Believe it or not, just beside my house (like just a few steps away), there’s a field where these bird owners gather every Sunday for a bird singing contest. At first, just a few people, now there are many. A free bird concert every Sunday. Hahaha…
I’ve never been there, only hearing it from my house. And sometimes, there’s a bird that gets loose from the cage and stays or flies around my house, and at that moment, I know, this is that bird… LOL.
I’ve never known how the winners are chosen. Just because of your post, I decided to Google how they pick the winners. And it turns out, it’s so complex yet objective. Even more complex than human's singing contests or pageant contests.
The bird singing contest is usually judged based on rhythm, tempo, sound stability, sound quality, variation, duration, loudness, etc [1][2].
Speaking of the gambling side, it’s just like any other activity. As long as there is a possibility, there’s a chance for gambling. If I am not mistaken, I also saw a video on social media of gamblers betting on the number of cars passing a specific road. IMO, this is easier to manipulate. You can pay people to pass the road, right? In this case, bird singing contest is more make sense.
Source:[1]
https://journal.jis-institute.org/index.php/jnik/article/download/505/303[2]
https://repository.dinamika.ac.id/id/eprint/1846/3/BAB_II.pdf