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.