Media playback is unsupported on your device Media caption The captured birds are often sold as pets - or for their meat

Untold numbers of migratory birds are being caught and killed every year in Afghanistan, helping drive species like the Siberian Crane to the verge of extinction. Hunters say other bird populations are also declining rapidly, raising fears among environmentalists.

Noar Agha loads small stones into the leather pouch attached to his homemade sling shot. "This is like a big transit airport for birds," he says, pointing to a lush valley ringed by snowy Hindu Kush peaks in Parwan province, about 160km (100 miles) from Kabul.

Syed Khel district's wheat fields and orchards offer a perfect resting point for migratory birds.

"Thousands of White-naped Cranes, flamingos, ducks, falcons and sparrows migrate from India and Pakistan when summer temperatures begin to rise there. They make a stopover here before taking off for Russia. That's when we make a move," Mr Agha says.

About a dozen of his grandchildren nod in appreciation. Then a boy perched on a tree top waves a piece of cloth and Mr Agha orders everyone to scatter. Soon a huge flock of sparrows descends on the valley.

He and his grandchildren fire a volley of stones from their sling shots. As dozens of sparrows crash to the ground, three hunting dogs are let loose. They quickly retrieve the injured birds, including those that dropped far from sight.

Image caption Houbara Bustards are caught and smuggled to the Gulf

Like Noar Agha, many Afghans hunt birds for meat. There is also a thriving trade in canaries and finches which are trapped, sold and smuggled to Iran, Pakistan and Gulf countries, where they are popular as house pets.

The head of Afghanistan's Environment Protection Agency, Mustafa Zahir, recently told a local TV news channel that nearly 5,000 birds are smuggled out of the country every year. That may be a conservative estimate. Many of these are falcons and Houbara Bustards - the latter widely prized as quarry by hunters in the Gulf.

With the Afghan economy in tatters, hunting and trading in birds offers a welcome source of income for many struggling Afghans. Markets selling birds of all shapes and sizes - dead or alive - are fairly common in remote areas like Syed Khel and Kohistan.

"This is how I make a living," says one hunter in a bird bazaar in Kohistan, pointing to a sack full of dead sparrows. "There is no work here. What else can I do?"

The sparrows in question are probably Spanish Sparrows, whose numbers are not thought to be at risk, Taej Mundkur of Wetlands International tells the BBC. But he adds: "The harvest could well extend to other species as well."

In truth, no one really knows.

In a country which has seen decades of war, the welfare of birds is low on the list of priorities. The Siberian Crane, once a regular visitor en route to India, has not been sighted in Afghanistan since 1999. It is now listed as globally critically endangered.

Image caption Siberian Cranes have not been seen in Afghanistan this century

Other birds are also now less common, say Afghan hunters.

"My elders used to talk about cranes, flamingos, wild ducks and quails. These birds were very common in this part of the country. But now it is no longer so," says 27-year-old Mohammad Wahid.

That view is shared by Mohammad Agha, 70: "There are just too many hunters… so the birds have fled."

A few kilometres along the Panjshir river in Kohistan district, Haji Dost Mohammad has hunted ducks for half of his life and says every house in his village has a shotgun. Mr Mohammad, 40, starts his days early during the migrating season.

"Every day before sunrise, we put stuffed ducks in our village pond. When a flock of birds arrive, drakes are attracted to the pond. We wait as the drakes try to pair up with the stuffed ducks. Just when they are about to settle on the pond, we fire," he says.

People used to use [guns] to fight the Soviets - now they are used for hunting Haji Dost Mohammad, Villager, Kohistan district

Huge nets are used, too, to trap entire flocks of birds.

"They spread themselves across the gaps in the mountains carrying the ends of these nets. When a flock passes through the gap the ends are pulled, forcing hundreds of birds to fly straight into the nets," one village elder said.

In some places, large wooden bird boxes with paraffin lamps are hung on trees. The warmth draws the birds, which fall into the concealed trap.

Such methods can result in huge catches. I saw one hunter bagging up birds by the hundred for a local party. Two local shopkeepers caught at least 500 birds each in a single day to sell at market.

Image caption Afghan hunters say numbers of all kinds of birds are in decline

It is impossible to know how many birds are being killed in Afghanistan every year, but the kind of hunting I saw is going on right across the country - so the figure would appear to be in the hundreds of thousands every year, at least.

The authorities say they are aware of the situation, with one senior official in Parwan even calling it a "genocide of birds".

"But you have to understand that this is the way of life here. Hunting of birds has been going on for hundreds of years. Besides, many government officials are themselves hunters. Who will speak against them?"

The government banned the hunting of migratory birds five years ago in a presidential decree but the law is still to go through parliament - and the ban is barely enforced.

"We are also working with religious scholars and other influential members of society to start an awareness campaign on the ill effects of excessive hunting," Ghulam Mohammad Malikyar, deputy director-general of Afghanistan's National Environmental Protection Agency, told the BBC.

Image caption Many birds are caged to be sold as pets

Educating Afghans is one challenge. Another is the lack of reliable information - because of decades of instability, no comprehensive survey on bird numbers has ever been conducted.

Qais Agah of Save the Environment-Afghanistan said there had been "significant conservation efforts", but told the BBC a recent study had identified almost 150 endangered species of birds in Afghanistan, which is not a party to the Convention on Migratory Species.

Some birds may have changed their migration routes, which would explain the apparent drop in their numbers in Afghanistan. Or Afghans may be wiping them out.

Without proper scientific data, it is impossible to know. What is clear is that hunters say some birds are now seen more rarely, or not at all.

"Thirty years ago, I used to shoot 500-700 sparrows a day with my sling shot," says Haji Shakoor, 57, from Salang valley. "The sky used to be full of birds. But now it seems so empty."

Conservationists hope that doesn't mean more birds go the way of the Siberian Crane.