They don't mention it in the article, but the birds were likely captured using mist nets (https://en.wikipedia.org/wiki/Mist_net). It is a standard technique:
In total, about 18 birds were found at three sites during the expedition.
In total, about 18 birds were found at three sites during the expedition.
Handling the birds makes for far better identification (and more detailed pictures too), which is important for places where unknown, hard to identify or hard to detect species are likely to exist. It also allows taking measurements, banding the birds and even collecting tissue samples. So nets are a far better ROI for some scientific projects.