Credit To: CrimsonGallade
You are approached by the leader of your chosen team. They inform you that a legendary Pokemon has been sighted in your area and ask you to investigate the sighting.
After the event starts, Pokestops have a chance to yield ‘Legendary Feathers’. Yellow electric feathers for Team Instinct, Red fiery feathers for Team Valor, and icy blue feathers for Team Mystic.
After you discover your first feather, you need to then report to a gym your team controls in order to deliver it to your team leader. They ask you to continue the investigation.
After your third feather, the legendary bird represented by your team lands at the gym you delivered the feather to, initiating a battle that any teammates in the area can help fight. This Pokemon will be strong, but not too strong so it is deferrable. The battle yields candies and CP for any Pokemon used in the fight.
Once the battle is complete and the legendary bird is defeated, your team leader hands you a special Poke Ball, named after your team. The ‘Valor’ Ball, ‘Instinct’ Ball, and ‘Mystic’ Ball. This ball will have an 100% catch rate to capture the legendary bird you just defeated.
The three legendary birds will be captured at 1500 CP and will not have a way to get candy. This ensures that they are strong, but not game-breaking.
Before the event begins, trainers will be given a 1-time chance to change their team, so they may capture the bird they want. The other legendary birds will not be able to be captured and will only be obtainable by trading in an upcoming update.
Warning: I did in fact get this theory from Twitter, so if it looks familiar, that’s why. I just thought that it was a really cool way to incorporate the legendary birds into the game.
Thanks for reading! Crimson out!