Where It All Goes Down
We'll find two contrasting settings in "Sympathy": the confined space of the cage that the bird is trapped in, and the wide open spaces of nature. The cage where the bird hangs out (not voluntarily, of course), isn't a nice place. The bars that frame this cage are described as "cruel" (9), and the bird has nowhere to go except to its little perch inside the cage. It can't fly around. It can't jump from tree to tree inside the cage. It's totally, utterly stuck—bad times.
By contrast, the wide open spaces of nature that the poem depicts are beautiful. Outside in these spaces, the sun is "bright" (2) and the grass is "springing" (3). Flower buds let out a beautiful "perfume" (6). Unlike the cage, nature is an unconfined space—it's big and open.
The contrast between these two settings is important because it highlights to us readers just what a miserable time the little birdie is having in its cage. How can it be happy in this tight little space, especially when there is all of this beautiful open space outside that the little birdie doesn't have access to? So these settings are both symbolic: the cage represents the bird's imprisonment, and the natural setting represents freedom.