In Susan Glaspell's Trifles, the dead canary that Mrs. Hale and Mrs. Peters find does two very important things in terms of galvanizing the plot.
Up until this point in the play, Mrs. Hale is uncertain as to Minnie Wright's guilt in the alleged murder of her husband, while Mrs. Peters is very much in line with the letter of the law. Mrs. Hale is the more sympathetic of the two women at the play's beginning. However, as the women listen to the chauvinistic remarks of the men (women worry over "trifles," the very things that keep a house going—bellies filled, and clothes and homes cleaned...hard work), there is a noticeable change in Mrs. Hale's attitude, who actually speaks up to defend a woman's worth in the home. This foreshadows her eventual change of attitude with regard to Minnie's plight.
HALE. Well, women are used to worrying over trifles. (The two women move a little closer together.)
COUNTY ATTORNEY ...And yet, for all their worries, what would we do without the ladies? (...Starts to wipe [his hands] on the roller towel, turns it for a cleaner place.) Dirty towels!...Not much of a housekeeper, would you say, ladies?
MRS. HALE. (stiffly). There's a great deal of work to be done on a farm.
COUNTY ATTORNEY. To be sure. And yet...I know there are some Dickson county farmhouses which do not have such roller towels...
MRS. HALE. Those towels get dirty awful quick. Men's hands aren't always as clean as they might be.
COUNTY ATTORNEY. Ah, loyal to your sex, I see.
(The attorney's last comment also foreshadows the women's eventual support of Minnie Wright.)
The women gather some things to take to Mrs. Wright in the jail. Then they discover first an empty birdcage, worse for wear:
MRS. PETER. Why, here's a birdcage. Did she have a bird, Mrs. Hale?
MRS. HALE. Why, I don't know whether she did or not--I've not been here for so long. There was a man around last year selling canaries cheap, but I don't know as she took one; maybe she did...
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.
Here is the first sign of John Wright's anger, something Mrs. Hale had noticed missing before. Then they find the bird:
MRS. HALE (lifting the silk.) Oh, Mrs. Peters--it's--
MRS. PETERS. It's the bird.
MRS. HALE. But, Mrs. Peters--look at it. Its neck! Look at its neck! It's all--other side to.
MRS. PETERS. Somebody--wrung--its neck. (Their eyes meet. A look of growing comprehension of horror...Mrs. Hale slips box under quilt pieces, and sinks into her chair...)
This is a moment of enormous significance for the women: they have found proof of John Wright's brutal nature, but they have also found evidence that could lead to Mrs. Wright's conviction for murder. Finally understanding what Minnie had to live with—we can infer that he was also abusive with Minnie—the women choose to say nothing. The men have made it clear that they believe a woman's day is spent dealing with "trifles," unimportant things—rather than extremely hard work that makes up a woman's entire life. Certainly they won't understand Minnie's despair not at being brutalized herself, but seeing the only joy in her life, the singing bird (symbolic of Minnie herself), broken in another of John's fits of anger—for although she has been married for years, it seems the bird's death breaks her.
The bird marks the turning point in the play.