As you read this, flocks of bar-tailed godwits may be departing from the south coast of Alaska. These brown wading birds may seem nondescript - like smaller cousins of curlew - but they are embarking on one of the greatest migratory journeys.
The godwits will fly non-stop for 11,700 km, the distance from Hong Kong to Los Angeles, in the longest known flight by any creature. After eight days, they arrive at their destination, coastal mudflats in New Zealand. Church bells will ring to welcome flocks touching down in Christchurch, where they are seen as harbingers of spring.
Bird migration is among the wonders of the natural world. It has evolved in response to the great rewards of being able to exploit food in a place like Arctic tundra, while also fleeing winter conditions that make life impossible for many birds.
Yet there are immense risks and countless birds die en route.
Migratory birds have a host of remarkable adaptations that enable them to undertake such journeys. To prepare for their autumn marathon, the bar-tailed godwits store energy until fat comprises up to 55 per cent of their body weight. Their livers, kidneys and intestines shrink, becoming almost useless, as the birds' bodies focus on flying.
The godwits also have innate weather forecasting skills.
"All the departures we've observed were associated with low pressure systems," said Bob Gill, a wildlife biologist with the US Geological Survey's Alaska Science Centre. "The birds get on the back side of these lows and get 900 to 1,200 kilometres of pretty strong tailwinds."
Built-in weather knowledge could also help the godwits avoid being buffeted by Pacific typhoons. But no one knows for sure, just as no one fully understands how birds navigate.
Experiments have revealed that birds can follow familiar landmarks, such as rivers, coastlines and even roads. At least some migrants have mental star maps for orientation on clear nights. The height and position of the sun can help birds judge the direction they are headed.
Yet this is no help in cloudy weather. Nor can simply observing the sun fix position when there are no landmarks - a task which proved so challenging for humans it was not until the late 18th century that mariners could determine longitude at sea. To precisely determine their position and direction, birds would need a sense we don't possess: the ability to gauge the earth's magnetic field.
Studies have shown birds can orient using magnetic fields and a change as small as a thousandth of the earth's magnetic field can affect the navigation ability of European robins.
Yet it is how they sense magnetism that is a mystery. Notions that iron-rich cells near pigeons' beaks serve as tiny compasses were dispelled when researchers could only find white blood cells there, which do not produce electrical signals.
Perhaps the answer is more bizarre. Robins' magnetic sense requires them to see clearly, in turn leading to notions it depends on a weird property of matter known as quantum entanglement. Possibly, light excites two electrons on a molecule of a chemical, leading to one electron departing for another molecule of the same chemical.
Though the electrons have separated, their "spins" are inextricably linked for a short time, during which they are affected by the earth's magnetic field.
The magnetism could affect the chemical's properties, resulting in subtle changes across the eye that lead to a bird "seeing" the earth's magnetic field. A possible chemical responsible for sensing magnetism is the protein cryptochrome. Fruit flies with cryptochrome receptors can navigate using magnetism; those without fly as if oblivious to it.
While the ways birds navigate eludes us, we are well-informed on the routes they take, thanks to satellite tracking of birds such as the bar-tailed godwit E7.
E7 is a female godwit that was tagged with a satellite transmitter in New Zealand in February 2007. Back then, no one was certain godwits really flew the length of the Pacific in one flight.
That year scientists watched as she left Alaska on August 29, passed Fiji and on September 7 landed eight miles from where she had been captured. In March, E7 made another other huge flight to coastal mudflats in north China, 10,300km away.
Here, E7 spent five weeks refuelling for the breeding season ahead. Tens of thousands of other godwits were doing the same at wetlands around the Yellow Sea - one of the world's greatest areas for intertidal mudflats.
The godwits are among an outstanding variety of shorebirds relying on the Yellow Sea wetlands. Some species are unique to east Asia, others face extinction; one, the spoon-billed sandpiper, numbers less than 500.
Yet while the Yellow Sea should be a key region for conservation, its wetlands are being casually destroyed. As habitats dwindle, populations of the migratory birds depending on them will decline. Year by year, there will be less wonder in the world.
Perhaps a season will come when the church bells in Christchurch are silent, as godwits no longer arrive - before we understood how they ever got there.
Martin Williams is a Hong Kong-based writer with a PhD in physical chemistry from Cambridge University