If you’re trying to claim Team Mystic’s Legendary bird, Articuno, you better act fast. The majority of players have already been able to capture one, but if you don’t act soon, you won’t have a chance for a while. Tomorrow, July 31, Niantic is going to be replacing the Ice-type bird with Moltres in all Tier 5 Raids. It’s unclear at what time they’re making the swap, so you may have a little bit of time tomorrow, but I wouldn’t risk it.
We also have no idea when Articuno will be back. After Moltres is gone, Zapdos will have the chance to take the throne. Perhaps after all three birds have taken over the Tier 5 Raids they will all be released randomly throughout the world. Niantic hasn’t said much regarding their future plans for the big trio, but I would imagine that would be the route they would take.
Since Articuno is in every Tier 5 Raid, Moltres will take the spot, followed by Zapdos. Once the each have an equal amount of time in all Tier 5 Raids, the Raids will go back to normal and you will have a random chance of encountering any of the three. This little period right here will ensure that you get the current bird, but if you end up missing it, you will have plenty of chances in the future.
I would definitely try to lock it down soon though. Even if Articuno does come back soon, there’s no telling when it’s going to be taking flight over your town.