
So, Humanity has driven off the Xeelee for the time being, as they did in Exultant.
But after the Xeelee leave, some cosmic entity hits Xeeleeverse Humanity with a Hercules–Corona-Borealis-Great-Wall-sized clue-by-four, and they realize something:
"Hey, we've had some independent discoveries of dark matter acting quite strangely, including records in the Venusian chalk crust. Maybe we should look into that?"
So they investigate and surprise! They find the Photino Birds and what they're doing.
Of course there's the little fact that the Xeelee, the baryonic lords themselves, couldn't defeat them. What hope would Humanity have?
Hopeless? Maybe not. There are two phenomena that, I think, could actually help Humanity out.
First: the reality dust entities. First time we saw them they casually shifted Callisto virtually instantaneously a few thousand kilometers or so. This killed a Xeelee that was attacking due to the fear that the dust entities were powerful enough to hinder the Xeelee (!). The Xeelee was killed by being trapped under the ice (a method that seemed to act by both encasing the Xeelee in the ice and impaling it thanks to the fact that Callisto suddenly occupied the same space the Xeelee was in). They also helped out in the Prime Radiant conflict by giving the Coalition the designs for the black hole gun when Pirius was sent to contact them. So they are powerful enough to beat single attacking Xeelee (maybe more if the situation demands it?), and Humans can communicate with them by uploading/copying their mind to the configuration space reality.
Second: the Snowflake technology. We're not given hard parameters on what Snowflakes can do. It's based off of the Mach principle which, in brief, states: "matter over here affects matter over there". The original Snowflake damaged a Human vessel with "coherent gravity waves originating outside of our galaxy". The Human made Snowflake in The Siege of Earth restructured Earth's space-time to produce a sort of temporal gradient (with time at the top/outside of the restructured space-time running the fastest, and time at the bottom/inside at the center of the Earth running the slowest, with a smooth gradient between the two). Judging by these events it's dangerously close to being flat out magic.
[NOTE: the Snowflakes created by Humans were only used as weapons during the siege of the Solar System, but not during the first or second Xeelee/Human wars that occurred earlier. There is conflicting information and guesses on whether Snowflakes were weaponized by then, or only much later. For the sake of this scenario we'll assume that they were being weaponized during or shortly after the Prime Radiant conflict.]
There is a range of possibilities on how this turns out. Worse case for Humanity is that it only royally pisses off the Photino Birds without making a dent in their plans. Best case is that the Photino Birds are purged from, or flee, the Milky Way Galaxy. A middle scenario being that they're driven away for a while (possibly long enough for Humanity to prepare for a longer battle with the Birds).
My guess would be on driving the Birds away for a bit while Humanity tries to develop new technologies and tactics for better defending against them.