Much to the annoyance of animal welfare groups, men from a rural Scottish community will this weekend keep alive a tradition dating back more than 500 years.

The annual slaughter of thousands of juvenile seabirds has been likened by animal rights activists to the clubbing to death of baby seals by Canadian hunters, yet they are powerless to stop it. For centuries the men of Ness on Lewis in the Western Isles have embarked on an annual expedition every August to scale the rocky 300ft cliffs of the remote island of Sula Sgeir and harvest the birds, which are considered a culinary delicacy.

It has been a rite of passage for generations of the Gaelic-speaking fishermen since the early 15th century, when guga, as the gannets are known locally, was a popular dish among the island communities, which relied on the salted birds as a staple part of their winter diets. Each year about 10 men sail the 60 miles across the rough Atlantic waters to the rocky outcrop and spend a fortnight on the uninhabited island.

Using 10ft poles with nooses on the end, they pluck the birds from their nests on the cliffs before throttling, gutting and salting them. They are then preserved in an empty beer keg filled with pickling brine.

Once the catch is complete, the men return as heroes to Ness, where they are met at the quayside by their friends and neighbours who, despite the grotesque olive-green and stubbly appearance of the gutted gugas, queue for a share of the harvest for about £20 a brace.

However, many islanders admit that it is an acquired taste. Usually boiled, the rich oily meat tastes much better than it smells and, devotees say, is best served with potatoes and a glass of milk.

Although the hunt is licensed and the islanders' right to carry it out is enshrined in law, the expedition to the narrow, guano-covered rock with a ruined chapel built by an old hermit and a few prehistoric stone shelters remains controversial. For years animal welfare campaigners have tried to have it banned.

Despite keeping the exact date of their departure a secret, John McFarlane, the leader of the hunt this year, confirmed that by this morning the team will be on the island and keeping tradition alive. 'As far as I'm concerned we're licensed to do it and that's the end of the story,' he said in response to campaigners who call the hunt barbaric and outdated.

Western Isles MSP and resident of the Isle of Lewis Alasdair Allan said the hunt enjoyed his full support. 'It's not being done for sport, it's being done for food purposes,' said Allan, a member of the Scottish National Party. 'The arguments against it are essentially vegetarian. People can be vegetarian if they want, but I don't think they should impose that on other people. They should be careful not to associate it with hunting or sport, because that's not what this is. The guga hunt is part of the tradition of the island. It's continuing existence is because it's a traditional food, traditional to the island, a traditional activity. It's not done out of entertainment, but because it's a local delicacy and a foodstuff.'

However, Advocates for Animals has called on the Scottish Executive to stop what it has labelled a 'barbaric' tradition. 'We believe this is a brutal practice,' said Ross Minett, the group's campaigns director. 'We don't believe it can be justified on the grounds of tradition. It isn't right in the 21st century in a civilised country that young birds should be killed by strangling, clubbing or having their necks broken.

'We hope that the new SNP Scottish government will be prepared to stop giving the annual licence that allows this slaughter to continue.'

Sula Sgeir is home to about 10,400 breeding pairs of gannets, of which the islanders harvest no more than 2,000 juveniles each year, the maximum specified in the licence issued by the Scottish Executive. Since 1954, when the Protection of Birds Act made it illegal to harm the gannet population, the men of Ness have been granted special permission to continue with their harvest, with the backing of the Royal Society for the Protection of Birds and Scottish Society for the Prevention of Cruelty to Animals.

Both organisations believe the hunt does not cause unnecessary suffering to the birds and do not share the same concerns expressed by Advocates for Animals and People for the Ethical Treatment of Animals (Peta) which have been encouraging supporters to lobby the Scottish Parliament.

'Clubbing helpless baby chicks like cave people is right out of the stone ages,' said Anita Singh, spokeswoman for Peta. 'Baby gannets may not be as cute and cuddly as puppies and kittens, but they too suffer and feel pain. It's time to relegate this cruel, outdated activity to the history books, where it belongs. There is desperate need to protect these animals. We are encouraging all caring members of the public to raise their voices in protest at this cruelty by contacting the Scottish Parliament.'

However, it is unlikely the Scottish Executive will intervene, as its own advisory group, Scottish Natural Heritage, has said there are no grounds for trying to stop the practice. 'We would only interfere if it was shown to be hitting the gannet population as a whole and was no longer a sustainable harvest, but there's been nothing to indicate that is the case,' said a spokesman.

Five centuries of guga hunting

· Guga is the Gaelic for the pressed, salted and pickled chick of the gannet, or solan goose. A century ago, islanders culled guga from all over the Hebridean coast.

· It has been variously described as 'unspeakable' and tasting like something between rotten leather and turkey laced with motor oil. Gordon Ramsay declared it a cross between duck and mackerel.

· The sturdiest 10 men of Ness - as far north-west as you can go in Europe, at the Butt of Lewis - have a special licence from the Scottish executive to continue the tradition. Each summer they sail for six hours across the Atlantic to the remote rock of Sula Sgeir and land by leaping against a small cliff from a rocking boat.

· The ancient drystone cells on Sula Sgeir are so earwig-infested that the men sleep with scarves wound around their heads.

· The cull is ancient - in 1549, Dean Munro wrote that the men of Ness would 'fetche hame thair boatful of dry wild fowls with wild fowl fedderis'.

· The baby gannets, pictured, are caught on open cliffs using a long pole. It requires agility and nerve.

· In the 1980s a hunter was lost overboard but the most famous drama was in 1912, when the expedition did not return. A search patrol reported the rock was deserted and Ness mourned but several weeks later the men returned. It turned out the rescue party had searched the wrong island.

· These days, locals queue on the quayside for the hunters who sell the guga, rationed to a brace per person, for £20.

· The practice has been attacked by animal rights activists for years. 'When the English stop killing five million turkeys for Christmas,' a supporter declared two decades ago, 'we'll stop catching guga.'

· Guga is boiled and the rich, oily meat is traditionally eaten with potatoes and a glass of milk. An old joke goes: 'To best enjoy guga, boil it with a stone. Boil it until the stone is tender. And then eat the stone.'

Tracy McVeigh