will the legendary birds come back?

I hope so, still looking for a good Lugia.

Maybe eventually. But there are a lot of other legends that they will probably do before then.

The three beasts are going through November. Mewtwo is slowly rolling out. Ho-oh, Mew, and Celebi might each have special events. Delibird and Smeargle are not legends but should be coming out too.

Sometime the end of this year or early next year they will probably release Generation 3, and then there are ten more legends.

I could see them coming back shortly during some event, but it's tough to say. There are a LOT of legendaries/mythics coming up.

Yes, 2019

Maybe we'll see them during type-events, e.g. the temporary return of Zapdos raids to spice up an electric event.

I find it highly unlikely that Niantic would give us a Pokemon during a few weeks and then never let it appear ever again.

But I think it will appear during a short amount of time and not very often. Perhaps next summer.

I sure hope so because I have none ?