The little bird is shut up in a cage, away from fields of air. All the same, she sings, praising—somewhat sarcastically, it should be said—God for making her such a prisoner. But then, that's all she has to do in that confined space. What else is there for her to do but sing? And sing she does, in the certain knowledge that it pleases God.
No prizes for guessing that the poem is in fact a succinct allegory of how women were treated by male society in the 19th century. Like the little bird, women were also imprisoned, confined to the home to act as wives as mothers. Just as the bird sings all day in her little cage to please God, women were still expected, despite their confinement and lack of opportunities, to please their husbands and to be sweet, smiling, and constantly attentive to their needs.