With no windows, the gloomy, gray building looming four stories above the rice fields in a remote village in Indonesian Borneo resembles nothing more than a prison.

Hundreds of similar concrete structures, riddled with small holes for ventilation, tower over village shops and homes all along Borneo’s northwestern coast.

But these buildings are not for people. They are for the birds. Specifically, the swiftlet, which builds its nests inside.

Zulkibli, 56, a government worker who built his giant birdhouse in the village of Perapakan in 2010, supplements his income by harvesting the swiftlets’ nests and selling them for export to China.

The nests, made from the birds’ saliva, are the key ingredient in bird’s nest soup, an expensive delicacy believed by many Chinese to have health benefits.

Left to their own devices, swiftlets usually make their nests in coastal caves, where harvesting them can be hazardous work. The key to attracting the birds to a man-made home, Mr. Zulkibli said, is treating them like “rich humans” and guaranteeing their comfort and safety. Mr. Zulkibli, like many Indonesians, goes by one name.

“Comfort, by regulating the temperature,” he said. “Safety, by keeping pests and predators away. The swiftlet house must be really clean. They don’t even like spiders.”

Government officials say Indonesia is the world’s largest exporter of swiftlet nests. Sambas Regency, the county-sized region in West Kalimantan Province where Perapakan is located, is a major producer, with the birds thriving in its marshy coastal areas, rich with insects.

The bird nest business can be lucrative. Over the past decade, so many property owners in this sparsely populated region of coconut palms and banana trees were eager to cash in that the number of birdhouses here jumped fivefold, Mr. Zulkibli said.

In a twist on condo conversions, some people even remodeled the upper floors of their homes — blacking out windows and drilling ventilation holes — to make them habitable for swiftlets.

Swiftlets are fast-flying, insect-eating birds that can cover vast distances in a day, using echolocation to navigate in low-light environments. They build as many as three nests a year, Mr. Zulkibli said, frequently changing their nesting sites.

With the region’s glut of birdhouses, many now have vacancies.

“The birds have many choices,” Mr. Zulkibli said.

So owners compete to lure the swiftlets by playing recordings of the clicking sounds they make as they echolocate.

The small, delicate nests are carefully harvested with a specialized tool similar to a paint scraper and then cleaned. Intact white nests bring the best prices.

The theft of birds nests is a common problem. Mr. Zulkibli said his birdhouse has been burgled 20 times, with the thieves sometimes breaking through its concrete walls.

Birdhouse owners say that they wait until the fledglings have left the nest before they harvest and that neither the parents nor their babies are harmed. But sometimes, burglars steal nests prematurely, killing hatchlings in the process.

Inside Mr. Zulkibli’s 50-foot-high birdhouse, wooden joists crisscross the ceilings, creating places for birds to make their nests. Each ventilation hole is covered with mesh to keep out vermin and is connected to a short, curving pipe that blocks the light, helping replicate a cave’s gloom. A pool of water at ground level helps cool the building and gives the birds a place to bathe.

The swiftlets enter at high speed through a rectangular opening at the top and reach the lower levels through 8-by10-foot holes in each floor.

Though the swiftlets provide an income, Mr. Zulkibli said he was passionate about birds, as his parents were. They raised free-range pigeons and never served fowl as food.

“We never ate duck or anything that could fly,” he said. “That’s one reason I want to protect the birds. Many birds build their nests around my house here, maybe because they feel safe with me.”

Once the swiftlets are settled in their nests, he said, they let him pet them.

Just south of Sambas Regency, the coastal city of Singkawang was once a major nest producer. But today it suffers from the local version of empty nest syndrome.

Known for its large ethnic Chinese population and colorful Buddhist and Taoist temples, Singkawang now serves as a trading center where businessmen buy nests and ship them 500 miles south to the capital, Jakarta, for export.

Dozens of large birdhouses, some as tall as five stories, still dot Singkawang. But as its human population has swelled to 250,000, fewer swiftlets have come into the city.

The birds were plentiful as recently in 2010, when Yusmida converted the top two floors of her house into a home for swiftlets. But a few years later, Singkawang’s largest shopping mall was built next door. Since then, her bird nursery has sat empty.

“No birds have come in a decade,” she lamented.

On Singkawang’s outskirts, about 60 miles north of the Equator, a farmer, Suhardi, 52, built some of the region’s earliest birdhouses in 2000. For more than a decade, the birds were plentiful and his business was profitable.

At its peak, he said, he could produce 10 kilograms of nests a month, or about 22 pounds, which he could sell for $20,000 — huge income for an Indonesian farmer. Now, if he harvests a little more than three pounds a month and sells it for $1,500, he considers himself fortunate.

He doesn’t blame the overbuilding of birdhouses so much as the rising temperatures from climate change and the cutting of nearby jungle to make way for palm oil plantations, which wrecked the ecosystem the birds relied on for food.

“The earth is getting hotter and the sun’s intensity is scorching hot,” Mr. Suhardi said. “In the past, there were forests to cool down the heat. And with the forest disappearing, their food source is also gone.”

It doesn’t help that the government now requires that nest exports go through a handful of traders in Jakarta, cutting into the price farmers got when they exported directly to China.

“With this situation, many of the bird nest farmers have quit,” Mr. Suhardi said. “They sell their houses and the land at a cheap price.”

Now, many of the birdhouses around Singkawang stand unused. Unlike human homes, the birdhouses go unpainted, adding to the pervading sense of melancholy.

Mr. Suhardi, not expecting the swiftlet situation to improve anytime soon, has shifted to planting avocado and durian.

“But I will still keep the bird houses,” he said, “and check them every month or two.”

This article was produced with support from the Round Earth Media program of the International Women’s Media Foundation.