It is late afternoon in the Wiang Lo Wildlife Sanctuary in northern Thailand, and a protected forest like this should be alive with birdsong at this time of the day.

But no birds sing.

Somewhere in the distance, a single barbet calls, but the rest is silence except for the rasp of cicadas. The hills are picturesque and the forest is verdant, but it is not trackless. Local hunters penetrating deep inside the hills, for generations and even after the place was officially protected, have emptied out the forest, killing birds and other animals.

For the birds that survive, and would usually thrive in the tenuous overlap between forest and field shared with humans, today even the crop fields that edge into the forest fringes don't offer them much. Previously largely planted with rice or fruit and vegetables, these are now rubber and cassava plantations, drenched in herbicides and pesticide, with almost no natural vegetation, offering little to grain- and insect-eating birds.

Where there are rice and corn fields, a few mynahs, munias and egrets manage to get by. But even these fields are now often cropped all year, with no fallow periods left as respite from the chemicals that kill off the insects that birds feed on. With the bottom knocked out of the food chain, there are no more black drongos, no more black shouldered kites, and no more shrikes.

This is typical of most of South-east Asia and especially Thailand and Laos where hunting has been a tradition for generations. The carnivorous black-necked stork, up to 1.5m tall and with a 2.1m wing span, has long vanished from Thailand. So has the stately Sarus crane, at 1.7m the world's tallest flying bird.

Around the world, bird populations are falling, in some cases dramatically. Globally, 15 per cent of all bird species are critically endangered, according to Birdlife International and the International Union for Conservation of Nature (IUCN). In Asia, 59 per cent of water bird species are seeing declining numbers.

"Thailand has very good coverage in terms of protected areas, around 20 per cent of the country's land area, but most of it is upland," says Dr Philip Round, country representative of The Wetlands Trust and associate professor of Mahidol University's biology department.

"The lowlands have been trashed. And they are being trashed throughout South-east Asia. Rubber and palm oil are a cancer for biodiversity," he says.

Of a list of the world's 10 most endangered birds compiled this year by scientists, five are in Asia. Some number just in the couple of hundreds, and some are in dispersed groups with little or no success at breeding. Some like the Bengal florican and the imposing great Indian bustard may be extinct very soon, possibly in less than three to five years.

Most of the declines can be attributed to human activity. Birds are a barometer, sending us signals about our environment and what we are doing to it.

Birds regularly crash into tall buildings in Singapore's business district, from their perspective a mid-air maze of concrete, glass and steel, with strange and disorienting reflections.

And even in rural India, the birds are not spared from the vagaries of development.

In the once-vast grasslands of western India where the great Indian bustard has for generations roamed unhindered, the low-flying birds now occasionally collide fatally with high-tension electricity cables. On the ground, their grassland habitat is being ploughed under or turned into industrial land; in what remains, they run the gauntlet of roving packs of semi-feral dogs.

In the late 1990s, it was found that Egyptian vultures in India were dying en masse, poisoned by diclofenac, a drug - since banned - used to treat cattle, that found its way through the food chain to the livers of the scavenging birds.

There have also been episodes of mass deaths of birds from pesticide poisoning.

In Britain and the European Union - where intensive mechanised farming brought a drastic plunge in farmland bird numbers - the status of certain bird species as an indicator of environmental quality has in recent years been recognised by governments at the policy and administrative levels. In Britain, this is partially in response to a vocal and powerful lobby for birds; "backyard birds" have been discussed in Britain's Parliament as an indicator of quality of life. European wild bird indicators are officially adopted by the EU as indicators of biodiversity.

But there is no such recognition in Asia. Perhaps tropical Asia, rich in biodiversity, has taken it for granted. In the Manila-based Asean Centre for Biodiversity's 2013 report, much of the focus is on the marine environment. This is understandable in the Asean context, where many member states have extensive marine territories - but birds get only a passing mention.

"Governments in Asia do not yet regard birds as indicators of human well-being," says Dr K. S. Gopi Sundar, director of the US-based International Crane Foundation's Sarus crane research and conservation efforts.

"Bird declines seem to be taken seriously in Asia only if aggressively lobbied, but even then only if there is a clear and perhaps relatively easy solution readily offered," the India-based scientist wrote in an e-mail.

Can the birds make a comeback? Dr Sundar's work in the plains of north India and southern Nepal offers a clue to what has been lost - and also what could be saved.

In some cases, some birds do better with people than without them, he noticed. The Sarus crane, for instance, is celebrated in folk tales and tolerated by farmers, in the same way they tolerate the spectacular peafowl and an array of other birds that share the land with them. Religion helps; in Hinduism, the peacock is associated with the goddess Saraswati.

In his surveys, Dr Sundar also found that the black-necked stork, a species thought to need undisturbed, large wetlands with interspersed trees, seemed to be doing fine in land planted with rice and wheat. The bird is still rare, but in India, it is not as rare as once thought.

Wildlife scientist Umesh Srinivasan, who has studied plantations and logged forests in southern and north-east India - in situations similar to South-east Asia's - has found some bird species doing well in forest that has been logged.

"Huge tracts of logged forest in the tropics have significant conservation value for tropical birds, especially in South and South-east Asia," he wrote in an e-mail.
But he cautioned that logged forests are being rapidly put to other uses such as oil palm planting, which have "next to zero value for tropical biodiversity".

In Thailand, however, recent trends offer some hope, says Dr Round. "There is generally less hunting now; kids now play computer games instead of wandering around with guns. Some large birds like the painted stork and the spot-billed pelican are coming back."

The lowlands of Cambodia and Laos are increasingly being used for large-scale commercial agriculture, just as in Thailand, Malaysia and Indonesia. But at Tonle Sap, Cambodia's largest lake, locals have now begun earning tourism dollars in return for protecting breeding birds, some of which, like painted storks, are re-colonising parts of the region, he said.

And while the overall situation is dismal, Thailand deserves some credit for protecting the endangered green peafowl, which, breeding deep inside the Western Forest Complex that straddles the western border with Myanmar, is slowly coming back.

In Wiang Lo, as evening set in, I saw two green peacocks tentatively wandering in a cassava field. When they saw me, they immediately ran and took flight with cackling squawks of alarm and vanished into the trees at the edge of the forest.

The contrast with peacocks in India, where they are usually not so nervous in the presence of people, was stark. Here in Thailand, where they are now protected but may still be hunted, the green peacocks still distrust humans.

Birds signal the state of the environment, shining a light on our own impact on it. At a very basic level, when they run or take flight at the sight of humans, it probably means they have been hunted by people.

On a larger scale, when one by one they disappear from forest and field, it means people have killed too many for too long, or poisoned the food chain, or changed the environment so much that they cannot survive.

It is up to us, for our own sake, to read the signs.