What does a bird represent for many people? A bird can fly, so we perceive that it has a kind of freedom we cannot have. Many poems that include birds have freedom as a theme. In this poem, the narrator is telling us that capturing a bird cannot give us freedom! That makes sense, doesn't it? When we capture a bird that is free, what happens? Quite often, the bird does die. So one message is that we kill freedom when we capture those that have freedom. In the case of a bird, we cannot offer the proper support to nurture the bird, so it can be free. As the narrator says, "You are not a nest...."
But there is another theme in this poem, also concerning freedom. What does the author say can fly in the last verse? He is talking about words, isn't he? Can we use words to gain freedom? One example is the Declaration of Independence, which you have certainly learned about in school. Lane lived in South America for several years, where freedom was at risk or nonexistent in many countries. There were and are Latin American writers who use words, in stories, poems, novels, and articles, to fight for freedom for their people. Notice that he specifically talks about a poem "that talks about the end of cages" (18). Which is more likely to create freedom, caging a bird, or writing something to help people be free?
This is an interesting little poem, and I hope my discussion of it helped you to understand it. People sometimes disagree about what poems mean and what their themes are. The idea when you are analyzing a poem is to read the poem carefully, use your own ideas, and rely on the words in the poem to support your ideas.
Good luck to you.