“If you tried to keep this bird in a cage, it died.” According to this quote, being caged—without freedom—can cause something to die. If you look at the concepts of death and lack of freedom as figurative language, the “something” that dies could be a person, a skill, a trait, or just about anything. To broaden the statement, you can liken “death” to “loss.” The question becomes Where else in the novel does a lack of freedom—a limitation or restriction—cause something to die or be lost?
Here’s a generic example: A parent decides to not allow their child to play sports (the restriction, or lack of freedom). As a result, the child loses several things—not only the chance to develop athletic skills, but the chance to make friends and to learn to work as part of a team. Each loss has the potential for additional negative effects in the future.
A direct example of this is what happened to Estevan’s daughter, Ismene: Being metaphorically caged by the government’s actions—forced to either give up the names of members of the teacher’s union or lose his daughter—would have caused a loss no matter what he decided to do. As it stands, Estevan and Esperanza lost their daughter.
The theme of human rights—and the loss of them—is applicable here. Since the quetzal is "the national symbol of the Indian people in Guatemala," I'd recommend focusing on places in the text where those people have lost freedoms.