The quote finishes with "...mocker poured out his repertoire in blissful unawareness of whose tree he sat in...."
The theme of "it's a sin to kill a mockingbird" has not only been established by chapter 28, but back in chapter 21, Tom became a mockingbird symbol when he was convicted of something he did not do. He did nothing but help Mayella and had the "nerve" to feel sorry for her. That was his demise. So in this chapter, which is only a couple hours before Boo becomes a mockingbird symbol, this "mocker" is significant because Lee is pointing him out. Not only do the kids hear the bird singing, Scout's narration tells us that he's in Boo's tree. This may in fact be foreshadowing the upcoming events. That is most likely Lee's overall purpose of putting the bird in this scene.