In the quote "He told me that the national symbol of the Indian people in Guatemala was the quetzal, a beautiful green bird with a long, long tail. I told him I had seen military macaws at the zoo, and wondered if the quetzal was anything like those. He said no. If you tried to keep this bird in a cage, it died." Indicate what the novel gains from this image—that is, explain how the image relates to the larger themes running throughout the work, thereby enriching the experience offered readers.

The idea in the given quotation is that freedom and independence are essential to the lives of the Indian people. This is why the quetzal is the national symbol of the Indian people in Guatemala.
The protagonist of The Bean Trees is Taylor Greer, who leaves her family home in Kentucky and heads west to find adventure, freedom and independence. In relation to the quotation, Taylor begins the novel as the macaw bird and then becomes a quetzal. In other words, she begins the novel in a small town in Kentucky, metaphorically caged (like the macaw birds at the zoo) by the limited prospects there, and she becomes free and independent like the quetzal bird. Taylor leaves her hometown because she is afraid that her spirit, and her independence, will die if she stays. She fears dying in her own cage, just like the quetzal.
The idea in the quotation that the quetzal will die if it is kept in a cage also relates to the characters Estevan and Esperanza, who are political refugees from Guatemala. Estevan and Esperanza flee to America in search of freedom. The idea that one might metaphorically die if one's freedom is taken away helps explain Estevan and Esperanza's decision to leave their daughter, Ismene, behind in Guatemala. The obvious reason why they do this is because they do not want to give to the authorities the names of other members of the teachers' union, knowing that if they did, those members would lose their freedom and perhaps even die. Estevan and Esperanza make enormous sacrifices to protect the freedom of their friends, knowing how essential freedom is to life.
Nonetheless, when Estevan and Esperanza essentially sacrifice their daughter for the freedom of their friends, it still seems like a difficult decision to fully understand. It is easier to understand, however, if we appreciate that Estevan and Esperanza's own freedom is synonymous with and essential to their lives. They left their daughter behind in part because if they had stayed, they too would have died in cages, metaphorical and literal.
When Estevan and Esperanza move to America, they still lack freedom, because they are undocumented immigrants, meaning that they are denied the rights and freedoms afforded to American citizens. They only become free in any meaningful sense when they travel to a Cherokee reservation, where they are much less likely to be apprehended by the authorities. They, like Taylor, make sacrifices and endure hardships to acquire their freedom. And they, like the quetzal, need freedom to live.