Fascinating. My first thought when I read your initial posting was that these were Savanna Nightjars according to your description, but I then figured they weren't because these birds make a series of calls, not just isolated bursts. They call as they're flying. I was living in Pingtung when they arrived en masse, and they made quite a stir. One year they didn't exist, the next they were everywhere and at night they were really, really noisy. (They came out at dusk and were noisy for about two hours.) As the Taipei Times article you cite points out, they were somewhat of a disturbance, but, to me, a welcome one. They were hard to see at night, but eventually I got good at spotting them. The trick is to look ahead of where the sound is coming from, sort of like fighter jets.

Their call though is a series of short bursts of high pitched calls, not just one sound... or at least it's a series of calls a few seconds apart. If that was a nightjar you've been hearing, I'm certain we haven't heard them here because I would have recognized it. It's very distinctive.

The nightjar call is a bit like a nighthawk I've heard in the US. It's sort of like a chirping sound. You might be in an area where this bird is moving into. That would explain why we haven't heard or seen it here. As I said, it appeared in Pingtung City literally overnight. And why is that?