The migratory bird has been feted by everyone from Elvis to Shakespeare.

However hunting, intensive farming and pollution have left it close to extinction in the UK.

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. 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. 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.

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.