Under the torrential rain of a fickle British summer, 25 soggy pilgrims gather in front of the Plough, an ancient oak-beamed pub in the West Sussex village of Rusper. Above the drone of planes bringing travellers into Gatwick, the walkers – variously devotees of trekking, folk music or wildlife – raise their voices to sing an old English folk song that is told from the point of view of a globetrotting visitor to these shores: “Fare you well, my dear, I must be gone / And leave you for a while / If I roam away, I’ll come back again / Though I roam 10,000 miles, my dear / Though I roam 10,000 miles.”

The song, The Turtle Dove, was discovered by Ralph Vaughan Williams when the Plough’s publican sang it to him more than 100 years ago. It is a forgotten lament for a bird that has inspired writers, musicians and country-dwellers for centuries. This small, delicate wild dove, with grey feathers blushed pink and ginger, is a symbol of renewal in the Old Testament and an emblem of love and constancy for Shakespeare. The romance embodied by the turtle dove has fluttered into countless songs, from The Twelve Days of Christmas to Elvis Presley’s Baby If You’ll Give Me All Your Love and Cliff Richard’s Bachelor Boy. But this celebrated bird is slipping towards extinction in Britain.

The turtle dove is a global citizen, spending winter in the Sahel region of Africa before flying to Europe to breed. The Victorians observed great flocks of turtle doves assembling to begin their migration south. As recently as the 60s, there were 125,000 pairs in Britain. Between 1967 and 2016, their numbers plummeted by 98%. Each year, population estimates are revised downwards: there may be fewer than 2,000 pairs left. Most worryingly, there is no agreement about how we can reverse the decline.

The pilgrims, led by the singer Sam Lee, are beginning a two-day walk from the Plough to one of the turtle dove’s last strongholds, the Knepp estate in West Sussex. In a quixotic act of faith, they plan to sing the song to the doves. If we all paid more attention to this enigmatic bird, thinks Lee, perhaps we could save it from disappearing.

More than 40 million birds have vanished from this country in 50 years

“We’re casting a magical spell,” says Lee. “Vaughan Williams was passing this pub on his bicycle 113 years ago and he popped in and said: ‘Anyone know any songs?’ The landlord sang him The Turtle Dove, which he’d never heard before. He took it away to London and he made a global hit.” Vaughan Williams’ arrangement of the song has been recorded, in different forms and with different titles, by artists including Joan Baez, Marianne Faithfull and Nic Jones, yet the song’s original meaning has been forgotten, explains Lee, especially in its Sussex heartland. “We’re going to rewild this song,” he says. “The amazing connection is that the song itself is about extinction.” The turtle dove sings of its migration, its disappearance and its devotion to its unnamed love, which could be for a place as well as a person.

The turtle dove is a symbol of the extinction crisis unfolding in Britain. Our intensively farmed, densely populated country can still look pretty, but experts agree that lowland Britain – the turtle dove’s home – is one of the most nature-depleted landscapes in the world. More than half of Britain’s plant and animal species are in decline and one in 10 is severely threatened. More than 40 million birds have vanished from this country in 50 years.

Pilgrims on their two-day quest to find endangered turtle doves. Photograph: Dominick Tyler/The Guardian

The Victorians may have seen flocks of turtle doves, but today they are elusive; the best chance of encountering one is to hear a calling male, which makes the soft “tur-turr” that gives the dove its name. The sound is gentle, summery and somnolent – one of the most soothing in nature, according to Victorian admirers. When I search for it this summer, I hear it only beside the old walled courtyard at Pensthorpe, a Norfolk nature reserve that has run a trial to breed the doves in captivity. “It’s so sad that the only place to hear them is in our aviary,” says Chrissie Kelley, head of species management at Pensthorpe Conservation Trust. “It used to be a common sound.”

Kelley takes me through the turtle dove’s troubled recent history. It has been affected badly by recent droughts in Africa and the Mediterranean penchant for shooting migrating birds each spring and autumn (traditionally for food, but now more for fun). It has been estimated that 3 million turtle doves are shot each year. While EU law bans the hunting of birds during periods of breeding and migration, the turtle dove is still shot in many countries during autumn. In addition, BirdLife International estimates that 600,000 are killed illegally each year.

But these trends do not explain entirely the loss of British turtle doves. Although the bird is declining across Europe (down more than a third this century), only in the similarly intensively farmed Netherlands is there a decline to match Britain’s. Turtle doves eat mostly grains, living on wild plant and weed seeds. Since the 50s, Britain has destroyed almost all its wildflower meadows, while chemical pesticides have removed arable weeds. “Back in the day, when turtle doves were very numerous, they’d feed on farmers’ grain spills,” says Kelley. “Fifty years ago there was a lot more mess – stubble fields and open barns. We’ve got so clean in our farming practices now, that there’s no spillage, no waste.”

The Knepp estate in West Sussex is one of the few places where turtle dove numbers are increasing. Photograph: Dominick Tyler/The Guardian

Britain has also lost dense hedgerows where the turtle dove likes to nest. “They need thick, scrubby hedgerows, because as far as turtle doves are concerned they think a couple of twigs is a nest,” says Kelley, pointing out the feeble construction her captive doves have made in a thicket inside the aviary. “They need food and water fairly close by, sucking up water to make crop milk for their young.”

Pensthorpe is working with a group of mid-Norfolk farmers to preserve hedgerows, restore farm ponds and offer the birds supplementary food. It is a formula being repeated across the turtle dove’s last strongholds in the south-east and Yorkshire, as part of Operation Turtle Dove, a partnership by charities including the RSPB and Natural England, the government’s advisory body on the environment. But earlier, similar, schemes have not worked. One Norfolk farmer who planted field margins with wild seed mixes succeeded in boosting bird populations, but not turtle doves.

Isabella Tree, the co-owner of the Knepp estate, is critical of recent attempts to provide the doves with additional food. Knepp is one of the few places where turtle doves are increasing: from none at the start of the century, the 1,400-hectare (3,500-acre) farm now has 20 calling males. In the book Wilding, Tree’s account of how she and her husband, Charlie Burrell, returned their farm to nature over two decades, she argues that our categorisation of the turtle dove as a “farmland” bird is misleading. What did it eat before there were farms? What does it eat in sub-Saharan Africa? Tree believes turtle doves that have been observed eating spilled arable grains in Britain are starving and desperate. Any dove breeder will tell you about the dangers of feeding wheat and corn to the birds: cracked grain can tear the throat, while in the gut it absorbs moisture and can cause fungal diseases.

‘Losing the turtle dove is to lose the intelligence of that creature, which understands the landscape around it.’ Photograph: Getty Images/iStockphoto

When Operation Turtle Dove launched in 2012, the RSPB wanted to scatter a mix of wheat, oilseed rape, millet and canary seed at sites including Knepp. Tree and Burrell politely refused. Knepp’s farm produces no such seeds, yet their turtle doves are thriving. “Why can’t modern conservationists go back to the meticulous records of the Victorians to find out what turtle doves were actually eating?” asks Tree.

While conservationists agree that turtle doves love weed seeds such as common fumitory (which has returned to rewilded Knepp), most weeds do not produce seeds until the middle of summer; the turtle dove arrives in Britain, undernourished, in April and May. In fact, says Tree, there are old accounts of turtle doves eating snails. They may peck at hawthorn shoots, too. Ornithologists have observed continental turtle doves eating berries, fungi and invertebrates.

One theory is that Knepp’s turtle doves are thriving because they can easily find seeds on the bare ground created by the rootling of the wild pigs that Tree and Burrell have restored to their farm. “We know the turtle doves like bare earth, where their little legs aren’t having to cope with anything too scraggy or long,” says Tree. “They want areas where they can see what’s coming and fly off quickly.” Turtle doves at Knepp this summer are being caught and fitted with a radio tag that should help scientists identify where they have been feeding and deduce precisely what they need.

Sam Lee says the decline is ‘part of a greater silencing of the natural world’. Photograph: Dominick Tyler/The Guardian

Increasingly, however, conservationists fear there are other, even more complicated causes for their demise. Carles Carboneras, an RSPB research scientist who specialises in migratory birds, says that global heating is causing quail, another migratory “farmland” bird, to move further northwards. But the turtle dove is showing no sign of expanding its range. “Everybody agrees the main cause for the decline is the transformation of landscape,” says Carboneras. “The increase in monocultures is very, very bad for the species. In this country, you either have very intense agriculture or very dense woodlands – and neither of these is good for turtle doves.”

But Carboneras points out that this shift to a “simplified” landscape in Britain occurred 50 years ago. So why have turtle dove populations fallen so dramatically more recently? Carboneras fears that the rapid accumulation of nitrogen from farm fertilisers and traffic pollution in the soil is causing vegetation to grow too quickly for turtle doves. “If the grass is too high, they can’t access the food. In the south-east, we’re probably seeing the effect of nutrients, but it’s a hunch. We don’t have the evidence for that yet.”

It will be difficult to save the turtle dove without understanding the reasons for its decline but, some conservationists warn, endangered species have been “studied to extinction” in the past. We cannot always afford to wait. At Pensthorpe, Kelley and her team have successfully demonstrated how to breed turtle doves in captivity. Here, turtle dove eggs are fostered by Barbary doves, because these common birds are less sensitive and happy to be kept inside smaller cages. Releasing captive-bred turtle doves into the wild, says Kelley, could be “an insurance policy for British wild birds before they go extinct”.

There’s something calming about the turtle dove’s song. It appears and disappears – a metaphor for its own situation

The RSPB is opposed to breeding turtle doves in captivity, saying captive-bred birds are unlikely to possess a migratory instinct; they would not depart every autumn and return each spring.

“This is like the British solving their own problem – and so what? You still have a declining population internationally. You cannot have millions of captive-bred turtle doves being released across Europe and expect them to behave naturally and link breeding and wintering areas as they have done since they evolved. It might solve the problem for this island, but not for the species,” Carboneras says.

Other conservation scientists disagree. Carl Jones has used captive breeding to save the pink pigeon and other critically endangered birds from extinction on Mauritius. Now, as chief scientist for Durrell Wildlife Conservation Trust, an international charity, he is looking at whether he can help save British turtle doves with a similarly intensive rescue plan. The RSPB is, he thinks, being “narrow-minded” and cites other examples of the successful release of captive-bred migratory birds, from the white stork across Europe to the orange-bellied parrot in Tasmania.

“There’s no better way of understanding these animals than breeding them in captivity and really getting hands on with them, rather than just standing back and observing decline or putting fences around things,” says Jones. He argues that a captive-breeding programme, alongside the management of wild populations through supplementary feeding, is the best way of saving endangered species and quickly understanding their needs.

Back at Knepp, the pilgrims arrive, wet and exhausted, at their journey’s end. At 4am, they rise from their bell tents and head to great green pillows of sallow and blackthorn scrub in the rewilded landscape. Here they finally hear the tur-turring of the turtle dove and softly sing Vaughan Williams’ folk song.

“There’s something immensely calming about the turtle dove’s song,” says Lee. “It has this low frequency that suddenly appears and disappears. It’s almost the flicker of a film – and such a metaphor for its own situation. The silence around it is as powerful as its presence.” Singing about this bird, he says, makes it “become more powerful and significant. Suddenly the bird is fuller and you realise every creature has that potential – we’ve just stopped listening”.

Most of us will never notice if this one small bird with its soft song disappears from Britain. How would we lose if it does? “There are so many ways,” says Lee. “The disappearance of its song is part of a greater silencing of the natural world. Losing the turtle dove is to lose the intelligence of that creature, which understands the landscape around it and plays a part in it.” He pauses. “It’s the burning down of a library. To take these books off the shelves and lose them would be to weaken us as human beings.”

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