Earlier today, Niantic revealed the “Ultra Bonus” reward for Pokemon GO players, and it’s safe to say that it has lived up to the hype. As part of the Ultra Bonus, Gen 1 regionals like Mr. Mime and Kangaskhan are currently available in 7km eggs through the end of the month, and in a few days, the first generation legendary birds will be returning to raids.

Starting September 13 at 1:00pm PT, the legendary birds Articuno, Moltres, and Zapdos will be returning to Pokemon GO in legendary raids. The birds will remain in legendary raids until September 20 at 1:00pm PT, at which point they will be replaced by the psychic-type legendary Pokemon Mewtwo.

The addition of Mewtwo later in the month seems like it will be a bit more exciting for players than the return of Articuno, Moltres, and Zapdos. After all, Niantic has given Pokemon GO players plenty of opportunities to add the legendary birds to their collection, even making them obtainable outside of raid battles through the Field Research breakthrough rewards. It also hosted special days dedicated to each bird, with Moltres Day occurring just this past weekend.

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Even so, it’s hard to say when the legendary birds will be available in Pokemon GO beyond this event, so anyone still needing to add them to their collection should be sure to participate in as many legendary raids as possible throughout the month. In the meantime, some Pokemon GO fans may be wondering which legendary Pokemon will be added to the game after the events with the legendary birds and Mewtwo are over.

It seems likely that we will see one of the fourth generation legendary Pokemon added to Pokemon GO at that point, of which there are nine possibilities. Since Mewtwo will be exiting raids on October 23, likely in the midst of Pokemon GO‘s annual Halloween event, one distinct possibility is that Giratina, a dragon- and ghost-type legendary Pokemon, will be making its debut next month.

Of course, that’s just speculation at this time, and Niantic may not be ready to add fourth generation Pokemon to Pokemon GO just yet. If that’s the case, then it could very well replace Mewtwo with other legendary Pokemon that have already appeared in the game before, like Ho-Oh or Lugia, for instance.

Pokemon GO is out now for iOS and Android mobile devices.