When the red bird died it caused bad luck. This made the characters assume a "plague" or bad event about to come.
How do you think they affect the development in plot?
They affect the story because the characters are more tedious and nervous over the course of the book. They are frightened and believe that the death of the bird will curse them.
Can you think of other superstitions you believe have head others believe?
The superstition of spilling salt is a big superstition. This causes bad luck and a seven year curse.