Up to 4 million birds trapped illegally this year face a slow, sticky end before being sold to restaurants for tapas, say campaigners

Up to four million migrating birds will be killed by illegal Spanish hunters this year, with many dying a slow, sticky death in traps that literally glue the animals to the ground, according to campaigners.

Thrushes flying south for a warm Mediterranean winter this week will, as usual, run a gauntlet of illegal hunters who kill some two million birds in their peak hunting season: the six late autumn weeks in which Spanish skies fill with migrating birds.

Hundreds, if not thousands, of hunters in Castellón, eastern Spain, and neighbouring areas will have already set their so-called parany traps – copses filled with glue-covered twigs and spikes.

Most of the illegally trapped birds will end up as tapas in Spanish bars, fried tidbits that locals claim are part of a cultural heritage stretching back to Roman times. "There are pictures of parany traps in the mosaics of Pompeii," said Miguel Angel Bayarri of the trappers' Apaval association. "This is a tradition that has existed for centuries and that we will continue to fight for."

Hunting of song and mistle thrushes and their cousin the redwing is not illegal, but the methods used are, despite attempts by legislators to introduce exceptions.

Campaigners say the painful deaths suffered by the birds, whose wings are glued together before their necks are broken or their heads squashed, contravene European wildlife laws.

"There have been sentences against this in the courts in Madrid and at the European court in Luxembourg," said Mario Giménez, head of SEO/Birdlife in the eastern region of Valencia.

Up to two of every five birds that fall into the parany traps will not be thrushes. Hundreds of thousands of other migrating insectivores such as robins, blackcaps, chiffchaffs and black redstarts will die. Local birds, including warblers and owls, also fall prey to the parany trick.

"That only happens if the trap is badly operated," said Bayarri. "Our members only catch thrushes. This is just banning for banning's sake."

But Giménez said few parany operators went through the laborious process of cleaning glue off birds that may not be hunted.

"Even those cleaned up with dissolvent often don't survive," he said.

Campaigners say it is time politicians, whose attempts at legalising the traps in Valencia's regional parliament have been stymied by Spain's higher courts, publicly disavow a tradition that contravenes EU law. But protecting local traditions wins votes in rural areas where setting and emptying traps may also involve evenings or weekends of food, drink and partying.

Hunting continues, even though Apaval has this year asked its members not to set the traps, where recordings of birdsong are used to lure passing birds into thickets of trees. Unable to use their glued wings, the birds fall to the ground and are killed by hand.

At least 340 functioning parany traps have been identified in the past few weeks, according to Spain's Association of Environmental Officers. "They are only the first ones discovered so far this year," said a spokesman. "In previous years we have found some 2,000 traps."

"The hunting continues, it is just not so flagrant," said Giménez. "We have seen the traps and heard the birdsong recordings that they use. Now when they hear a car, they do at least turn the machine off."

Those caught using the illegal parany method can be fined up to €200,000 (£170,000) and may also end up with a criminal record.

Previously, special whistles were used to attract the birds, but these were replaced by tape-recordings and, now, digital devices.

While the thrushes were once an important food source in the countryside of eastern Spain, birds are now mostly sold secretly to bars and restaurants. Many are stored in freezers to be sold later.

Campaigners fear that with a change in Spain's government due after a general election on 20 November, authorities will once more turn a blind eye to illegal hunting of migratory birds.

"The People's party is expected to win, and that is the same party that has tried to legalise this in Valencia," said Giménez. "We don't want a new government turning the clock back."