What is the significance of the symbol of the broken cage door in Trifles?
The broken cage door signifies that Mr. Wright brutally snatched the canary out in order to stop its singing. This act symbolizes his invasion of the last refuge Mrs. Wright has from the cruelty of her husband.
Much like the little songbird, Minnie Foster Wright has loved to sing. But, because she lives on a remote farm, Mrs. Wright no longer sings in the choir as she did before she was married. So, when she is able to acquire a canary, Minnie Wright finds solace in listening to its lovely songs. Then, when it is ripped out of its cage and strangled, Mrs. Wright herself probably feels a symbolic strangulation of her spirit as her desperate loneliness overpowers her.
MRS HALE. If there'd been years and years of nothing, then a bird to sing to you, it would be awful--still after the bird was still.
While the broken cage door signifies the brutality of Mr. Wright, as well as his cold inattention to Mrs. Wright's feelings, it also signifies how he has no respect for her property and what she holds dear.