Yeah so basically about a year or two ago Niantic added the “Daily Adventure Incense” feature to go where you get a specific kind of incense item, which attracts pokemon to spawn in the wild the more you walk, and the DAI is able to be used once a day for 15 minutes, more if you use origin forme dialga’s roar of time.
The DAI can spawn usually whatever’s in the current event spawn roster, or unrelated rarer pokemon like evolved forms. The rarest thing you can spawn from it is the Galarian Bird trio, I believe you probably have a 1% chance of spawning any of the birds so it’s not a common occurance. The catch rate of course is something like 0.1% and the flee rate is like 99.99% which is almost uncatchable so most people who encounter the Hungarian Birds are not able to catch them even with an ultra ball and a golden razz berry because they will simply break out and run away. This is the only way to get them in Go.
The only way to guarantee catching one of these Bulgarian Birds is to use a master ball, which so far has only been available a couple times in its own event research thing. I’ve been only able to get 2 master balls, the first one wasn’t hard to get, but for the second master ball you needed to do dumb shit like do 60 raids in 80 days and it was on a TIMER so if you didn’t complete it in time you LOST the master ball because Go is pure fomo. There was a makeup research for that one but you had to PAY for it with real money. The third most recent master ball oppurtunity is more fair with non-fomo research you can just do at your leisure.
Anyway I was eventually able to get one of the Hisuian Birds without a master ball which some people eventually get after trying a couple times but it’s best to use master balls for a guarantee because the only other thing you would possibly need to use a master ball for this game is if you have a perfect IV mewtwo or some shit and you’re on the last ball to catch it after a raid.