Symbolism is hard to find in Miller's The Crucible. Two things to make note of are the doll (poppet) that is brought forth as evidence against Elizabeth, and the bird the girls "see" in the court room.
The doll (or poppet ,puppet) was made by Mary Warren while sitting in court:
DANFORTH, turning to Abigail:
A poppet were discovered in Mr. Proctor's house, stabbed by a needle. Mary Warren claims that you sat beside her in the court when she made it, and that you saw her make it and witnessed how she herself stuck her needle into it for safe-keeping. What say you to that?
ABIGAIL, with a slight note of indignation:
It is a lie, sir...Goody Proctor always kept poppets.
The doll, or poppet, was believed to be a representation of a human being that could be harmed through the "harming" of the poppet. (This is based on a misconception that dolls are used in vodou.) Abigail has accused Elizabeth of stabbing her in the stomach with a needle, by way of the poppet.
CHEEVER, wide-eyed, trembling:
The girl, the Williams girl, Abigail Williams, sir. She sat to dinner in Reverend Parris's house tonight, and without word nor warnin' she falls to the floor. Like a struck beast, he says, and screamed a scream that a bull would weep to hear. And he goes to save her, and stuck two inches in the flesh of her belly, he draw a needle out. And demandin' of her how she come to be so stabbed, she—to Proctor now—testify it were your wife's familiar spirit pushed it in.
While the poppet represents a tool of Elizabeth's "familiar spirit," it is also symbolic of the way the members of the town and the court officials are being manipulated—like a child controls a toy. Abigail accuses and all listen to her—never questioning the veracity of her claims. Even the most "saintly" citizens in town are called to defend themselves against her allegations: a number are eventually executed.
Birds have many associations in literature and cultures around the world. In this case, I believe the bird is used by Abigail (when she— and then the other girls—"sees" a bird in the courtroom) to show a connection between Mary Warren and the spirit world—the devil's domain. In doing so, Abigail draws suspicion away from her and the others, side-tracking court officials who may doubt them after Proctor and Hale accuse them of lying. They pretend that the bird is actually the spirit of Mary Warren.
Why—? She gulps. Why do you come, yellow bird?...
...to the ceiling:
My face? My face?...
...to the ceiling, in a genuine conversation with the "bird," as though trying to talk it out of attacking her:
But God made my face; you cannot want to tear my face. Envy is a deadly sin, Mary...Oh, Mary, this is a black art to change your shape. No, I cannot, I cannot stop my mouth; it's God's work I do.
The other girls join in with the charade; soon they begin to mimic Mary as she appeals to them to stop. Mary is slowly defeated by the united front of the girls in their "pretense."
While literature speaks to readers differently, I think the bird here is symbolic of the truth. Yellow is symbolic of disease or illness. So here the truth rests above the heads of all those present—where no one can reach it. The fact that it is yellow signifies that the words of the girls are lies.
(Yellow can also signify jealousy or envy: this is what drives Abigail to "attack" Elizabeth Proctor in the first place. In this scene, she is defending herself against Proctor's accusations.)