I was really confused when I did it too, but after looking at a bunch of streamers doing savage the blue/red portals mechanic seems logical enough once you realize that the damage comes from the birds warping through the portals.
If I understood it correctly, when the portals initially spawn, there will always be a blue/red pair at south, followed by the two sets of bird dashes that will first hit one color and then the other. The color that gets hit first will then go off first on the other side.
It works the same way when the portals repeat with the white/black birds - the birds of a specific color will fire off from the same portal they entered.
Idk, it's kind of confusing when written in text and there's no good videos out there yet, but this one kind of shows it:
https://www.youtube.com/watch?v=_nSX_38_OUY&feature=youtu.be&t=169 (at around 2:50) - the first set of birds hits the red portal so that's the one that goes off first.