What does Mercy Lewis do to help abby, and further discredit Mary Warren?

When Abby begins to act witched, Mercy jumps right in and begins to see the yellow bird and wonders if it is attacking her as Abby suggests it might be. Since they say Mary is the one creating the bird, then they tell her to go away since they are telling the truth and they will not back off even if she "witches" them.