There are huge concrete bunkers all around Pangkalan Bun and elsewhere, emitting loud birdsong. The birdsong comes from loudspeakers, with the intent to attract shallows to nest in the bunkers. When the young have left the nest, the nests are collected and sold to China, where they are eaten as an expensive delicacy.
The nests are made of bird spit. The most expensive ones are almost transparent. Think of it as the oral version of Kopi Luwak (look it up on Wikipedia, this post is weird enough as it is), or, if you like Monty Python, lark vomit. The Chinese eat everything. 