Symbolism, Imagery, Allegory
Bird Is The Word
Birds are everywhere in Sula, and they're not just flitting from tree to tree or hanging out on telephone wires. They're associated with certain characters. When we meet Rochelle, she wears a "canary-yellow dress" and has the "glare of a canary" (1920.40-3.41). And we already know that a "plague of robins" (1937.1) arrives in Medallion just before Sula does.
But why?
Well, robins are often associated with the spring, the season of rebirth and growth. And, although Sula brings with her a lot of pain and destruction, we learn that her presence also generates a renewed sense of purpose in the Bottom...even if it is directed against her.
As for the canary-like attributes of Rochelle, we think that hints at canaries in coal mines. In order to check for sufficient oxygen, miners would lower caged canaries: if the cages came up with the birds alive, the men would follow, if the cages were filled with dead birds, the miners knew not to go down until more air shafts were built. Rochelle symbolizes this sort of "danger test" for Nel: she exudes sexual confidence and callousness, much like Sula later does. Once Nel has realized that this type of personality isn't immediately horrible—the metaphorical canary doesn't die because Nel doesn't recoil from Rochelle—she seeks the same kind of personality out in Sula and Sula's mother.