How does Abigail turn the court against Mary Warren in "The Crucible"?

In Act 3, Mary Warren is forced by John Proctor to come back to the court to tell the judges that the girls are lying and always have been and that everything that they are claiming has taken place never really happened.  When Mary comes back to tell the judges this, they invite Abigail and the other girls into the courtroom.  At this point, Abigail pretends to see a yellow bird on one of the rafters in the room.  The other girls then pretend to see it as well.  Abigail talks to the bird and asks it why it has come, begs it to leave her alone, and calls it "Mary" -- Abigail is trying to incriminate Mary Warren by making the judges think that she is sending out her spirit on Abigail and the girls in the form of a yellow bird that is trying to attack them.  Unfortunately, the court does believe that there is an invisible bird and instead of getting herself in trouble, she sides with Abigail and calls John Proctor a witch.