There's logic with what you are saying, but don't forget that ymir passed her power to eren, so personally I believe that bird has something to do with ymir and she knew that Mikasa can distroy eren, so yup technically it's tied to eren now.. And for historia's baby now I am curious to know who's the dad since eren said he couldn't make up his mind between breeding her or genocide, and by now we all know what he picked.
you're righttt. I think the birds are a metaphor for freedom. Because that's a common theme throughout the whole manga. It's what Eren and Ymir want.
Let's just hope this is all just eren's way of ending stuff before they start and he let Mikasa see his future memories and take a pre-step on him, that's all I wish for now!
SO for all the people who are confused, here is my theory. (it's based off the time loop theory so you might wanna do some research on that) If you go back and look at chapter 1, almost the exact same thing happened between mikasa and eren but rolls reversed. She even said "see you later eren" in chapter 1 just like she did in chapter 138. So I personally think that everytime Ymir isn't freed the loop restarts. Ymir can only be freed when the "shiny thing" is destroyed because that was the thing that attached to her when she fell in the tree all those years ago. By freeing Ymir, the curse on Eldians would disappear and there would be no more titans as well as no more time loop. I think in the end of 138 the time loop was already starting which is why Ymir was there and Mikasa said the same thing she did in chapter 1. The question is what will happen in the next chapter. If they destroy the shiny thing before the time loop loops again then maybe the curse will be broken, if not....Then the time loop will start again, most likely right before the wall breaks in Shiganshima. Also this just a theory who knows ┗( T﹏T )┛