What does birdys mom mean in Catherine Called Birdy when she says why not cease your fearful pounding against the bars of cage and be conttent?

Lady Aislinn is telling Birdy to stop resisting the inevitable and accept her upcoming marriage. She uses the metaphor of a bird beating its wings against the bars of a cage to illustrate Birdy's futile struggle against her fate. Lady Aislinn believes that Birdy should resign herself to her marriage and make the best of it, rather than waste her energy fighting against it.