In Chapter 7, "1937," Sula returns to the Bottom and is accompanied by a plague of robins.  Sula has been away from home for ten years and, when she returns, the townspeople are not happy to see her.  The birds are an evil omen according to the townspeople, but they can do nothing about it and resolve to simply put up with any bad things to come.  In addition to the birds, the birthmark over Sula's eye also appears to change according to others.  Nel believes that the rose-shaped pattern is getting darker, and Jude thinks the birthmark looks like a snake's head.  All these superstitions surrounding Sula cast her as a bad omen, and later in the chapter, bad things do happen.  Particularly, Sula has an affair with Jude which crushes her friendship with Nel.