It is early morning in the heart of Provence, and somewhere behind the tall black pine trees a rousing dawn chorus begins. We are crouching out of sight among the rosemary bushes and wild asparagus listening to the melodic musical phrases of song thrushes and blackbirds.

This is Marcel Pagnol country, rich in flora and fauna and of exceptional natural beauty; but there is no sign of the singing birds anywhere in the rustling foliage, trees or sky.

Yves Verilhac, of France’s Ligue pour la Protection des Oiseaux (LPO), knows why. “The singing you can hear is from caged thrushes and blackbirds who are appellants (callers). They’re caught and kept in the dark for months so when they’re taken out into daylight they sing their hearts out and attract other birds.”

He points above the treetops where clusters of sticks attached to vertical poles glisten in the nascent sunlight. “Those are verguettes: sticks covered in glue. The callers call, other birds come, land on a verguette, and they’re stuck. The more they struggle to get away, the more they become stuck.”

The trilling Provençal songbirds are unwitting decoys to lure more birds into a death trap, he says. Once enticed, the birds are either blasted out of the sky by hunters hidden in camouflaged cabins, or find themselves stuck on the sticks.

It is a scene with which readers of Roald Dahl’s The Twits – in which the Twits coat branches with glue to catch birds to bake in a pie – will be familiar.

La chasse à la glu – glue-trapping – was banned in the EU by a 1979 bird directive, except in special circumstances where it is “controlled, selective and in limited quantities”. Since 1989, France has invoked these circumstances to permit glue-trapping in five south-east departments on the grounds that it is “traditional”.

A robin trapped by a French hunter’s glue-stick. Photograph: LPO

Bird campaigners, who say the practice is unspeakably cruel, have secretly shot photographs and film that they say proves it is not selective and, worse, poses a threat to endangered species. They have brought numerous legal cases in national and European courts over the past 30 years. All have failed.

“We’ve concrete evidence that sometimes the bird is struggling for 20–30 minutes. To remove them from the sticks, they spray them with petrol or acetone, which is toxic, and if it’s not a species they’re allowed to trap they often throw the bird away like a stone,” Verilhac says.

He stabs at his mobile phone to bring up a photograph of a dead robin, with its unmistakable red breast, stuck by its wing, legs and beak to a glue stick. “This is what they call ‘tradition’, but it’s a practice from the middle ages and barbaric,” he says.

The curlew is among millions of birds killed by French hunters that are on the International Union for Conservation of Nature’s red list of endangered species. Photograph: Andrew Parkinson/Alamy

The LPO claims French hunters kill an estimated 17 million birds every year – more than any other country – from 64 species. Of these birds, many of which are migratory, 20 are on the International Union for Conservation of Nature’s red list of endangered species in Europe, including the turtle dove, rock ptarmigan, violet thrush and curlew. About 1.4m song thrushes and more than 2m partridges are killed annually. Glue-trappers have permission to catch 42,500 song thrushes and blackbirds this year, half last year’s quota. Last year the French president, Emmanuel Macron, gave in to pressure from the powerful hunting lobby and halved the cost of a hunting licence.

The hunters claim glue-trapping dates back at least several family generations, and reject accusations of cruelty. They insist they are quick to remove any stuck birds and release those they are not permitted to trap – such as robins, blue tits and warblers – back into the wild. This is the official quid pro quo for being allowed to hunt with glue.

There are an estimated 6,000 licensed glue-trappers of songbirds in the Provence-Alpes-Côte-d’Azur region who are allowed to hunt between 1 October and 15 December, from one hour before sunrise to 11am every day. They must log and officially report their catches and kills, and birds shot must be for personal consumption. Those caught on glue sticks must be immediately unstuck and either caged or released.

Eric Camoin, president of the Association de Défense des Chasses Traditionnelles à la Grive (Association for the Defence of Traditional Thrush Hunting), accuses the LPO of “faking” photos and says those filmed breaking the rules are a minority.

“Birds caught using glue are never killed. If they are one of the five species (four thrush and one blackbird) we are allowed to catch, they are kept as callers. If not, they are released alive and freed,” Camoin says.

“As with any activity there are always those who do illegal things and they should be punished. I am clear about that. It doesn’t mean the whole activity should be banned. You don’t outlaw driving just because some people speed at 200kph,” he said.

A hunter holds up a dead thrush. Photograph: Kim Willsher/Guardian

There are just 50 “nature police” from France’s National Hunting and Wildlife Agency (ONCFS) working across the five departments covering an area of 31,400 sq km, and they have other tasks aside from getting up before the crack of dawn to check on songbird trappers, says Verilhac.

“The hunters are supposed to keep a log of how many birds they have caught and present it to the authorities at the end of the season. What sort of control is that?” he adds. “How do we know they release the callers at the end of the season? How do we know they are not selling them?”

At 11am, the bird trappers behind the pine trees are packing up to go home, closing up their camouflaged hides, some fitted out with rugs and heaters. A hunter holding two dead song thrushes quickly hides them behind his back as we approach. He is reluctant to show us either them or the caller birds that he has in cages in the back of a battered car. He says he shot the birds in his hand and that they will “go in the freezer for Christmas”. Thrush brochette is a regional speciality, though hunting the birds is often just for sport.

Surely there is not much meat on them, we ask. “We eat the whole thing. Delicious,” he says.

Another hunter, in a clearing filled with poles and perches for glue-sticks, warns us we should go back to the main path as there is an animal shoot in progress. He says he has caught no birds today. Inspecting his patch after he has gone, we find freshly removed feathers among the leaves on the ground. It is the same at all the clearings we visit, throwing doubt on Camoin’s insistence that glue-trapped birds are not killed.

“It’s a massacre,” says Verilhac, picking up a gluey clump of feathers.

“These are from a warbler and they’re definitely not allowed to trap them. They claim they release the birds alive but if they’ve lost these tail feathers they can’t fly and will starve.

“Even if they let them go, the birds are so traumatised or injured by the petrol or acetone used to remove them from the glue-sticks they’re as good as dead.”

Feathers from a warbler trapped by glue. Photograph: Kim Willsher/Guardian

Later, Verilhac shows us a collection of gruesome photographs and videos secretly taken of glue-trappers in the Provençal hills in the past six weeks.

They show robins, blue tits, warblers and finches struggling and dying on glue-sticks or being pulled off and discarded like litter. Last year, LPO activists found a dead kestrel, its wings gummed with glue.

Camoin, who has been hunting birds since he was a boy, when he went with his grandfather, is sceptical. He calls the photographs montages. “If these pictures are real – and I’m not saying they are – then the culprits must be punished. I’m passionate about birds and of course it pains me to see them suffer,” he says.

“I say to people: ‘You don’t like hunting, that’s fine. I respect your view, but leave me alone. I probably don’t like some things you do but that’s your decision, your life,’” Camoin adds.

French hunters claim to be defenders of ecology. Photograph: Serge Mouraret/Alamy

Verilhac’s response is resolved and clinical. The LPO will keep fighting what it considers an anachronistic barbarity, he says.

“It’s patently clear the birds suffer, but suffering and cruelty are not enough for the EU to force France to ban glue-trapping. I have to argue on the grounds that France is breaking EU conditions for allowing it, that we have absolute proof that it is not controlled and it is not selective,” he says.

Birdlife International says almost a fifth of European birds are at risk of extinction.

“France likes to present itself as a world leader in matters of biodiversity and give lessons to others,” says Verilhac. “La chasse à la glu is one of our famous French exceptions. And not one of which we are proud.”