For the 400+ sounds that Merlin (the user-friendly, offline-functional sister app to BirdNET) can now identify with 80-90% accuracy, it took a team of dozens of bird sound ID experts several years to annotate tens of thousands of individual audio spectrograms. On average, they needed about 1,000 recordings per species). Here is a bit more about how they did it: https://www.macaulaylibrary.org/2021/06/22/behind-the-scenes...