In Susan Glaspell's Trifles, what is the role of the bird in the play?

The bird in the Susan Glaspell's play,
Trifles, symbolizes happiness, and the ability
of one person to rob another of that happiness.

Mrs. Minnie
Wright has been accused of killing her husband, and as the play begins, several men
enter to search for evidence that will convict her. Two women, Mrs. Hale and Mrs.
Peters, also enter, seemingly aligned with the opinions of the men, there to gather some
belongings for Mrs. Wright who is in jail.

As the men
search for clues, the women gather some things for Mrs. Wright. In their search, they
come across a damaged birdcage in a cupboard, its door half wrenched off the cage. They
are curious about the cage and its missing occupant. As they continue to look around,
they have to listen to the derogatory comments the men make about what a poor
housekeeper Mrs. Wright was, and how she worried about "trifles," which the women know
are anything but unimportant, meaningless concerns or
tasks.

The women begin to feel some sympathy for the sad
life Minnie lived with her "hard" husband. They discover her sewing box, and in it, the
body of the dead bird: wrapped in silk and placed in a box…to be buried, they assume.
Its neck has been wrung: a brutish horrifying act; the realization the women come to is
that Mr. Wright killed the bird, killing Mrs. Wright's only joy in life. Mrs. Hale
comments on the reality of Mrs. Wright's existence.

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MRS HALE (examining the
skirt). ...She used to wear pretty clothes and be lively, when she was MInnie
Foster, one of the town girls singing in the choir. But that--oh, that was thirty years
ago...

MRS. HALE. Not having children makes less work--but
it makes a quiet house, and Wright out to work all day, and no company when he did come
in…

MRS. HALE. She--come to think of it, she was kind of
like a bird herself--real sweet and pretty, but kind of timid and--fluttery.
How--she--did--change...

MRS. HALE (with a slow
look around her.) I wonder how it would seem never to have had any children
around. (Pause.) No, Wright wouldn't like the bird--a thing that
sang. She used to sing. He killed that,
too.

All of these statements
show a progression of change in Mrs. Wright, and John Wright's subjugation of her. Not
only did the woman change from the lively and lovely woman of thirty years before, but
she had no children. It's logical to assume that she was very lonely. However, she found
some joy, some relief from the "stillness," in the form of a little bird. It was a
bright spot in her dark existence, and John Wright killed it, as (Mrs. Hale observes) he
killed every other good thing in her life. In killing the bird, Wright killed Minnie's
happiness. Is it irony or poetic justice on Mrs. Wright's part that her husband died
being strangled in his sleep, while the bird died with his hands
around its neck?