The bird is symbolic of the repression and abuse that the main character endures during her dysfunctional marriage. However, the bird is also important to set the tone and atmosphere of the story due to the impression that it causes upon Mrs. Hale and Mrs. Peters.
The play reads:
MRS. PETERS. (examining the cage). Why, look at this door. It's broke. One hinge is pulled apart.
MRS. HALE. (looking, too.) Looks as if someone must have been rough with it.
MRS. PETERS. Why, yes. (she brings the cage forward and puts it on the table.)
MRS. HALE. I wish if they're going to find any evidence they'd be about it. I don't like this place.
MRS. PETERS. But I'm awful glad you came with me, Mrs. Hale. It would be lonesome of me sitting here alone.
MRS. HALE. It would, wouldn't it? (Dropping her sewing). But I tell you what I do wish, Mrs. Peters. I wish I had come over sometimes she was here. I-- (Looking around the room.)--wish I had.
As we can see, it is obvious that the birdcage and the whereabouts of the bird create a strong sensation in the psyche of the ladies. They even feel the oppression of their friend's marriage and the harshness of the situation. The tones in both characters, Mrs. Hale and Mrs. Peters, change completely once they realize what must have happened in the house. This is especially obvious when Mrs. Hale says:
I wish I had come over when she was here. I wish I had.