Silva Gu's eyes dart back and forth across miles of tall grassland, scouring it for signs of life in the darkness.

He speaks in less than a whisper as we try to find a spot to hide in the fields. Behind us, the vast metropolis of Beijing has yet to wake. As we wait, we hear only our own breath.

As the sky starts to lighten ahead of sunrise, we hear footsteps. The poachers are here.

Slim and stealthy, Silva heads out first. We eventually follow with our cameras.

Slowly, we tread through a line of trees, into a small clearing. We only spot the bird net when it is a few inches from our faces.

Each year, tens of thousands of birds are caught in nets across China for the pet trade or for meat.

The pandemic and a property crisis have turned the economy sluggish - so catching and selling songbirds on the black market is a low-cost and often low-risk way of making a large profit.

A pretty songbird, such as a Siberian rubythroat, can often sell for nearly 2,000 yuan (approximately $280), which is more than many farmers earn in a month.

I want to protect them on this Earth controlled by humans, Silva says. For him, birds are a passion.

I often dream. And in my dreams, I'm always flying.

Trapped

In the skies above us, billions of birds, many so small that they can fit in the palm of your hand, are migrating south for winter.

They have taken advantage of the long summer days in Siberia or Mongolia, feasting on bugs and berries. As the year comes to a close and icy winds bring the first frosts of winter, they are flying to warmer places to nest and feed.

This was back in October, when flying through China is the equivalent of rush hour for migratory birds heading to Australia, New Zealand or southern Africa.

China is home to over 1,500 bird species, which is about 13% of the global population - more than 800 of those are migratory birds. Four of the nine major routes they follow intersect in China.

These are long, often perilous journeys, where the birds navigate through storms and evade predators while looking for the ideal spot to spend the night.

The patch of grassland where we were is an oasis for small birds - any further and the city skies offer few options to rest among towering rows of concrete.

It is also an oasis for the poachers and their mist nets, so thin you can barely see them.

The one we nearly walked into was stretched across half the length of the field and propped up with bamboo poles. In the middle, a small finch was desperately trying to free his legs, but the more it moved, the more its claws became tangled.

It was a meadow pipit, a protected bird in China, and an important indicator species - meaning if its numbers are thriving, so is its environment.

The poacher spotted us and started to run. From a small pouch on his hip, he threw around half a dozen small birds into the air before sprinting deeper into the shrubs.

Our cameras caught the moment he was stopped by Silva, whose years of experience have taught him how to detain poachers while he calls the police. He stops the poacher from leaving simply by continuing to block his path.

At the beginning, I had no experience and at that time I was quite afraid, he later recalls. But if you really want to do something, those fears will all be forgotten.

The police arrived about 40 minutes later to arrest the poacher.

Hunting the hunters

Silva, in his 30s, does this work for free using his own savings. He has given up many nights of sleep to set songbirds free and has spent the last 10 years persuading the police in Beijing to take this crime seriously.

Back in 2015, no one cared, he says.

So he recruited volunteers who did care and launched a group called the Beijing Migratory Bird Squad. He held public meetings and invited the heads of the local police and forestry bureau. These small and persistent acts of persuasion appear to have worked, as the police discovered that catching poachers also led to tracking down other kinds of criminal activity in Beijing.

We found our goals were partially aligned, Silva explains, adding that enforcement is still patchy.

Busted

On a long low wall alongside the Liangshui river in Beijing, a trader has several small cages with tiny twittering birds.

Another man stands outside the nearby vegetable market holding a bird cage shrouded in a black veil. He tells passers-by quietly that his songbird is rare, worth nearly 1900 yuan (about $270).

This is a glimpse of an old Beijing where small unofficial traders have created their own market.

The path by the river stretches for several miles, and on a sunny weekday morning, there were shoppers browsing everything from vintage jewellery to false teeth, all laid out at makeshift stalls.

Today there would be no sales because the police had arrived. They were questioning the bird owners and taking names. Defiant, one man said he was taking his caged bird for a walk. This does happen in many Beijing parks, where songbird owners gather with their caged pets to chat and compare notes.

This police visit was part of a wider campaign by the Ministry of Public Security that was announced earlier in the year.

Despite the renewed efforts to catch poachers, Silva worries that they face few penalties. However, he is also encouraged. He has rescued more than 20,000 birds on site over the past 10 years and disrupted the nets of countless poachers.

I think there's hope, he says, pinning his future hopes on a generational change - when more young people will understand and appreciate China's rare songbirds and the need to protect them.

Until then, Silva pledges to continue his relentless patrols during migration seasons: This is my ideal. If you have this ideal, you must persist. You can't not.