if one link in the chain is strong, if another link is smashed there is no chain any more.”

Facebook Twitter Pinterest Slated for rezoning: Lake Wollumboola. Photograph: Elly Kopytko/Wikipedia

And the weakest link is Australia. Like Toondah Harbour, Lake Wollumboola on the south coast of New South Wales is an internationally significant site that is slated for rezoning – into a development of a possible 600 homes.

“Eastern curlews are extraordinary creatures and the fact that we are allowing this species to go extinct under our watch is a total violation of the treaty,” Wood says. The eastern curlew is very site-specific; it always returns to the same place.

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When South Korea built the world’s longest seawall, Saddler says: “It killed the entire estuary. The birds didn’t go somewhere else, they landed where the estuary should have been and the food was no longer there and they died. When they land they simply don’t have the energy left to go to other food sources. With that one construction the great knot went from least concern to endangered.”

Every time one is frightened into flight by a human or a dog, Saddler says, that “is a wingbeat that is robbed from the upcoming migration. They don’t wander around because they need to save all the energy they can get.”

Even China, Saddler says, after many years of rapacious development and land reclamation, has “had a huge turn of attitude. China has got a new green policy. They have just had huge swathes of land nominated for world heritage status on the coast. They are forging ahead with the conservation movement in China – we are stepping in the opposite direction.”

Facebook Twitter Pinterest Shorebirds ‘show us a barometer of global health’. Photograph: Mario Suarez Porras/2018 Bird Photographer of the Year

He adds: “This perpetual growth model of the economy is fundamentally incompatible with life on Earth.”

There are estimated to be about 25,000 eastern curlews left which, given the dangers they face migrating, is not a lot. But everyone, Saddler says, including scientists around the world, “who is working on shorebirds is deeply and extraordinarily passionate about them”.

Says Wood: “We think we can lose a species here and there and perhaps we can but what they are actually showing us is a barometer of global health – and we ignore that at our peril. There is a ticking clock here and it is an extinction story and we are marching towards it.”