Curlew Moon. Mary Colwell

They seem to stretch as far as the eye can see. Much of the land is below sea level, in some places by nearly three metres. It can be disconcerting to contemplate that the low wall along the coast seems to be all that stops the North Sea piling in on top of you.

      The north Norfolk coast is separated from southern Lincolnshire by The Wash, over 15 well-defined square miles of shallow sea and tidal mudflat. Four rivers – the Whitham, the Welland, the Nene and the Great Ouse – flow into the North Sea here, depositing tonnes of sediment. When the tide is out this vast expanse of worm-filled, shellfish-rich mud forms one of the most important feeding areas for waterbirds in Europe. In winter, it is filled with avian life from all over the continent – 350,000 birds at any one time, and as many as 9,000 curlews.

      As soon as I arrive in Norfolk I head for one small section of The Wash, the RSPB nature reserve in Snettisham, on the coast. The whole day has been mild and damp, everything about it redolent of wet, musty sacks. By mid-afternoon, the light is already losing its sharpness, slipping into that unsettling, crepuscular zone between thin winter light and cold darkness. In mid-winter this comes so early in the day it feels like theft.

      Through binoculars, shapes reveal themselves to be a ‘herd’ of around thirty curlews. Herd is one of the collective nouns, and is not a bad description, as winter flocks generally move together and take flight as a group. The other collective is a near homograph: a curfew of curlews, possibly relating to their gathering at dusk.

      Like sewing machines stitching the sky to the Earth with invisible threads, the curlews prod the ground and generally move in the same direction. They form a fragile alliance. Though focused on finding food, their nervousness is palpable. Perhaps being watched stirs some ancestral memory of being legal quarry in England before 1981, when they were regularly shot on their wintering grounds. Birdwatchers say that they are noticeably more tense in areas where they were once hunted, and Norfolk is one such place. Some call softly, a whistling sound that floats over the damp air, soothing to the ear and a balm for the soul.

      My small camera is balanced on a fence post to keep it steady at maximum zoom. The picture is boring, the birds are too far away, but the sound alone fills the screen. But as I press the ON button, right on cue, the throaty rumble of a microlight edges into the soundscape and rapidly becomes a roar. Within seconds it drowns out the music of the field. The giant petrol-powered set of wings takes an age to move along. If only the creators of the combustion engine could have made it sound like a wind harp or grand piano, maybe even a xylophone. Anything would be preferable to this grating, intrusive drone that fills the sky for what seems like hours.

      In any case, the moment is lost. The curlews take flight, piping as they make their way towards the sea. And perhaps the microlight isn’t entirely to blame; it is just as likely the birds can feel the shifting tide in every cell of their bodies and register the failing light as a signal to move on to a safer place for the night. I pack up and follow them.

      Mud and ankle-deep puddles form the narrow path to the shingle ridge that overlooks the vast expanse of shimmering tidal flats. It offers up a strange and beautiful sight. Channels of water meander across the exposed mud, silvery sinews on a dark surface, at times taking on the sheen of mother of pearl. There seems no end to the mud; it stretches on and on towards the darkening horizon, rippled and ruffled by the water, strangely enticing and mysterious. Even the thousands of birds here are dwarfed by the vastness of the estuary. The air is filled with bird calls and a caustic, rich smell. The curlews are out there somewhere, a part of this mighty throng, big and small.

      Bulky shelduck patrol the surface of the mud like avian minesweepers, their distinctive upturned bills swishing to and fro, scooping up tiny molluscs. Pink-footed geese stream in from feeding in fields: an airborne, honking choir. Neurotic redshanks strut and fuss, then take fright at the slightest disturbance, sailing into the air, emitting their high-pitched wails. The ‘Sentinel of the Marshes’ they are called, the first to warn of danger; red-legged bird versions of Lance Corporal Jack Jones in Dad’s Army – ‘Don’t panic!’

      The real star of The Wash winter show is the knot. Thousands of small brown and white waders seem to be held together by magnets. They rise as one, twisting and spiralling through the sky, like a creature in agony. The display confounds predators and mesmerises humans.

      The sea creeps closer, pushing the birds towards the shore. They bunch together like commuters on a constantly dwindling platform. On days when the tide is very high, they can run out of space completely and tens of thousands take to the air and stream inland. You can find yourself standing beneath an aerial river of birds. It is one of the greatest wildlife spectacles that Britain has to offer. But today there is only a middling tide, and the birds stay on the seaward side of the wall.

      Just a handful of us birdwatchers are dotted along the shore, each alone with our own thoughts, bathed in evening light and surround sound of calling waterfowl. I am imagining how different this will all seem in six months’ time, at the end of May. I will arrive on this eastern coast footsore and tired after six weeks on the road. There will be greenery and leafy trees, warm air and the hum of insects. The sun still high in the sky. There will also be far fewer birds. All being well, curlews should still be on their breeding grounds and guarding their growing families. If they manage to hatch young they will be kept busy until late summer. If not, if they fail to find a suitable nesting site, or if predators eat their eggs and chicks, they will already be gathering into small flocks, preparing for winter once more. The pattern of previous years doesn’t bode well. For most curlews in large parts of Britain and Ireland, far too many will, yet again, gain the depressing title of ‘failed breeders’ and arrive back on the coast way ahead of time. They will have lost the chance to produce young once more. The world is an increasingly uncertain place for them, as it is for many other birds.

      All over the Earth the great pendulum of bird life swings to and fro in time to the beat of the seasons. Some migrants are well studied, but others are only just revealing their secrets as different kinds of tracking devices are developed and become smaller, lighter and more reliable. Hopefully, we will learn more about curlews in the future. It is amazing how little we know about their lives, particularly considering they were once so numerous and widespread. The everyday rarely gets the attention it deserves, until, that is, it becomes increasingly hard to find.

      Organic farmer, naturalist and former wildlife camerawoman, Rebecca Hosking, described to me on Facebook her experience of catching a glimpse of winter curlew in Devon:

      The north winds blew a pair of curlew to the farm this morning. Their unmistakable calls rooted me to the spot, as I watched them spiral downward to feed in one of the lower meadows. To me, their cry is the essence of wildness, both haunting and beguiling. I was astonished at the level of emotion those bubbling trills invoked. Pure joy and excitement, for this was the first time I’ve ever heard them at Modbury. Yet that joy soon turned to sorrow and lament, knowing how drastic their decline has been. My knee-jerk reaction was to run and find Dad so he too could hear. As we stood in the garden listening, the pair lifted back to the sky, calling their plaintive, lonely ‘cour-leees’ as they circled overhead. I looked across at Dad to see his reaction; his eyes were beginning to well, ‘I haven’t heard that bird in this valley in over 40 years,’ he said. We watched them slowly reel their way towards the coast, until they flew out of sight.

      A fifth of Europe’s curlews visit the coasts and estuaries of the UK and Ireland in the winter, a number that is steadily declining. It is an awesome responsibility to hold the fate of these world travellers in our hands. Although most of the birds are unstudied, there are some hotspots for research, and The Wash Wader Ringing Group has been busy fitting identification rings onto the legs of curlews for decades. It has amassed some interesting results. Most of The Wash curlews breed in Scandinavia, France and Germany; only 25 per cent are British birds that stay all year round. Some visitors travel a long way. One curlew ringed in The Wash in September 2000 was shot in Russia the following May, over 2,000 miles away. It was fifteen years old. Surprisingly, this