So we know Marcy's possesed or wintersoidered and then there's corruptianne theory which could be possible. After all her powers seem to brought out of anger/rage also make her go feral.
Some people have pointed out that the colors of these two herons being green and blue could be foreshadowing. And heron imagery/symbolism is used often
Anne being on the opposing side
Sasha has two swords with herons that might be something.
So like what if in a paralleling turn of events the now corrupt Anne and still possesed Marcy invade Wartwood. Sorta like these herons did.
And like that night where Spring and Polly's parents were killed this time another Planter is taken.
Will this happen most likely not, but hey it's just a very lose theory. Maybe a good fanfic idea though!