"
If you knew, knew what the blue birds sang at you,
You, would never sing along.
Cast them out because this is our culture,
These new flocks are nothing but vultures
In "If you knew, knew what the blue birds sang at you, you would never sing along," the singer/subject are saying that if you knew what the haters said about you, that you would not agree with them. In "Cast them out because this is our culture," the subject is saying that you need to ignore them because "you are what you love" (quote from "Save Rock and Roll"), and that you shouldn't let them do anything about it. And in "These new flocks are nothing but vultures," the subject is telling you that all these people who are rejecting everyone are just vultures, meaning rude and vicious.
Well done, Fall Out Boy, well done!